I have 125 amp service, and bought two #1 AWG black-colored wires for the electrician to use in his replacement. I couldn't find a white wire that size, or bare copper. Neither at Lowes nor Home Depot.

In my search, I found somewhere that said you don't necessarily have to buy the same size conductor for the neutral wire. That the neutral wire has to be at least 70% the size of the hot wire.

Is this true? And if it is, what size in AWG would this be? And does the wire have to be insulated?

My state follows NEC.

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    Is this an overhead service drop, or an underground service in conduit, or a direct-buried underground service? – ThreePhaseEel Jan 12 '15 at 4:29
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    Is there a reason why you need color-coding on the service drop? 99% of the time, all wires are black jacketed. Wire larger than 6 AWG is typically always black and is identified using a labeling scheme such as colored electrical tape or even adhesive printed labels, in an equipment enclosure for example. – cathode Jan 12 '15 at 7:22
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    Why are you buying wire for the Electrician? If he doesn't have the wire in his truck, call a different Electrician. – Tester101 Jan 12 '15 at 11:25
  • I TOTALLY agree. If an "electrician" is telling you to buy wire, especially service wire, then you need to find a REAL electrician. – Speedy Petey Jan 12 '15 at 12:59
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    @wallyk The OP said so. "bought two #1 AWG black-colored wires for the electrician to use...". – Tester101 Jan 12 '15 at 19:19

The maximum current for 1 AWG used in power transmission is 211 amps.(source) 70% of that is 147.7 amps.

2 AWG is rated at 94 amps, and 3 AWG is rated at 75 amps.

So there is no smaller wire rated at 70% or larger. You'll have to use 1 AWG (or larger) for all wires.

You could try an electrical supply house for other colors of 1 AWG, but I've found they only sell big spools (250+ feet) of such wire.

Try contacting a commercial electrician. They often carry those big spools around and might be willing to sell you a few feet of it. Of course they'll also want to charge you a $75 delivery fee and a $95 installation fee, but you should be able to avoid the first by going to them to pick it up, and the latter by firmly insisting that you don't want them to do it. Standard counter is they'll tell you it is illegal/dangerous/immoral for you to do it yourself. Stand your ground.

  • Sorry @wallyk, none of this has any relevance to the OP's situation. Generic charts like the one you kinked leave out a lot of other variables. In fact, the numbers I see in that chart are way off regardless. IMO the ONLY real source for numbers like ampacity are straight from the NEC. Problem is you need to know what you're looking at when using the NEC. There are no cheat sheets. – Speedy Petey Jan 12 '15 at 12:58
  • @SpeedyPetey: I don't follow how you could say none of this has any relevance. While I don't answer the "Is it true?" portion of the question, I render it moot by showing there is no viable solution. Furthermore, I provided several avenues to pursue for obtaining non-black insulated wire. – wallyk Jan 12 '15 at 16:53
  • The wire amperages you provided are all incorrect. Supply houses certainly DO cut wire of this size, most in colors like black, red, white and green. Most electricians will not sell wire off a spool, this is what supply houses are for. Also if they did they would NOT charge a "delivery" fee or an "installation" fee, not unless you hired them to do a job obviously. – Speedy Petey Jan 12 '15 at 19:14
  • Also, to say that their counter will be that they think it's "illegal/dangerous/immoral for you to do it yourself" is a bit silly IMO. Most of the guys who come to these DIY message boards are licensed and qualified electricians like myself. – Speedy Petey Jan 12 '15 at 19:14
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    @SpeedyPetey: Ah, I see my mistake in the ampere chart: That is for RF power uses. Cerro's ampacity chart does disagree substantially and invalidates my conclusion. – wallyk Jan 12 '15 at 20:32

Determining the size of the grounded (neutral) conductor isn't as easy as saying, hey let's take off 70% of the load and look that up on a chart.

I imagine the 70% that was spoken about refers to the allowable reduction of certain circuits named in NEC 220.61 (B)(1) which deals with cooking units and dryers. Those circuits, when the maximum unbalanced load is determined in accordance with 220.55 and 220.54 respectively, can be reduced by 70%.

But that doesn't equate to a total reduction of 70% because we're only speaking of a few of the circuits in the dwelling. Also, that doesn't include the 100% of circuits you can leave out do to the fact they are not supplied a grounded conductor and therefore, perhaps not obviously, do not contribute to the ampacity of the neutral anyway.

All in all, yes, you could have a total reduction of 70%, but I'd make the electrician prove it. For one, I wouldn't put the burden/responsibility on the homeowner to get my feeder cables - maybe a 250' spool of 14/2 romex but, at least by my experience, they'll screw that up too. So, I question that decision; I won't go as far as to call him names for it though.

If you did indeed have an actual 125 amp service or feeder rating you would need a #2 cu. or 1/0 al. or al. cu. (copper-clad aluminum) feeder according to TABLE 310.15(B)(7). Now, if you actually had a 70% allowable reduction you'd have...

125 * 0.70 = 87.5 A

...and according to TABLE 310.15(B)(16) you would need a #3 cu. or #2 al. or al. cu. cable.

Note that even this answer makes assumptions. You speak of a 125 amp service which doesn't equate to the actual minimum ampacity required, only the next largest standard breaker size. This leads to the fact that you may have an actual load less than 125 A. Taking 70% of the breaker size, if larger, would result in a larger reduction than is actually allowed.

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