12awg wire on 15A circuit


I added 14-gauge (15A) romex cable to a circuit that had 12-gauge (20A) wire, indicating I had made a mistake, but it turns out to be a 15A breaker on the circuit with 12-gauge wire. Is that OK/safe? I don't have to correct my mistake, right? Thanks
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Em wrote:

Correct. However, if the #12 wire goes to the panel you may end up confusing the next person looking at the electrical system--they may assume that since it's #12 wire its safe to put a 20A breaker on the circuit, not realizing that the circuit includes some #14..
Chris
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On Mon, 26 Mar 2007 11:01:56 -0600, Chris Friesen

Some AHJ would have you lable the circuit 15amp only.
tom @ www.Consolidated-Loans.info
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I have before seen a municipality's code webpage saying that it is against code to have a branch circuit having wires differing in allowed breaker amps. However, I did not find it clear whether that was NEC or a local building code.
If you have mixed size wiring in a branch circuit, I would equalize it if possible. If not, I would have visible tags everywhere the larger wiring would be seen saying that the branch circuit is a mixed wire size circuit and what the breaker amps should be - and hope the home inspector does not see that being a problem when you sell the house.
Personally, I think there should be no permanently installed wiring thinner than 12 in a house, as common as 20 amp breakers and fuses are.
- Don Klipstein ( snipped-for-privacy@misty.com)
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On Sun, 1 Apr 2007 05:41:27 +0000 (UTC), snipped-for-privacy@manx.misty.com (Don Klipstein) wrote:

Sounds like you have stock in copper mines. In truth 14# is 'safe' at 25 amps with today's romex (for pundents, look at 2005 NEC table 310.16 THHN. However, NM-B (be meaning 90C insulation) must be treated as 60C insulation. This lowers the amperage rating down to now 20 amps. Then when you look up 240.4D, you have further derate 14# down to 15 amps.
So what is good/safe 25 amps to 15amps is a fat margin for safety. So 12# in homes is kinda overkill, and not required per the NEC.
Disclaimer: DIY'ers this is a discussion, not a how-to, read the electrical codes yourself. ;)
tom
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Long post.
Short version: KISS principle. #14 = 15 amp breaker, #12 = 20 amp breaker.

Sounds like you need to take heed in NEC(2005) Section 90.1(C) Intention: "This Code is not intended as a design specification or an instruction manual for untrained persons."

#14 is _NOT_ 'safe' at 25 amps with today's Romex, or any other wire for that matter, _unless_ the terminals for the electric equipment are rated for 90 degrees Centigrade AND the conductors do not need to be derated for temperature! That's 194 degrees F. You will _not_ find Residential circuit breakers rated for 90 degrees C. Any wire ran in an attic must be derated for temperature.

Yeah, let's do that. The ampacities listed in Table 310.16 are for an ambient temperature of 30 degrees C, that's 86 degrees F. Try looking at the TOP of the column where you got that 25 amps. What's that? 90 degrees C? Like I said, good luck finding Residential circuit breakers and devices with terminals rated at 90 degrees C. For "pundits" see NEC Section 110.14(C)(1) and (2), which basically state that conductors cannot be used at the higher temperature rating unless ALL of the equipment and terminations are is listed and identified, as in "UL", for such use.
The only practical use for the 90 degree C column in Table 310.16 is for derating conductors due to temperature and other factors.

At best one can find Residential circuit breakers rated at 75 degrees C. Many older circuit breakers are only rated at 60 degrees C. IF you look at the 75 and 60 degree C columns in Table 310.16, #14 is rated at 20 amps. If ya try to crank a fully loaded 90 degree conductor at full 90 degree C rating on a 60 or 75 degree C terminal, something _is_ eventually going to burn.

Yeah, and IF you bother to look at the BOTTOM of Table 310.16, you'll see (temperature) correction factors. Is it safe to say that a typical attic can reach 105 degrees F or higher? The correction factor in the 90 degree column for 105 degrees F is 0.87. Is _not_ Romex commonly installed in attics? Your 25 amps just went from 25 amps to 25 amps x 0.87 = 21.75 amps.
If you bother to look at NEC Section 310.15(B)(2) you'll also see that adjustment factors must be made for number of conductors in conduit or bundled, etc. These adjustment factors are even more drastic than for the temperature correction factors in Table 310.16. The smallest factor (for 4 to 6 wires) is 80%. 25 amps x 80% = 20 amps. Put 'em in an attic and that 20 amps just became 20 x 0.87 = 17.4 amps.

As shown above, that "fat" margin disappears in a hurry. I call it idiot proofing...see NEC Section 90.1(C).

Some areas will not allow wire smaller than #12 in houses. See idiot proofing.
No, it's NOT overkill. Not only that, but a lot of the older Romex and circuit breakers were only rated at 60 degrees C. Some of it was 75 degree C. When derating for temperature, one then has to derate from the 60 or 75 degree columns. 20 amps x 0.87 = 17.4 amps, let's hope it doesn't need further adjustment factors. That's probably why Romex started getting manufactured circa late 80's with 90 degree C wire. IMHO, there is no "fat" margin once considerations for temperature are made. Romex is finally where it should be.....idiot proof.
When houses are "re-wired", sometimes some of that old wiring is reused by some electrical contractors. All the more reason, should one sign a contract to get a house re-wired, to make sure it has a clause that says something like: "All wiring to be new. Remove or abandon in place ALL old wiring."

Yes, #12 is required by NEC for circuits for kitchens, dining rooms, bathrooms, etc.

I don't need a disclaimer. My comments can be verified by qualified people anywhere on the Internet, or by your local electrical inspector. Here's a site that is administered by an electrical inspector: http://www.selfhelpandmore.com/index.htm
For online viewing of the NEC: http://www.nfpa.org/itemDetail.asp?categoryID0&itemID!227&cookie%5Ftest=1
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On Sun, 1 Apr 2007 05:41:27 +0000 (UTC), snipped-for-privacy@manx.misty.com (Don Klipstein) wrote:

I'm fairly sure that NEC only requires you to size the breaker for the smallest wire in the sequence. I'll admit I can see the point in wanting them all to be the same, though, because it would be easy for someone to look at the wire leaving the service panel, notice that it's 12 AWG, and up the breaker to 20A.

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There are times when you will have bigger conductors for voltage drop reasons and as the load starts getting distributed along the way the wire size can go down to the 310.16 minimum. Certainly an untrained installer could assume the wire was the larger size the whole way but the code does not try to prevent things a dumb person might do.
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