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I'm planning on installing a new electric oven and solar panels on my house soon. Given the location of everything relative to my electrical service panel, both the panels and the oven will need to run along the same path, so I'd prefer to install a single conduit to run all the wires, and branch off near the end of the conduit to either the oven or the panels. The other end will branch off to either the AC shut off (for the solar panel wires) or go directly to a breaker (for the oven wires).

The solar panel wires are already at 240 VAC, because the panels have microinverters mounted directly underneath them. The oven is a 240 VAC appliance.

Does NEC allow such an arrangement?

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Normally you're not allowed to mix service wiring and branch circuit wiring, but solar is special.

Derating

It's almost never a problem with 2-3 circuits, but when you put 2+ circuits in conduit, you must pause to do a "derate" calculation. Other rules prevent you from using wires at their thermal maxima, but you derate off of that. For THWN-2 wire, that is the 90C column of table 310.15(B)(16). So for instance if you're running 8 AWG Cu wire, your 90C number is 55 amps.

Your derate comes out of table 310.15(B)(3)(a). In all split-phase 120/240V circuits, there are only 2 wires that count, so you are always 2 wires per circuit. So 2 circuits is 4 wires, which calls for an 80% derate. 80% of 55 amps is 44 amps.

You then round up to the next size of breaker. They don't make 45A breakers, so you go to a 50A breaker. Lucky you!

This does not liberalize any other rules. If this was #8 Cu NM cable, that's only allowed to run at 60C, so it is limited to 40A regardless.

Up to 4 circuits per conduit (70% derate) is fine for 15-30A circuits. However for 50A and above circuits, it starts to pinch. #8 THWN-2 can only carry 40A. If you ever see installations with many conduits next to each other, that's why. They are keeping it to 3-4 circuits per conduit.

Can you use the 90C column if your termination is only 75C? Depends if the termination is inside the conduit! (unlikely to be legal to splice in a conduit, but perhaps in a raceway, gutter etc.? Anyway if it was a 60C splice, yes, #8Cu would derate off 40A at that point.) If the splice is outside the conduit, then conduit/raceway derate rules are inapplicable and you apply whatever rules exist for the space the splice is actually in.

Branching

Anywhere wires split, there must be an accessible point, such as a "T" conduit body or a junction box. (Places where splices happen have much larger cubic-inch requirements; junction boxes are sized to allow splices, conduit bodies are not).

Every accessible point most remain accessible in perpetuity. For instance if you get a mind to deck over your attic to use it for storage space, you better install a junction box so it'll be flush with the ultimate floor, not bury a conduit body under flooring.

Also you can't have more than 360 degrees of cumulative bend between accessible points; for a novice DIY puller, even 180 is ambitious. I prefer 90-135 at most.

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How many current carrying conductors total? More than 3 and you have to start derating per table 310.15.B.3.A So it may be worth it to have separate conduits. Since you have micro inverters you already have 240 ac , the code really focuses on dc connections for pv systems as the ac is standard and you still have the same rules. The requirements are spelled out in article 690 but since you already have converted to ac you would use the standard wiring practices.

  • The derate is almost never a problem with up to 6 wires (3 circuits). You have to crunch the numbers to be sure, but if you are expecting to use ampacities in the 60C column, you'll be fine. – Harper - Reinstate Monica Oct 30 '19 at 14:14
  • There will be 4 total current carrying wires (2 per circuit) - unless I have to split the solar wires into two parallel circuits to handle the current. There will also be a neutral on each circuit, I suppose, and the panels require a GEC, but those aren't normally current carrying, in my understanding. – rothloup Oct 30 '19 at 16:37
  • @rothloup Neutrals are normally current carrying, but don't count toward conduit derate because any current on neutral means proportionately less on a hot, so neutral flow only improves the situation. – Harper - Reinstate Monica Oct 30 '19 at 19:53
  • We would need to verify what the total harmonic distortion is on the microinverters. the last system I installed the THD required neutrals to be current carrying but depending on the configuration it may not matter , there are a few tricks out there. – Ed Beal Oct 30 '19 at 20:02

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