Electrical Myths, Part 2: Showing that Heater Current doesn’t go up when Supply Voltage Drops

Last time, we talked rather abstractly about why a space heater doesn’t draw more power when supply voltage drops.  This time, we’re going to talk about it with a real space heater, current and voltage measurements, and a short, poorly produced video (I made it, so I can say so).  More pictures and fewer formulas this time!


  • Ordinary 1500W space heater.  Mine is several years old, here’s something similar.
  • Transformer and switch box.  This will get its own post soon, but it’s a circuit that can drop supply voltage by 10%.
  • Multimeter.  I’ve already written about them in general, and the one I’m using in this post.
  • Line splitter, from this post, will let me use the clamp-on ammeter function of my multimeter to measure current.

First Measurements

The  first thing we’ll do is measure voltage, and see that the switching circuit works as intended.  The voltmeter is set to measure AC volts, with the line splitter plugged in to the receptacle.  On the end of the line splitter, there are two small holes for the meter’s leads–which is a much easier and safer place to measure voltage than by sticking the probes directly into the receptacle.  One lead in each and we’ll see about 120V on the meter:

Meter probed inserted into line splitter.
Meter showing 120.7VAC with switch in normal position.

If we flip the switch, bringing the transformer into the picture and subtracting about 10%, we expect to see voltage between 105V and 110V:

Meter showing 106.5VAC with transformer circuit enabled.

Next step is plugging the space heater into the line splitter.  If we turn on the fan and flip the switch, you’ll hear the sound chance (video at the end of this post).  The motor is slowing down with the reduced supply voltage.  This is not a synchronous induction motor, like we have in our air conditioning compressors, but a fractional-horsepower universal motor–it’s basically a DC motor, where speed is a function of input voltage and load.

Turning on the heat, let’s see how current changes.  With the switch set to pass through supply voltage, we measure 7.5 amps:

With heater on, we’re measuring about 7.5A.

If we wanted to measure current on something not drawing so much power, or wanted a more accurate measurement, we can use the 10X loop on the line splitter–what this does is run the power lead through the loop 10 times the same direction, making the current sensed appear 10 times larger.  Doing this, we essentially get another significant digit from our meter:

Now we’ll flip the switch, dropping the supply voltage, and measure current again:

Now the Math.  Does it Make Sense?

Now for just a little bit of algebra to see if those numbers make sense.  Recall Ohm’s Law:

We’ll add subscripts to tell the two pieces of information apart, and rearrange solving for R.  First just with the subscripts:

Solved for R:

Let’s plug in some numbers, and see how close those come out:

The numbers are a little different, but within 3%.  There is a slight temperature effect on resistance, which will account for most of that difference, but for now we can say that resistance is approximately fixed.  In that case, we can rewrite the two equations above as a single relationship, and ignore what R actually is:

With that relationship, we can estimate what current will be drawn for a hypothetical supply voltage as long as we have a supply voltage and current measurement.  But perhaps more interesting is to look at power.  We know that power, P, is current, I,  times voltage, V.  So we know that with normal supply voltage

With the reduced supply voltage,

The difference is a lot more obvious with these numbers.  That’s a result of the quadratic nature of voltage with a resistive load.  Recalling the equations from last time, relating power to either current and voltage or voltage and resistance, we should get about the same result:

It’s basically the same math, but it’s a little easier to see the effect of voltage on power both directly and as it affects the current flowing.  I’ll save it for later, but that’s also a big part of why your lights quickly get dim as voltage falls.

Here’s the video:




Electrical Myths, Part 1: My Space Heater’s Plug Gets Hot

Since it’s still winter time, let’s start with one dealing with space heaters:

My RV’s wiring isn’t big enough to handle a space heater.  If I plug one in, the heater’s plug gets hot.  I should blame the campground, then open the box and hose it down to cool it off.

Ok, that might be a little much.  But let’s start with the most basic of principles in circuit protection.  Whenever a wire is run in your RV or in the campground wiring, it is protected by a circuit breaker.  That circuit breaker has a rating in amps, and its most basic job is to kill power if the load on the circuit exceeds what the wire can safely handle.

That’s why the wires on your 50-amp RV’s shore power cord are bigger than on a 30-amp RV.  It’s also why the wires are the same size on a small 30-amp travel trailer as they are on a larger 30-amp motorhome.  The breaker can’t allow more than the wire can safely handle, so if you need more power, you have to step up to heavier gauge (smaller AWG size number) wire.

Once inside your RV, at the main power distribution panel, the wiring coming in splits out to power a number of devices–things like air conditioners, battery chargers/converters, refrigerators, dishwashers, and all of the household-type receptacles.  The wires feeding those things aren’t as big as what’s coming in from the campground pedestal, and are protected by circuit breakers, with smaller ratings, in the distribution box.

For a typical branch circuit, that means either a 15 or 20-amp circuit breaker.  Again, that sets the wire size going to each outlet.  The standard receptacle in your RV is a 15-amp receptacle.  Each receptacle is rated for 15 amps too–think about that for a minute.  With say 10 places to plug something in, any of them individually able to fully utilize that capacity, how do we not overload the wiring?  The circuit breaker serves as protection against that, though some planning is useful to prevent nuisance trips.

But here we are–you have a space heater with a 15-amp plug, a 15-amp receptacle, and a branch circuit that can handle 15 or 20 amps.  So we plug the heater in, and turn it on.  After a little while, the plug is quite hot (a little warm is normal, but here, it’s almost too hot to touch).

Why is the plug getting hot?

Now let’s dig into why that might be.  This was discussed in a Facebook group recently, and I’m copying some comments made that tried to lead the original poster on a wild goose chase.  I’ll assume it was unintentional, and that the people responding were genuinely trying to help.  Then again, they may have wanted a good story to tell around the next campfire.

The problem may be the wiring in your RV being under sized for the current draw or the connectors to the outlet you were using (many RV manufacturers cut corners).

Let’s think about this for a minute.  The wire and receptacles are pretty standard–you’re not going to find a standard outlet rated for less than 15 amps, and they’re not made to accept wire smaller than 14 AWG.  It’s easy enough to figure out if the right side wire was run though–Romex-type cable is color-coded, so all you have to do is look at the outer wire jacket at the breaker box or an outlet.  Expect it to be white, which is what it should be.  So long as nothing is damaged, you should be able to plug any appliance into a 15-amp outlet and be perfectly safe, worst case tripping a breaker.

RV manufacturers love to cut corners, but when there’s a regulatory body that requires a certain specification, they generally oblige.  It’s technically a possibility that something wasn’t done right, but it’s far more likely that something is operating in a degraded state.

The problem is with your shore power. Many campgrounds operate at an under volt condition because they have poor and old and over utilized electrical distribution system (lower voltage causes larger amp draw and more heat in the wiring and connectors.)

We’re talking about a space heater, so the above statement is wrong every way to Sunday.  Just flat-out no-truth wrong. So it’s time for an electrical lesson.

What is a space heater, electrically speaking?

While most have some sort of fractional-hp fan motor, for all practical purposes the heater is simply a big resistor.  On a typical space heater, that resistor has a resistance value somewhere between 10 and 15 ohms.  It doesn’t matter whether it’s oil filled, infrared, ceramic, or wire-wound.   Nor does it matter whether it’s a $10 heater from Wal-Mart or the built-in “fireplace.”  They all produce exactly the same amount of heat.

In terms of energy consumption, there’s no free lunch.  The heater is inside your RV, so all of the heat it generates is in the RV.  All of the energy it gets out of the wall outlet is turned into heat–you’ll see some even advertise 100% efficiency.  So again, more expensive doesn’t mean better.  You may get other features, like oscillation, or a remote, or an anti-tipover switch, but you’re not getting any more heat.

So it’s a big resistor.  What does that tell us?  At the most basic level, we’re dealing with Ohm’s Law, and the power equation.  First is Ohm’s Law:

This says that the voltage is current times resistance.  With a little algebra, it can be rewritten as

Right away, this shows us that for a fixed resistance appliance, like our heater, when voltage drops, current drops.

What about heat generated in the wiring and connectors?

Now we’ll introduce a second equation, relating power, current, and voltage:

As applied to our heater, we know that current and voltage go up or down together, so we can see that if voltage drops, power drops as well.

But this is more easily understood rewritten slightly.  To see the relationship between voltage and power more explicitly, we can substitute the Ohm’s Law relationship for I in the power equation:

That lets us easily see what happens to the heater’s power as voltage rises or falls–a 10% increase in voltage results in a 21% increase in power; a 10% drop in voltage results in a 19% drop in power output.

Okay, but that’s power at the heater, not power (heat) in the wiring.  Power in the wiring, or in a connector, is governed by the same relationship.  But we don’t know right off what the voltage drop across the wire or connector is. We do, however, know the approximate current through the connector or wire, and how it changes as voltage changes.

Going back to the power equation and subsituting Ohm’s Law, this time for V, we get the following:

Like our heater, a wire or connection point has (for the most part) a fixed resistance.  If system voltage drops, let’s say by 10%, current through the heater drops by 10% as well.  But for our resistance in the wiring, power drops by 19%.

We could get into heat transfer, and a lot of other downstream effects that come into play if we wanted to estimate a temperature, but we now know a drop in pedestal voltage is not going to make things hotter!

The problem is trying to run a space heater on a 30-amp hook-up.

Uhh. No.  Last time I checked, 15 is less than 30.  There’s absolutely nothing wrong with running a space heater, or even two of them, on a 30-amp hook-up.  What is a problem is trying to run a space heater along with other stuff, combining to use more than 30 amps.

But putting more stuff on that 30-amp hook-up isn’t going to make the plug at the heater hot.  Remember, heat at the plug is related to current flowing through it, and current flowing through it goes down as the pedestal voltage drops.  If you had a heater running, and added a toaster to the mix, the heater’s plug would actually be a little cooler, as the voltage to the heater would be a little lower.  Not enough to notice, mind you, but bigger loads in the RV won’t make the heater’s plug hotter.

My heater is 1500W. P=IV, so if V goes down, I goes up.

Again, nope.  Remember, I and V are related by R, and in the case of a heater, R is fixed.  There are devices with regulated power output where this isn’t the case, but heaters are pretty simple devices.

It’s hot because you didn’t have a surge protector.

I’m starting to sound like a broken record.  Nope.  Surge protectors will get their own discussion, but aren’t even remotely related to the problem at hand here.

Ok, really.  Why is it getting hot?

First things first.  For the plug to be hot, there has to be more power being dissipated at plug than normal.  Recall the power equation:

Normally, the voltage drop across the plug should be very close to zero–maybe a volt or so.  With it, and a normal heater drawing on the order of 10 amps, power at the plug is on the order of a watt.

For the power (heat) at the plug to go up, either current or voltage at the plug has to go up.  If pedestal voltage goes up, or the heater’s resistance goes down, current goes up.  Both are possible, but not too likely.

However, if resistance at the plug increases, so does the voltage drop across it.  The resistance is still very small compared to the heater’s resistance, so total current isn’t significantly changed.  Using the last form of the equation above, we can see that as R increases, so does power.

But why would R increase?  There are a couple of possible causes.  First, there could be fraying or a poor connection in the heater’s plug end.  This isn’t too likely though, as it’s most likely a molded plug.  And the actual contacts are fixed pieces of metal, so unless there’s corrosion on the surface, they’re also probably not the problem.  To be sure, move the heater elsewhere and see if the same thing happens.

That brings us to the receptacle.  There are two current-carrying wires (commonly referred to as “hot” or “line” and “neutral”), and each one has two connection points at the receptacle: the connection meeting your heater’s plug, and the one on the back side connecting to the supply wiring.  The receptacle itself has spring-loaded terminals that are designed to maintain contact with your heater’s plug.  These could be worn, and not contacting with enough force.  On the back side, most RVs have insulation displacement connections, where the Romex cable is just clamped into the receptacle.  Either one could be the cause, but the solution is essentially the same–replace the receptacle.


This post has ended up about four times longer than I shoot for, but hopefully it makes sense. If you have questions, if you think I’ve erred somewhere, or if this doesn’t make sense, speak up!

Next time, I’ll include a demonstration where we monitor power to a heater, measuring voltage, current, and power, and tinker with the power supply.  I’ll be using my electrical meter, which can measure current inductively, and the line splitter that goes with it, so that I can probe voltage and current safely without exposing live wiring.

FYI — If you want to be notified when the next post goes up, there’s a box in the upper left that allows you to subscribe to the “newsletter”.  The only thing you’ll ever get is an e-mail notifying you when a new post is published.

Electrical Truths and Old Wives’ Tales Explained

As I approach 11 years of RVing, I’ve heard lots of things said about what you should or shouldn’t do, or explanations of a particular problem that have been passed along and accepted as truth.  The problem is many of them aren’t much more than Old Wives’ Tales, or are so far removed from the original situation that they applied to as to be counterproductive.  But there are some that hold water too–how can you tell the difference?

With this post, I’m announcing what will become a weekly post, taking various things I’ve heard relating to RV electrical systems and breaking them down so that they can be understood.  It’ll be a combination of write-ups, sometimes with mathematical formulas (as an electrical engineer, I can’t resist that), videos, and demonstrations.  I’ll pull from your comments, things I’ve seen in campgrounds, Facebook posts, etc. to educate you not just that you should or shouldn’t do something, but make sure you understand why.

Inevitably, some topics will be hard to grasp for the non-technical RVer just getting started.  If something doesn’t make sense, or you want to know more, let me know!

Fair warning:  I may exaggerate some of the titles of the myths, just to keep things a little more interesting.

Serious warning: The first one, dealing with a space heater’s plug getting hot, is going to be pretty long.  It’ll show up sometime within the next day.

What do you want to know more about?


Cleaning 240,000 miles and 13 years of dirt off the ceiling

It didn’t look that dirty for the most part when I got it, but I knew it wasn’t the cleanest.  The ceiling just had a slight grey hue, and if you took something down (like I did with the kitchen remodel), you could see a definite difference in color.

Some of that was yellowing, which I couldn’t do much about.   But the dirt had to come down. It’s a padded vinyl of some sort, so it shouldn’t be that hard to clean, right?  Well, as far as I can tell, the film is a mix of oily particulates, possibly from diesel combustion, dirt, road grime, and who knows what else.  But the oily particulate bit is what makes it an interesting animal–it’s not a typical dusting or kitchen cleaning operation.

I’ve had several “false starts” trying to clean it–I’d do one small section, only to get frustrated by how slow it wast going and stop.  It was also tough to see when you’d gotten it all removed–the difference wasn’t so stark as to be obvious when you’re up on a step stool.

But finally, last week, I got to a routine that seems to work pretty well…

Continue reading “Cleaning 240,000 miles and 13 years of dirt off the ceiling” »

Putting things back together

Since there was so much stuff out of the way, it seemed like a good time to get rid of some nasty looking wallpaper in the bedroom.  Every attempt at cleaning got a little dirt off, but it just never looked clean.  A few coats of primer and paint can’t hurt.

Here’s what it looked like before:

Before paint.

Before paint.And after a couple of coats of primer:

Looking cleaner already.

By this point, the floor wasn’t too wet any more, but it still had a ways to go.  Somewhere along the way, I worked my way around the plumbing, trying to cut the carpet as close to the wall as I could.

Lots less moisture than before, but still wet.

In the middle of this, I also decided to finally put up the backsplash in the kitchen, but I’ll do a separate post on that later.  Here’s a teaser:

Now back to plumbing.  I’ve grown to like PEX tubing, and the steel crimp rings that have overlapping bands.  It’s easy to see at a glance that you have a good crimp, and the tool isn’t too expensive. Here’s what I have:

Apollo Crimper

I bought mine at Lowe’s some time ago, and if memory serves me correctly, I paid closer to $50.  I just went to look for a picture and link for the crimp rings, and I’m cringing at how much more I paid locally–$15 for a 25-pack at Lowe’s, $35 for 100 on Amazon.  I lost count, but I used most of two 25-packs of crimp rings, which should give you a feel for the amount of re-plumbing that occurred.

Most of the brass fittings and all of the valves from the old configuration were reused.  The nice thing about the crimp rings I linked is that a side cutter can get them free.  The copper rings take more work, and the tool has to wrap around the pipe, which would have been difficult in the space I had.

In the picture above, you can see the plumbing starting to come together. The drain line from the tank on the right will continue to the tee over towards the left, stopping along the way to supply the pump.  The three valves connected to blue pipe control flow to the pump from each tank, and you can see the first of 2 drain valves installed toward the left.  Note that I’ve started to label the lines–it’s too easy, and will make it easy in the future to figure out what’s going on.

Now it’s more or less all finished up, except for a few loose ends tying all of the pipe.  The round black thing on the overflow line at the front of the tank is a vacuum breaker, to prevent siphoning when the tank is overfilled.  This was a problem previously, as it would sometimes siphon nearly half of the water out before drawing in air.

The way everything is set up now, with the drain and pump supply sharing the same tank outlet, I’m able to connect the pressure sensor directly to the tank (lower front corner), in a position a little less vulnerable than before.  After a check for leaks, it was time to fill the tank and get back to normal.

But I have plans for reconfiguring the side wall of the bed platform.  There’s going to be a little more open space, and a shelf for boots/shoes in a previously inaccessible space.  More on that next time!


Getting things dried out…

So I left off last time with a little bit of carpet pulled up, and an idea of what I was dealing with.  I needed to remove the side wall of the bed platform, which was a quick task of just removing a few screws.  After (mostly) finishing draining the tank next to all of the plumbing connections, it was light enough to pull out away from the wall.

View of the edge of the carpet with the bed platform’s fascia removed. I hate when flooring is run under walls and other permanent stuff.  All of that carpet is damp, so the tank has to come out.
It’s wet. But it’s definitely still solid, and there aren’t any signs of mold.

The carpet under the tank wasn’t too damp, but the wood under it was.  Some of that was surely the mess I made getting everything disconnected, and moving the tank with probably about 10 gallons of water still sloshing around in it.  While it was a mess, the wood was still solid, and there weren’t any signs of mold.  It was pretty clear this was a recent thing.

Slowly starting to get dried out.

It did take quite a while to get it all dried out.  There was still carpet around the pipes against the wall, and under the wall itself.  So as the floor dried, more moisture weeped out.  Ultimately, I let it go for about a week before all the signs of water were gone.

Plumbing manifold feeding water pump.

The goal of the plumbing modifications was to get things consolidated along the back wall, and get all of the valving within easy reach.  Here’s the start of that. The three valves connected to the blue piping are the supplies from each of the three fresh water tanks.  Also notice that I replaced hard (well, PEX) pipe with flexible lines.  For whatever reason I hadn’t gotten around to that on this RV–it makes a huge difference in the amount of noise the pump makes.  If you ever want to do the same thing, here’s what’s needed:

And while on the subject of other modifications that are pretty easy for anyone to do, I should probably mention the expansion tank you can see in that picture.  It’s a 2-gallon tank with a rubber bladder, with air pressure on one side and water on the other.  It also reduces pump noise, but more importantly reduces the pulsing water flow typical from RV water pumps (by absorbing pressure spikes that occur with each diaphragm cycle in the pump), and it allows the water to flow for a little bit before the pump cycles on.  A quick flush of the toilet, or even filling a glass of water can often be done without the pump needing to run, and it’ll reduce the number of times the pressure switch has to operate.  For about $40 (link here), it’s an easy and cheap upgrade.

Next time will cover a little bit of painting and starting to put things back together.

Drip, Drip, Drip…

You never want to hear the water pump cycle on when no one is using water.  There’s only one thing it could be…a leak.  Now we aren’t talking about something every few minutes, or even every few hours.  It was more like once every few days, but I knew I had a leak.

Looking down at the maze of plumbing and wiring (the carpet is on the floor).
Looking down at the maze of plumbing and wiring (the carpet is on the floor) after the mattress platform was removed.

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My FoodSaver turns 25, and fixes the Air Conditioning

FoodSaver V2244 — a more recent version of the original that I have.

With anything I carry in the RV, it’s nice when it can perform more than one function.  Such is the case for my FoodSaver vacuum sealer–it gets used regularly preparing food for the freezer, and for cooking sous vide.  But it also falls in the “tool” category as well.


How’s that?  Well, it’s actually a pretty good vacuum pump. I had to replace the air conditioning condenser on the car, and the FoodSaver has been called into action (on more than one car, no less) to pull a vacuum to get it ready for charging.  On the bottom of the lid, there’s a hose connection that will fit an 1/8″ hose barb, and with another fitting, can adapt to the flare fitting on the AC gauge set.

Evacuating the repaired system prior to charging.
Evacuating the repaired system prior to charging.

It may not be the fastest, but it’s able to meet the manufacturer’s  charging spec (-14.6 psig) in less than a minute.  I tend to run it a little longer for good measure, but it’s probably not necessary.


So what about my FoodSaver?  It was a hand-me-down when I graduated from high school, and has been with me ever since.  It’s not exactly pretty–the white plastic has yellowed some–but it still works just fine, now at a little more than 25 years old.  I’ve repaired the heat strip once during that time, but it was just a matter of re-gluing the protective cover in place.

So–if you want another gadget for the kitchen, and need to convince the mechanic it’s good for him (or her) too, there you go.  If you’re the tool buyer, expand your budget just a bit by showing it works in the kitchen too!




Replacing PowerTech Generator Mounts

Last time, I briefly mentioned that the mounts on my generator were in need of replacement.  You can see in the picture below that the rubber had deteriorated quite a bit, and was an oozy, greasy mess.

Old, partially collapsed mount.

PowerTech sells a replacement mount for my generator, but at over $60 each, plus shipping, I felt it was severely overpriced.  It looked generic enough, so I figured I could find what part they used or a compatible substitute.

Sure enough, a Lord (or Doosan) J-20922-2 mount matches up and does the job, at half the price.  Showhauler anchored it to the motorhome body with two bolts (3/8″ hex head), and a 14mm hex head bolt went up from the bottom into the generator.  It could have been easier, except that the hole to get to the bolt directly from underneath wasn’t big enough to get a 14mm socket through–the bolt into the generator had to be dealt with with an open-ended wrench.  Fortunately, the generator was light enough (ha!) that a pry bar could lift it enough to slide the mount out.

What’s left of one of the old mounts.

The old mount was obviously long gone, supporting the weight of that corner without any rubber in between.  With any luck, the new mount will be good for quite a few years.

New mount installed.
New mount installed.

While I was in there, I also replaced the fuel filter.  It was probably long overdue, as I’d never done it–and finally figured out that the number printed on the old filter was missing a digit.  I was able to cross-reference it to a Baldwin BF7648 or Luber-finer FP588F, and the fuel pump took care of filling the filter and re-priming the injection pump.

More 12-volt Tweaks

I mentioned last time that I wasn’t happy with the efficiency of the 12V power supply I installed for running the lights, water pump, furnace, and a few other smaller items.  How bad was it?  Well, putting out 2 amps at 12.3V, it was drawing 52W from the wall outlet.  That’s less than 50% efficiency, and while it isn’t a lot of power being wasted, I expect this power supply to be on 100% of the time–so in terms of total energy wasted, it would account for a pretty big chunk.  At 5A output, it was a little better, but still needed 100W in (60% efficiency).

Continue reading “More 12-volt Tweaks” »