to "derate" or not

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analogmusicman

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I'll be renovating one of my bathrooms. (don't know exactly when) I'd like to bring a 20a circuit from the main panel to that bathroom since the GFCI is only 15a. (actually, every "protected" outlet in the house goes to a single 15a GFCI out in the garage) The NM from the panel to that bath will be around 72 ft. long. my question is: since the run will be relatively long, should I "derate" and maybe go with #10 wire as opposed to #12? (you know,the "one wire size larger" method of derating)

tnx,
 
72 feet is not too long. 300 feet is long. You will be fine with 20 amp on #12.
 
Joe's right. With 120V, you're not going to measure any voltage drop until after about 100 feet. It's those 300 ft runs, like he said, that you would need to think about upsizing the wire.
 
analogmusicman said:
I'll be renovating one of my bathrooms. (don't know exactly when) I'd like to bring a 20a circuit from the main panel to that bathroom since the GFCI is only 15a. (actually, every "protected" outlet in the house goes to a single 15a GFCI out in the garage) The NM from the panel to that bath will be around 72 ft. long. my question is: since the run will be relatively long, should I "derate" and maybe go with #10 wire as opposed to #12? (you know,the "one wire size larger" method of derating)

tnx,
Click to expand...
Something that is not generally known to those that have not had extensive training is that wire gauges 14, 12, & 10 are already derated by the rules of the US National Electric Code (NEC). For all of the receptacle and lighting circuits in dwellings the NEC limits the Over Current Protection (OCP) of those wire gauges to 15, 20, & 30 Amperes respectively. The actual ampacity of those conductors in the tables of the NEC is higher in each case. There are applications' such as Heating Ventilating Air Conditioning (HVAC) and other motor circuits, were conductors in those sizes are used at higher ampacities then the NEC would otherwise allow. The Code Making Panel limited the overcurrent ratings in order to compensate for actual field conditions of installation and use. When an electrician calculates a derating for those conductor sizes they do not start with the NEC OCP ampacity. They start with the ampacity that is listed in the applicable table of the NEC. It is often true that the resultant ampacity after derating is still higher than the maximum OCP allowed by the NEC.
 
analogmusicman said:
I'll be renovating one of my bathrooms. (don't know exactly when) I'd like to bring a 20a circuit from the main panel to that bathroom since the GFCI is only 15a. (actually, every "protected" outlet in the house goes to a single 15a GFCI out in the garage) The NM from the panel to that bath will be around 72 ft. long. my question is: since the run will be relatively long, should I "derate" and maybe go with #10 wire as opposed to #12? (you know,the "one wire size larger" method of derating)

tnx,
Click to expand...

Here is a good V drop calculator there are 100’s on line pick one you like. In your case it looks like #12 @ 72’ will give you a drop of 2.86%. I have been told not to worry about it until you see 4% or more.

http://www.calculator.net/voltage-d...ance=72&distanceunit=feet&amperes=15&x=47&y=5
 
I have two questions, that don't really count for anything.
How long will it be before the whole house will be wired with #12
How much more would it cost to upgrade that run?
 
The copper industry would have you doing now. It is very common in USA to wire everything with #12. Not so common in Canada.
 
My 100 year old house was all rewired before I bought it 200A service it was all done in #12. As bad as the house was when we bought it seeing that was PTL moment.
 
JoeD said:
The copper industry would have you doing now. It is very common in USA to wire everything with #12. Not so common in Canada.
Click to expand...
Wiring most residential structures entirely with #12AWG Copper is a waste of effort. The ampacity of #14 AWG wire is Twenty Amperes but in most residential applications you are required to provide overcurrent protection at Fifteen Amperes. Fourteen AWG wire is more than adequate for the vast majority of residential wiring needs.
 
hornetd said:
Wiring most residential structures entirely with #12AWG Copper is a waste of effort. The ampacity of #14 AWG wire is Twenty Amperes but in most residential applications you are required to provide overcurrent protection at Fifteen Amperes. Fourteen AWG wire is more than adequate for the vast majority of residential wiring needs.
Click to expand...

AMEN.

However we have to deal with the powers to be.

My common sense exception would be in kitchen and the number of potential devices simultaneously connected. On occasion some meal preparation resembles a "symphony".
 
I agree totally but if you read some of the material supplied by #12 lover(people who sell copper) you will save a fortune in electricity because of less line loss. The USA has bought into this theory and wire much of their homes with #12.
 
To you pros:
When you quote a job do you charge more to terminate a #12 wire than a #14? Or run the wire thru the walls etc? I understand the cost of #12 is going to be higher because of the wire cost. As a homeowner or general contractor is the only difference in the final cost going to be the cost of the wire?

If you had to guess how many percent would you guess the cost of any overall job would be with stepping up the wire gage?

The wire itself is about 50% more for #12 than #14 but I think the wire cost is a smaller part of the overall cost of the job.
 
#12 takes up more space in boxes, so larger boxes could be needed. It is harder to work with. In Canada 20 amp circuits require 20 amp T slot receptacles.

My whole house is wired with #14 on 15 amp circuits and I have never issues with any circuits overloading so that excuse does not hold water.
 
We see it in custom built houses where the homeowner wants bragging rights and I am sure they are paying a lot extra for it, hell they even put in hot and cold hose bibs.
 
JoeD said:
I agree totally but if you read some of the material supplied by #12 lover(people who sell copper) you will save a fortune in electricity because of less line loss. The USA has bought into this theory and wire much of their homes with #12.
Click to expand...

1.5% difference in voltage drop based on the OP’s example and full load of 15 amps, we have no idea how much load per year he is going to subject that line to for example at 5 amps there is only a .5% difference in voltage drop on his 72’ run.

Chances are all the wall wart transformers everyone has plugged in are costing more per year in the average house.

The vast amount of wasted power is taking place before the power enters your house in transmission lines.

But then again the same comparison can be made in plumbing, framing, insulation etc. in the house .
 
nealtw said:
We see it in custom built houses where the homeowner wants bragging rights and I am sure they are paying a lot extra for it, hell they even put in hot and cold hose bibs.
Click to expand...

When I did my PEX manifold I ran cold off to the hose bibs and had left over spots on the manifold with hots. I ran a 2’ piece of PEX to a Tee and leave the hot shut off at the manifold. I did it 3 places and I use the hot feature quite a bit actually. Only cost about 2 bucks to do and man do I have bragging rights now with the neighbors. ;)
 
bud16415 said:
To you pros:
When you quote a job do you charge more to terminate a #12 wire than a #14? Or run the wire thru the walls etc? I understand the cost of #12 is going to be higher because of the wire cost. As a homeowner or general contractor is the only difference in the final cost going to be the cost of the wire?

If you had to guess how many percent would you guess the cost of any overall job would be with stepping up the wire gage?

The wire itself is about 50% more for #12 than #14 but I think the wire cost is a smaller part of the overall cost of the job.
Click to expand...

Generally it's by the circuit where you are only doing the rough and finish switch and receptacles and someone else is installing the fixtures.

The labor is relatively fixed, yet is more costly in the assembly of the boxes, with #12.
 
So on a $10,000 house wiring job with #14 would I expect to pay $15,000 if I asked for #12?
 
bud16415 said:
So on a $10,000 house wiring job with #14 would I expect to pay $15,000 if I asked for #12?
Click to expand...

When you work in a geographical area you are familiar with the building depts requirements so if you limit recep. at 6 per circuit, and you install 10 switches per circuit, it amounts too about the same wire cost per.

So a per circuit rate becomes the lesser of the every nut and bolt estimate.

The upcharge, in my case is 3 times the cost of a full roll of the conductor requested, IE. you need 348', the next full length bulk quantity is 500', that's 3X the 500' rl. price, and its paid in front.

The trade is by law, only required to perform to the requirements of the code adopted by the governing municipality.
 
In Canada 12/2 is almost twice the price of 14/2 at HomeDepot.
150m 14/2 = $99
150m 12/2 = $188
150m 14/3 - $188
 
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