I would also like to hear an expert opinion on this. Never really made sense to me either, but it also doesn't sound unreasonable.
this post was submitted on 26 Dec 2024
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I have an RV with 2x 10 gauge, 50' extensions running power to it. I have a built-to-purpose coverat the connection point. Its fine and safe enough. Just keep slack at the connection. It cant be under pull stress.
Check the neutral pin at the pedestal and the connection. After a few months running an AC unit, it'll be char broiled somewhere along the way.
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The longer the cable, the thicker it needs to be to carry the same amount of current without getting hot. This is due to resistance (Wikipedia does a good job with the details). Basically:
20' cable be long and thick.
10' cable be short and thin.
10' + 10' cables be long and thin.
Long and thin = heat then fire.
That being said, put an LED light bulb at the end, no problem. Put a gaming PC on the end, problem.
I don't think you're right about heat. Two cables in series have double the resistance and therefore double the total heat generation, but that heat energy is split between the two cables so each one does not get hotter than if it was the only cable.
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I think a lot of people are mostly on the money here. It's to do with resistance. Now, I'm not a qualified electrician, but I'm an amateur radio license holder and a lot of what you learn for that is applicable here.
The main problem as many have said is resistance. This comes about from both the length of the conductors but also from every plug/socket connection adds resistance. Also in the case of the non extension socket multipliers, as you add more the weight bearing down would also likely start to make the connections less secure causing more resistance and possibly adding to the problem through arcing.
Now the resistance alone on small loads likely wouldn't be a huge problem. But if you had a large enough load (specifically at the end of the stacked connectors/extensions), or a fault that caused a larger than expected load the current would cause the resistance to generate heat.
There's a lot of ifs and maybes involved, but really why do it? There's really no real world situation to need to have a dangerous amount of extensions like this though.
For larger loads here in the UK there's some very specific other concerns when dealing with ring mains. But really you'd need to do really weird/unusual things for that to become a problem.
On the contrary, practically every single person in this thread that mentions resistance is talking precisely backwards. Higher resistance will not cause overheating. Except for certain motor loads, it will cause malfunction due to voltage drop. The actual fire hazard is the increased chance of faulty cable, faulty connection, and, inevitably, overloading.
Resistance alone doesn't cause heat. Drawing current through resistance causes heat at the point (or points) of resistance. Which is why I clarified that it's not likely so much a problem on small loads.
This is why resisters come in different physical sizes. Because they have differing abilities to dissipate power as heat.
A good example is of dummy loads in radio use. Which needs to dissipate the power output of a radio. That can be anything from milliwatts to a kilowatt. Up to probably 50w they will have a basic heatsink. I've seen huge drums filled with oil as 50ohm resisters to handle up to a kilowatt of dissipation.
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Extension cords are rated for certain levels of resistive load based on their length. It typically has to do with the gauge of the wire used in the chord itself. Electrical resistance is typically converted into heat energy. If you daisy chain extension cords and have them plugged into appliances, then the resistive load of the chained cords can be higher than the rating of one or more of the extension cords. This can cause fire.
It's going to be down to the gauge of the wire and if it's rated for hard usage. There would be no difference if the 4m cable was the same gauge and insulation type as two 2m cables connected by cord and plug. You couldn't say how dangerous it is specifically without calculating the load, wire ampacity, ambient temperature, insulation type, distance, and on and on so it's general advice to prevent fires from overheating wires.
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I'm not an electrician, but I recall that this video explains it pretty well (along with other interesting things): https://www.youtube.com/watch?v=K_q-xnYRugQ
Here's another good example: https://www.youtube.com/watch?v=4cp28hIMP94
Stacking 283(!) double mains adapters and measuring the voltage drop. Because it's not just the cable, each plug/socket adds additional resistance.