# Does my extension cord have too much internal resistance?

We had a snow storm yesterday and today I found in my front yard the 25-foot 110-volt outdoor-rated extension cord had caused a melt depression in the 5-inch snow. (In the picture the curving line is the extension cord)

I'm not looking for system optimization here. I just want to make sure that this isn't a fire hazard and it's within safety tolerances of the extension cord.

Edit: I should add it got to 30 deg Fahrenheit (-1 Celsius) last night so it wouldn't take too much of a temperature increase to cause melt.

New Edit: thank you all for your help. I don't have a voltometer. I will check the temperature when the timer clicks on but I don't think it's too hot to hold as some snow was left on top of the cord. If more snow fall comes down (it's all melted away now) I will swap out cords to see if I can demonstrate the same effect.

• What load is connected to the extension cord, and how much power does (or did) it use? Dec 10 '17 at 15:10
• It's a projector displaying holiday patterns on my wall. It is on a timer so it stops at about 11 pm. It's an LED projector I believe (I threw out the box) so it shouldn't be loading very high I think.
– user1578295
Dec 10 '17 at 15:45
• Please edit that information into your question :) Dec 10 '17 at 16:03
• I did a quick back-of-the-envelope calculation; at 1.5mm² (a common wire size here, roughly 15 AWG) the cord has a total resistance of 0.0224Ω/m. If your load is 110W, that's 1A, and thus 0.0224W/m. That's very little dissipated power, and my intuition is that that would not be enough to melt the snow. Maybe it's a different phenomenon (e.g. the cord depressing the grass with its weight, causing the snow depression you see), or maybe something is wrong. It would be helpful if you could measure the power draw at the start of the cord with a kill-a-watt-type device. Dec 10 '17 at 16:04
• Read this question - so similar to yours.... electronics.stackexchange.com/questions/344179/extension-cords
– Solar Mike
Dec 10 '17 at 16:20

A few relevant facts:

1. The maximum current of the extension cord is the current where it produces some specified temperature increase above ambient, possibly 10 or 20 degrees C. (Or some specified voltage drop from one end to the other, hopefully not more than 10 V).

2. Snow is a fairly good insulator, so once it has covered the extension cord, all the heat produced by the cord will be contained around the cord. This means the cord temperature will rise more than it would if the cord were surrounded by air.

3. Snow (typically) falls only when the air temperature is in a fairly narrow temperature range around the freezing point of water. So it won't normally take much added heat to start melting new-fallen snow.

These three facts mean that what you saw does not necessarily indicate a problem.

But of course you should make sure the current load is no more than the cord is rated for, and that the two ends of the cord are both well protected from water intrusion if you're going to use it with snow falling or on the ground.

• Any current will produce some non-zero temperature increase, so one could expect some heat to be produced even when not drawing the maximum rated current. That's the first law of thermodynamics. Otherwise, these are good points.
– Jon
Dec 11 '17 at 17:17
• @Jon, edited to make my meaning more clear. Dec 11 '17 at 17:18

First off, if the cord is not overtly warm to the touch, it's safe. Some resistance is expected, normal and ok.

Second, check the rating: Cords come in different wire gauges, ranging from light duty cords unsuitable for heavy use at any length, to thick wires for running long distances with minimal heat or voltage drop.

Here this is a SJTW type cord with 16 gauge wire and 3 conductors. You can readily look up maximum length/amperage for a given cord wire gauge.

Since you stated the length in feet and the voltage is 110 volts, I will assume you are in the USA or Canada. The cord should have a UL, cUL, ULc, CSA, or ETL (USA only) mark. That provides pretty good assurance that the cord will safely carry the maximum current that any one appliance will draw if it has a plug for a 15 amp outlet. That is assuming that the appliance is also marked with one of those marks.

• I think so. I don't know if I can check on the cord, since I threw away its packaging a long time ago. I'll check on the device when I go out again.
– user1578295
Dec 10 '17 at 15:53
• I'd recommend testing a different cord anyway just to be safe. The cord could be defective or worn out or even counterfeit
– Jon
Dec 11 '17 at 17:21
• In the USA, listed extension cords are required to be marked along the cord itself, not just on an attached label, including the company name, UL number, AWGs and count of conductors, and the service type (e.g., SJTW) of the cord, among possibly other things. Dec 12 '17 at 2:54
• This answer is off base. Cords are available in various wire gauges, so you can't just assume "one appliance" is OK. Dec 28 '17 at 1:08

I just want to make sure that this isn't a fire hazard and it's within safety tolerances of the extension cord.

When I ask myself a question like that, I do the following ball-park estimation. I carefully touch the cord by my hand. If I can hold it for about 5 seconds or more (it's not too hot), then I guess there is no danger.