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Old 01-19-2010, 10:50 PM   #1
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Maximum vs safe capacity of 15 Amp circuit


I was on another forum and someone asked if it was ok to run a 15 Amp circuit at exactly 15 Amps and if it would cause any issues. I replied back that although technically you can, probably not a good idea. Down the line may cause overheating, physically effecting the overall condition of the wiring, device failure, etc... Best to run a new 20 Amp circuit if possible or simply reduce the load. I also remember something about an 80% rule of any branch circuit but don't have a copy of NFPA 70 to look that up. So I told him to run no more than 12 Amps on the circuit to be on the safe side.

Well some yo-yo replies to my post and basically said that I was full of it! That a 15 Amp wire can actually handle up to 32 Amps and all that would happen is a "little heat on the wire".

Someone please tell me I'm not going nuts!!!

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Old 01-19-2010, 10:55 PM   #2
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Maximum vs safe capacity of 15 Amp circuit


I'm not qualified to give an exact answer, but I'm sure depending on the grade of the wire and the quality of each termination it could technically handle more then 15 amps. But the ones that decided that you need 14 awg minimum for 15 amps probably did much more testing and research then anyone of us did, and in a more controlled environment. I'm sure there's lot of math behind it as well. There's what it can do, and there's what it can do safely, best to stick with what it can do safely. Also I'm sure you could pump a couple hundred amps through it if you want...., for a very short period of time! You want to be able to pump it's rated max non stop and the situation remain safe.

Kinda like fixtures that say 60w max, you can probably get away putting a 100w bulb in there and it wont melt under the right conditions, but still not a good idea to go over what is stated.

I personally would not feel comfortable running a continuous borderline load either, at the most I would at least change the wire to 12awg and still keep it at a 15 amp breaker, but may as well change the breaker too as it will probably eventually trip.

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Old 01-19-2010, 11:06 PM   #3
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Maximum vs safe capacity of 15 Amp circuit


The wire itself can handle a lot more than 32 amps. I have an electric wall oven manufactured by GE that draws 30 amps at 240 volts, and incredibly enough the STOVE wire is 16 gage. This is perfectly legal, since the manufacturer of the stove is not covered by NFPA 70. Note that the circuit wire must be #8 copper, since the circuit is governed by NFPA 70, but once you get into the stove the design of the wire is not controlled by NFPA 70.

The key to determining how much amperage a given wire can handle is almost always driven by the temperature rating of the wire insulation. In the case of my GE oven, GE is obviously using very high temperature rated insulation, much greater than the typical 75 or 90 degree C rating of branch circuit wiring.

So in answer to your question about what happens if you run more current than a wire is tested for, well your friend is right, it gets hot, and eventually the insulation could break down, but this is not going to happen immediately, and if you don't really overdo it you may never cook the insulation. That said, the code is there for a reason, it is conservative, and you are not permitted to deviate from it simply because you will not immediately burn down your house. If you are really interested in studying the effect of overcurrent on the longevity of wire, you will find that it is a complex subject, and an interesting place to start would be to examine the design of heating element wires, which are deliberately subject to high currents in order to produce resistance heat.
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Old 01-19-2010, 11:08 PM   #4
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Maximum vs safe capacity of 15 Amp circuit


Quote:
Originally Posted by Fire View Post
Someone please tell me I'm not going nuts!!!
Your not going nuts! The higher the amperage the higher the heat, the more heat the more degredation of the wire and the insulation. If you follow the NEC you will have a reasonably safe installation. In a house you have other things that come into play such as insulation, wires that are bundled, you just dont know all the variables. A conductor in open air will handel more current than one buried in a wall.

The 80% rule is a good choice. However in a house you dont know when you are at 80% or greater, you only find out when your fuse blows or breaker trips. Most people do not have the expertice to even think about how much load is on their wiring or even care. When the breaker trips they unplug things and reset the breaker.
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Old 01-19-2010, 11:46 PM   #5
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Maximum vs safe capacity of 15 Amp circuit


My wife used to run her hair dryer on a 15a circuit until I could run a dedicated 20a to the bathroom
The hair dryer alone took over 83% of the circuit
The same circuit had a bunch of lights & other stuff on it, tripped when we 1st moved in

I have multiple 20a outside circuits for my Christmas display
I load them up to about 90% Max, usually never over that
I did load one to 2625 & it did hold...long enough for me to go inside
Sat down to fill out my spreadsheet & relaized I loaded too many lights C-9's
Looked out & the breaker had kicked them off

I use a Killawatt meter to test as I setup
With more circuits the % has dropped down below 80% now
At least until I add more lights

I try to use high watt items only on a 20a circuit - heaters, hair dryers etc
But the vacuum takes 12a & that is plugged into any outlet available
Older house so I still have a few 15a outlet circuits

What were they running ?
I've never added up anything that I run in the house
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Old 01-20-2010, 12:04 AM   #6
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Maximum vs safe capacity of 15 Amp circuit


The code is designed for all situations nationwide. Best experience, this will be safe considering all installations everywhere, etc.

And how about when someone does something like the following, ignore the foam, I mean the tight bundle of wires...
(Hint: Air circulation...)

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Old 01-20-2010, 12:07 AM   #7
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Maximum vs safe capacity of 15 Amp circuit


Yeah...I've seen pics of even worse
And some of the people on Christmas web-sites bundle all of their extension cords together to keep the yard looking nice
Works OK if you are in a cold climate...but not down South

I prefer only 2 cables together, but near the panel not possible
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Old 01-20-2010, 01:48 AM   #8
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Maximum vs safe capacity of 15 Amp circuit


Here is some "ammunition" for you (OP) about the heat build-up in wires. Short and sweet on "derating".

And of particular interest in the following is "(3) the rate at which generated heat dissipates from the conductor".

And an "open to air" wire in something like the loose wires inside an electrical appliance as opposed to wires which *might* be in a tight bundle in a house...

http://www.southwire.com/support/DeratingAmpacities.htm
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Old 01-20-2010, 07:37 AM   #9
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Maximum vs safe capacity of 15 Amp circuit


Ok, so I'm not going nuts! But I do agree, pulling the maximum load through the wire, yes it will work, but is it a smart idea? I wouldn't do it in my home. In fact I've had a couple circuits added because I knew I was pulling close to the maximum.

The one big unknown here is the condition of the circuit. Nothing was said about how far away the receptacle is from the main panel, how old the wiring is, or if there are any junctions in between. He only stated that he believed there was nothing else on the circuit. Best case scenario the main panel is located in the same room, two feet of wire between the panel and receptacle, and all new equipment (breakers, receptacle, panel). But in that case why not just run a new 20 Amp circuit?!? What if someone tapped into this circuit to run a sump pump, what if there is 200 feet of wire between the panel and receptacle, or worse what if the circuit is already overfused or on a FPE breaker! Those questions are still unanswered and will effect the actual maximum capacity.

I've seen first hand what can happen when some idiot decides to replace all of his 15 Amp fuses with 30 Amp time delays. As soon as he starts pulling more than 15 Amps through the circuit he cooks the wire. Years down the road that same wire will just fall apart in your hands.
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Old 01-20-2010, 08:18 AM   #10
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Maximum vs safe capacity of 15 Amp circuit


The code (NEC) recognizes "continuous" loads like heaters and air conditioners which should not exceed 80% of the rating (12 amps for a 15 amp circuit) versus "intermittent" loads which do include audio equipment and hair dryers.

But in my opinion, when the device or appliance has been in use for half an hour, the amount of heat generated in the wires has already reached its maximum.

When you are dealing with long cable runs (like over 150 feet) it is voltage drop as opposed to heating of the conductors that becomes significant. The amount of heat per foot is the same as for a shorter run given the same number of amperes and the same wire size. Half the number of amps means half the number of volts lost in the wires and therefore deducted from the number of volts measured across (available at) the destination.
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Last edited by AllanJ; 01-20-2010 at 08:23 AM.
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Old 01-20-2010, 08:28 AM   #11
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Maximum vs safe capacity of 15 Amp circuit


The circuits I use for my Christmas display are all new 20a circuits
I would not have loaded my old circuits as heavily - older wiring
I've added enough new circuits in the house to distribute the loads
2nd floor is all new wiring, as is the basement
The only older wiring I have left is the overhead fans/lights - which are CFL

Most space/portable heaters are 1500w Max = 83.4% of a 15a circuit
Coincidence ?
Or do they know that if they get close to the 1800w it will trip older circuits ?

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