extension chord hazard

Thread Starter

Nathan Hale

Joined Oct 28, 2011
146
hello guys i was at homedepot today and bought a radiator heater. i was about to buy a really really long extension to use with it. i was told by the homedepot guy that the really long extension chord will be a fire hazard and not to use it. does anyone know why it would be?
 

praondevou

Joined Jul 9, 2011
2,942
No, I don't know why he said this. If the extension is rated for the current/voltage the heater needs (better a bit more) I don't see why it's supposed to be more hazardous than a short one.

Maybe he was referring to a model that didn't have the proper rating?
 

tommydyhr

Joined Feb 3, 2009
39
If the extension cord is wound up, and significant power is drawn through it, it could, potentially, become a quite significant heat source.
 

thatoneguy

Joined Feb 19, 2009
6,359
If you were looking at something such as a 100ft 16 gauge cord, I can see where he would have suggested something more beefy, such as 12ga. (which is more money).

For 100ft, at 10A draw, with 16 ga wire (Max Peak Current 18A), you'd be losing about 4V, and the cord itself would dissipate 35 Watts of heat. That's for a 1000W heater.

Going to 12ga wire (Max peak current 30A), you would only drop 1.5V, and dissipate 14W over the 100ft run for the same 1000W/10A draw heater.

Usually at the extension cord center, they have a UL card showing length to current at various gauge wires, with what is OK and what would fail code.

Many cords are rated by the "Peak" current, which they can sustian for 10 seconds without becoming overly warm. Stick to the continual current rating to be safe. The longer the wire, the more resistance the wire has, so the larger gauge the wire needs to be.
 

jimkeith

Joined Oct 26, 2011
540
I have also heard people like this salesman say that you must uncoil extension cords to reduce inductive reactance so your power tools can operate at full power...
 

mcgyvr

Joined Oct 15, 2009
5,394
People that work at Home Depot are always right... NOT.

Using the properly sized extension will not be a problem. It's the idiots who just use the cheap-o "lamp wire" extension cord and then wonder why their house burned down.
 

colinb

Joined Jun 15, 2011
351
I have also heard people like this salesman say that you must uncoil extension cords to reduce inductive reactance so your power tools can operate at full power...
Which is not true since the current flows in a loop, thus canceling any magnetic field, right?
 

mcgyvr

Joined Oct 15, 2009
5,394
Which is not true since the current flows in a loop, thus canceling any magnetic field, right?
You "should" uncoil an extension cord to decrease heat on the inner wraps of the cord. Just like a person in the center of a group hug is always the warmest :)
 

crutschow

Joined Mar 14, 2008
23,500
Which is not true since the current flows in a loop, thus canceling any magnetic field, right?
Nope. Coils make fine magnetic fields. It's how an electromagnet is made. But the effect of the small inductance of any reasonable number of turns of a power cord is negligible at power line frequencies.
 

jimkeith

Joined Oct 26, 2011
540
Nope. Coils make fine magnetic fields. It's how an electromagnet is made. But the effect of the small inductance of any reasonable number of turns of a power cord is negligible at power line frequencies.
Bifilar conductors and coils as in paired extension cord conductors tend to cancel the magnetic field thus minimizing self-inductance.
 

Jotto

Joined Apr 1, 2011
151
Outlets used are rated at 15 amps unless you purchase one that is rated higher. The 15 amp rating is for instantaneous load, not constant load. You can actually burn out the outlet if your not careful. These heaters will accomplish that. Most extension cords are not rated at 20 amps with 12g wire, most are 16g. If your going to run a long distance you should run 10g wire.

You will see the plug start to deteriorate over time because of the heat build up. Heat will be generated in the outlet box which can lead to a fire in the wall because you have slowly melted the insulation off the wire.
 

someonesdad

Joined Jul 7, 2009
1,583
Outlets used are rated at 15 amps unless you purchase one that is rated higher. The 15 amp rating is for instantaneous load, not constant load. You can actually burn out the outlet if your not careful.
Jotto: this is not my understanding; do you have some experimental or regulatory data to support this statement? I've seen two loads in my house that slightly exceed the 15 A rating of the typical NEMA 15 A 120 VAC socket, yet I've never seen any damage. The loads are a microwave oven and my air compressor. I know they exceed 15 A because the Kill-a-Watt can't measure the current. When I blow my sprinklers out every fall, the compressor runs continuously for a couple of hours and I've been using the same 120 VAC outlet socket and compressor for the last 25 years.

I also know from measurement that a well-made screw connection in an outlet with 12 gauge solid copper wire will have a resistance of around 1 mΩ or below; the push-in connections have a resistance an order of magnitude higher. At 15 A, this would mean a power dissipation of N*225*0.01 (N is the number of wire connections, usually 2) or perhaps 5 watts or so; thus things might get slightly above room temperature at 15 A. I haven't measured the contact resistance of the plug to socket connections, but engineering judgment would put those around the 1 mΩ level unless badly corroded or sprung. Thus, I'd be hard-pressed to see where the power (i.e., IR drop) will come from that will destroy these outlets when run at a continuous 15 A. Of course, I'm talking about a properly-installed one in good condition; certainly, a problematic outlet could dissipate much more power.
 

GetDeviceInfo

Joined Jun 7, 2009
1,642
the NEC speaks specifically to heater loads, circuit ratings, wire derating, etc, plus the allowed circuit voltage drop. Read up or get an Electrician to decipher it for you.
 

mcgyvr

Joined Oct 15, 2009
5,394
Jotto: this is not my understanding; do you have some experimental or regulatory data to support this statement?
There are typically 2 ratings for breakers and their enclosures depending on which type you buy.. There are ones that are rated to carry 80% continuous and those that are rated to carry 100% continuous. Most household breakers are suitable for 80% continuous load. So a 15A breaker should never be loaded to more than 12A for more than 3 hours at a time. This is covered in the National Electrical Code as well as many other places and should be marked or included in the instructions for your breakers/switchbox.

Pretty much all breakers will take 100% of their rating for some time (might be a second..might be 10 hours).. That time is stated/estimated in the time curve for a circuit breaker. Breakers like fuses are available in different time delays based on your needs.
 

Jotto

Joined Apr 1, 2011
151
Sorry about that, I don't have anything other then experience with others and once with something I had done years ago. The heaters turning on and off would start to arc between the plug and outlet, in turn burning the plug and the outlet after prolonged use they do end up drawing more current and then melting the wire inside the outlet box. I did not correctly type what I meant. So I will try to now.

Outlets are on a run usually have 4-8 different outlets. The rating of the outlets are 15 amps, 14g wire is correct for 15 amps, we exceed that with 12g, and most have 20 amp beakers, but the outlet is still 15 amp. Unless you get ones that are rated at 20 amps. Space heaters sometimes don't have a ground and is just the two prong type plug.

Also using a space heater should be on a arc fault breaker in case you do start to degrade the insulation of the wire.

The 15 amp rating is for instantaneous load, not constant load. You can actually burn out the outlet if your not careful.

Still don't think I am saying this right.
 
Last edited:

strantor

Joined Oct 3, 2010
5,158
one thing that effects the plugs is the condition of the inside of the socket. if the socket is old and it's springiness is worn out, and the cord plug just sits in there loosely you can tell because the weight of the cord will cause the plug to sag at a 45 degree angle at the top. a loose connection will cause the resistance to go up and turn the socket & plug into a mini heater. Also the condition of the plug prongs. if they are pitted, it can cause arcing in the socket and ruin the socket. a bad plug can ruin a socket; a bad socket can ruin a plug. I had a problem with these 50A plugs for 30A portable heaters in my plant catching on fire & melting everywhere we plugged them in. I think what happened is one socket got loose or a plug wasn't plugged in all the way and caused heat/arcing which pitted the blades of the plug; then the plug was plugged in somewhere else and ruined that socket, meanwhile another heater was plugged into the first socket and this continued until the plant was full of fire hazards (bad plugs & sockets).
 

thatoneguy

Joined Feb 19, 2009
6,359
one thing that effects the plugs is the condition of the inside of the socket. if the socket is old and it's springiness is worn out, and the cord plug just sits in there loosely you can tell because the weight of the cord will cause the plug to sag at a 45 degree angle at the top. a loose connection will cause the resistance to go up and turn the socket & plug into a mini heater. Also the condition of the plug prongs. if they are pitted, it can cause arcing in the socket and ruin the socket. a bad plug can ruin a socket; a bad socket can ruin a plug. I had a problem with these 50A plugs for 30A portable heaters in my plant catching on fire & melting everywhere we plugged them in. I think what happened is one socket got loose or a plug wasn't plugged in all the way and caused heat/arcing which pitted the blades of the plug; then the plug was plugged in somewhere else and ruined that socket, meanwhile another heater was plugged into the first socket and this continued until the plant was full of fire hazards (bad plugs & sockets).
Just looked, These don't come in 50A rating for single phase. Those are nice, solid plugs that do not unplug unless you want them to. Normally seen in data centers, hospitals.
 
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