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.  Circuit breakers protect wires run in your RV, and in the campground supply.  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.  It’s most likely a molded plug, and those rarely fail.  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 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.

Enough!

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.

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8 Comments Add yours

  1. Jim Lane says:

    Good explanation! I usually take the explanation further explaining that the oxidation on both the receptical and the blades of the plug set up the point of resistance which starts a snowball effect of destroying both the receptical and the male plug. The receptical is ruined by the heat generated which de-tempers the metal thus reducing its spring tension at gripping the blades on the male plug, as well.

    The short answer is usually, “If the plug is getting hot, you’ll probably need to replace both the receptical and the plug on the end of the heater.” 🙂

    1. Dave says:

      Absolutely, depending on how hot we’re talking about. That plastic burning smell? Replace both, for sure. Just a little warmer than normal? Probably OK with just the receptacle.

  2. B Irwin says:

    Good explanation. Many will still not believe you unless an electrician.

    1. Dave says:

      Haha. Not to knock electricians, but most of the time the trade is focused on compliance–doing things to meet a legal standard. What’s actually going on behind the scenes isn’t always an electrician’s forte. Then again, I know plenty of other electrical engineers that know next to nothing about power distribution.

      1. gdottie says:

        I enrolled in a basic electronics tech circuits course to learn electrical basics. It was rather disturbing they focused on how to test out a component or circuit and replace it, rather than the physics of how it worked and what was going on. While it was a good intro, I ended up enrolling in electrical engineering and taking two years worth of course-it was a hobby that got out of hand 🙂 . I can see both sides of your comment. The tech side didn’t teach enough of the engineering part that I wanted, while the EE courses lacked a lot of the hands on application on everyday practical elements. Great article though. I explain things like this and people look at me like I’m from Mars.

  3. francis Helaey says:

    I believe you have a mistatement near the beginning:

    “lower voltage causes larger amp draw and more heat in the wiring and connectors.)”

    1. Dave says:

      Hi Francis!

      That was one of the claims that someone put out there. I go on to explain that it’s not true for a heater (resistive) load.

      –D

  4. BillKelly says:

    …..and sometimes it could be the heater? I had two of them bought at Costco, that the plug gets so hot you couldn’t touch it. One of them actually melted the plastic at the reptacle a little. Moved to a different plug in – same thing. Took the first one back and exchanged for a new one – same problem on both receptacles. Now have a different brand plugged in and everything is fine!

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