The end goal is to add a 50A receptacle in the garage.

Whomever installed the subpanel ran 1 inch "smurf tube" stubbed out to the ceiling which currently has no conductors. For the new outlet I plan to add a 50A GFCI breaker (the equipment involved holds a lot of liquid). I'll then pull THHN wire (this stuff), through the smurf conduit through the attic to the wall I need the outlet on, then transition to the gray PVC stuff, and run it down the wall to a surface mount box with a 50A receptacle.

The equipment will at times draw 45-47 amps, but for short periods (15-20m), most of the time it will draw less than 30A. Total run should be around 20-25ft.

First, and main question, will 8 awg THHN get it done? I've seen things about derating that indicate runs in the attic need to be lowered to 75*, which might require me to upgrade to 6 awg.

I also see reference to "terminations" which may derate it to the 60* capacities, but I don't understand what that is. Is it what the outlet is rated at?

I plan to pull two black, and two white conductors. Two black for the hot conductors, one white for neutral, and the other white I'll label green somehow (paint, tape, whatever) for the ground. This is mainly to save on costs (a 50ft roll is only $4 more than 25 ft roll).

Is there anything grossly wrong about my approach?

  • Isn't there an 80% of rated amps rule ? 80% of 50A = 40A. Drawing more than 40A may violate code? – Rodo Jan 4 '19 at 21:54
  • I think plan your wire and breakers to carry 20% over the expected continuous load. What you "draw" with a device isn't really relevant, only the protection of the circuit. If I were to draw 47 amps on a 50 amp circuit for a solid 3 hours, I'd be in trouble - heat would increase, resistance would increase, and ideally the breaker eventually would pop. Momentarily getting close to 100% of the circuit design is acceptable, as far as I understand. If I run 6 awg wire, I'd be covered, but I'd probably still run a 50amp breaker. – slambeth Jan 4 '19 at 22:09

First, you are required to use the 60 degree table for household wiring below 100A. That will call out #6 copper or #4 aluminum wirewhich is 55A but you are allowed to round up. Yhis is a wobbler point where I wouldn't normally mention aluminum (it is cheaper) except for the asterisk below.

Your short 47A peaks don't inherently require a 125% (80%) derate for continuous load, but you didn't mention what your load was, and certain loads have a statutory requirement to be derated as a continuous load, e.g. Heaters. The load's sly choice of 47A for its peak suggests the builder knows it is one. Regardless, it does not matter to wiring since 6Cu/4Al wire good for 50A is also good for 60A.

Using two blacks for two hots is fine.

Your plan to re-mark the wires has a problem: in THHN you are not allowed to re-mark to change a wire's function At all, not even whites to hots, unless the wire is 4 AWG or larger in which case you can mark anything you please. And now you know why I mentioned aluminum.

#4 will just fit inside that 1" conduit, though it will be a challenging pull.

However, "bare" is a wire color. You could strip all the insulation off a white to make it a bare wire. Just don't get caught. And be careful, if the wire is too finely stranded, it may go all rats-nest on you once out of the insulation, and that would not be acceptable.

You are not required to use a #6 ground wire. For 60A, #10 will suffice.

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  • Great info, thank you. I had read somewhere that white could be colored to anything else, but nothing can be colored to create a neutral (white). I'm glad I asked the experts :). This will be powering a home brewing control panel. The main load is resistive, one 6500W heating element, and one 4500W. They will not both be on all the time, b may at times. It may also power a small pump who's draw is < 2A. I'm definitely going for copper wire. It sounds as though maybe I should just go for #6 wire, and if I can run a #8 or #10 for ground I'll just buy green wire for that. – slambeth Jan 5 '19 at 15:17
  • @slambeth white can be colored in cables where you don't have a choice. Learned that myself only after going "oh hey, I can just use white for everything". Aluminum's bad reputation came only in small branch circuits where it was fit onto copper-only terminations, not large #4 or larger going between made-of-aluminum or Al-rated lugs such as on a subpanel. Of course if you are coming into wire nuts, you'd need to transition to Cu at Polaris connectors, that blows any cost advantage. – Harper - Reinstate Monica Jan 5 '19 at 17:04

For residential we don't usually derate when running in the attic. Most all circuit breakers and outlets are rated at 75 if this is the case and you go to 100 amp. You could use the 75 degree table, but below 100 requires the 60 degree table to be used. If you live in an area that requires a temp derate you can use the 90 degree table for the ampacity and derate but the load can't excede the 60 degree value. Hope that helps.

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  • If I'm hearing you correctly, for the 50 amp wiring going through the attic for the new outlet, I'll have to use the 60* table? – slambeth Jan 4 '19 at 20:56

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