For electrically inclined Dopers: Use of extension cords

I use several such cords in my room, which has at least four wall outlets. The computer console, monitor, and inkjet printer are plugged into a surge protector plugged into an outlet. Other electrical things, including three lamps, an electronic keyboard, an electric typewriter, a boom box, a coffee-cup warmer, a cordless phone charger, a TV, a VCR, and a digital alarm clock, are plugged into other outlets, usually with a “power strip.” I also have an “oil-filled” room heater whose tag bears a notice requiring that any extension cord used with it (and I would need one to position the heater far enough from furniture in the room) have a minimum rating of 1875 watts and an AWG, or AGW, with a minimum rating of 14.
For one thing, I’d like to know: Granted the wattage of all these things, added together, is quite high, does the potential danger of excessive current exist only if the particular items are switched on? --Even though this sounds like a stupid question. :rolleyes:

Not a stupid question. No, if the devices aren’t on you won’t have any current flow and of course no power use (no wattage) There may be some if devices have a standby mode wlhich means they arent’ ever really off but this should be pretty small.

Extension cords can get warm under load so don’t skimp, especially if you have high power devices like the heater. IMHO you should get a heavier cord that it says you need.

Thanks, Padeye. :slight_smile: Everything else being equal, I’d rather ask a stupid question than make a stupid mistake… :frowning:

I’d like to add to padeye’s answer if I might.

Excess length of extension cords sould NEVER be coiled up in a holder or spool. Cords are sized to carry their rated load while lying in such a manner to allow air circulation. Coiling the cord or leaving the excess length wound up can lead to fires. This is particularly important with your heater or other appliances that draw other than just a little power.

If your home was properly wired the outlets in your room should be fed from at least two, and possibly three, different curcuits.

Therefore the likelyhood of circuit overloading is mininal.

If you’ve only got one circuit the probability is increased-----particularly if you have ALL appliances “On” at the same time.

The likelyhood that they’d ALL be “ON” at once is unusual.

Add up all of your wattages- per- outlet,convert to amps and check you wire size.

If I remember correctly #!4 wire is good for 14 amps max.

But don’t hold me to that value----check it out!

well, sort of.

tv’s vcr’s etc use power even when turned off… i think i read the average tv uses something like 18 watts when turned off. god knows why, but they do.

but compared to the power they use when turned on, its actually really minimal. so you dont need to worry about blowing yourself up unless you try to turn them alll on at once :slight_smile:

even then you’ll probably only trip the breaker.

i would be careful with that heater though, it probably sucks more current than all of the other stuff combined.

It probably would, Something. :slight_smile: I have rarely used it lately because it’s coldest at night, of course, when I’m asleep or should be; and of course I don’t even want it plugged in at that time, let alone swiched on.

I’m sorry, but that just isn’t true. The plastic that the insulation is made from (usually PVC) is an excellent thermal insulator, so that the temperature gradient across it could be quite large. In other words, the copper conductor inside can be very hot, while the outside will remain relatively cool to the touch. Air-cooling is only effective if the heat is allowed to flow to the surface relatively unimpeded, as it is in metals.

So, QED, where does the heat dissipated in an extension cord go, if not through the insulator?

Much of it does get dissipated through the insulation eventually, but due to the thermal insulating properties, it takes time to do so. In practice, an extension cord operated within its ratings won’t get very hot at all but as those ratings are exceeded, the temperature of the conductor will rise much faster than the surface temperature of the insulator.

Q.E.D.: I think GaryM is correct. The issue is that the copper conductor can produce a lot of heat when you have a large current density. The temperature of a unit length of conductor is determined by four things: a) I[sup]2[/sup]R dissipation, b) conductor’s surface area, c) thermal resistance between conductor and outside air, d) outside temperature.

An increase in a) raises the temperature of the conductor
A decrease in b) raises the temperature of the conductor
An increase in c) raises the temperature of the conductor
An increase in d) raises the temperature of the conductor

And there is a safe temperature limit, which is primarily determined by the type of insulation used. In other words, the conductor will increase the temperature of the insulation, which (if too high) can be a Bad Thing[sup]TM[/sup].

When an extension cord is wound on a spool, you are greatly increasing c), especially for the loops closest to the spool’s center. It is conceivable, therefore, that when lots of current is flowing through the conductor, the conductor and insulation temperature can go beyond its rated limit.

Oh, and to answer the OP: relax. Just add up all the currents and make sure you’re not pulling more than the limit. And just what is the limit? Every segment of the circuit has a maximum current rating, and the limit is determined by the segment with the lowest current rating. Does that make sense?

From this site:

And while we’re at it, let’s figure out the watts loss in the conductor specified in the OP assumming alength of 25 feet. According to the chart on the site I linked to above, the resistance of 14 AWG wire is .00297 Ohms/foot x 25 feet = .07425 Ohms. Total watts loss = I[sup]2[/sup]R = 15[sup]2[/sup] x .07425 = 16.7 W. That’s not very much. If you can find the thermal conductivity coefficient of PVC you can calculate the temperature rise in the worst-case scenario.

Q.E.D.: Whoever wrote that was an idiot. Temperature rise in conductors is a very important issue, at least if you believe what the National Electric Code and C.S.A. has to say. The NEC goes to great length specifying how to derate the ampacity of a current-carrying conductor based on ambient temperature, temperature rating of insulation, and the number of conductors bundled together (in a conduit, raceway, etc.) In fact, the NEC says ampacity must be derated when more than three current-carrying conductors are present in a conduit or cable. Can you guess why this is the case? Further derating must be performed when the ambient temperature exceeds 30 °C.

This is one (of many) articles describing the importance of derating ampacity based on thermal resistance, ambient temperature, and the number of conductors in a bundle. Interesting stuff.

Here’s an eyewitness who claims…

Of course, he could be lying… :rolleyes:

According to the Naval Safety Center, “Don’t overheat the cord. Uncoil it, and don’t cover it with anything.”

According to Nevada Power, “Uncoil an extension cord fully before use and ensure that the amperage marked on it is adequate.”

According to the Texas-New Mexico Power Company, “Uncoil an extension cord fully before use. Find the amperage marked on it.”

Leviton sez, “It’s a good idea to uncoil a long extension cord before you use it to prevent heat from building up in the cord itself.”

There’s also another reason to unroll the extension cord: for some equipment the inductive effects (though small) can cause havoc with the appliance. This effect is mentioned here, here, and here.

It’s actually 33.4 watts. And if you put enough insulation over a 33 watt heater[sup]a[/sup] it might get toasty, depending on thermal resistance and ambient temperature.
[sup]a[/sup][sub]It’s actually less than this since the outside coils see less thermal resistance. But whatever…[/sub]

There is one easy answer to the above debate and it has a very easy answer. If the extension cord you are using is warmer than the ambient temperature, there is more current being drawn throught the extension cord than it was designed for. Extension cords should not give off any heat, when stretched out or coiled.

Ooops. I forget to account for both conductors. :smack:

And I’ll cede the other point to you.

I am confused by what you are saying here. Are you saying that the room should have outlets served by different circuit breakers? The houses I am familiar with would put one or two rooms on a circuit. Maybe some large rooms would have more than one circuit or perhaps rooms with special requirements like the laundry room or kitchen.

That’s not really true, Racer72. A conductor that is not carrying any current will be at ambient temperature. A conductor carrying current, no matter how small, will have a temperature higher than ambient. The heating power produced by a conductor (which drives the temperature differential) is proportional to the square of the current.

A good rule of thumb is that if the extension cord’s external surface feels warm it’s probably carrying too much current…

Many issues here.

Going back to the OP, extension cords were never meant to take the place of installed wiring in a dwelling or commercial occupancy. They are of smaller wire gauge size than the branch circuitry, and are subject to frequent physical abuse, rendering them a likely point for creation of an electrical hazard.

Most inspectors invoke the 90 day rule of NEC Article 527.3 (B) regarding Temporary Installations. This covers Christmas lights, and similar purposes.

I apply the same logic to my home as that of friends and customers. If you need to plug something in using an extension cord for greater than 90 days, have a proper duplex or quadriplex installed.

NEC Article 210 covers branch circuitry, and regarding the query of a previous poster, the kitchen, per 210.11 ©(1) requires a minimum of two (2) small appliance branch circuits. Other minimums are related to calculated load for a dwelling.

Sorry to seem like I’m thumping the tub, but I’ve been a firefighter for almost 30 years, and I’ve seen too many homes and businesses destroyed where faulty temporary wiring was the cause.

You’ll never think it will happen to you until it does. Be safe