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Discussion Starter · #1 · (Edited)
2inch metallic/pvc coated liquid tight conduit from main panel to metal junction box. conduit is 25 feet. All wires are 30 feet to the junction box. connections at the main panel and junction box are industrial grade and water tight. This conduit is in the envelope of the home, so yeah I went overboard on that.

All wires are THHN/THWN-2 dual certified.

One 6 awg ground wire in that conduit, grounded at junction box (mounting a ground bar to the metal junction box)

One 50amp 6 awg circuit, 50amp gfci breaker

Three 20amp 12awg circuits, with 20amp dual function breakers.

My question pertains only to this run from the main panel to the junction box. Not about anything I plan to do with these. It's really just a, does anybody else see something I don't question.

longtime lurker and reader here.

Any current carrying conductor issue here? By my count it's good to go. and there is nearly no chance of even two of these lines being used at the same time.
 

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If there's no chance of even two of those being used at the same time, then just put them all on the same circuit LOL!

Assuming your power supply is 120/240V split-phase and not 3-phase, then the thermal derates work out pretty simply: at 4 circuits in a conduit, ampacity is derated to 70% off the highest possible thermal rating of the wire. Per 310.15(B)(3)(a). THHN is 90C thermal, so you use the 90C column in Table 310.15(B)(16).

12 AWG = 30A x 70% = 21A. (but 240.4(D) limits you to 20A.)
6 AWG aluminum = 55A x 70% = 38.5A.
6 AWG copper = 75A x 70% = 52.5A

Note that this is not to be confused with the continuous-load derate. Suppose you have a Siemens EVSE which is programmed to charge at 30A actual and instructions state a 40A circuit. You must derate the 30A by 125% giving 37.5A. That fits in the 38.5A of your #6 aluminum. Since they do not sell 37.5A breakers, you round up to the next size, 40A.



Now if this is inconvenient, merge two 120V circuits into a multi-wire branch circuit or MWBC. Because for this thermal derate, a MWBC counts as 1 circuit and you have 3 circuits. Which gets you an 80% derate.

12 AWG is still 20A lol
6 AWG AL = 55A x 80% = 44A
6 AWG Cu = 75A x 80% = 60A
 

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Discussion Starter · #5 · (Edited)
If there's no chance of even two of those being used at the same time, then just put them all on the same circuit LOL!

Assuming your power supply is 120/240V split-phase and not 3-phase, then the thermal derates work out pretty simply: at 4 circuits in a conduit, ampacity is derated to 70% off the highest possible thermal rating of the wire. Per 310.15(B)(3)(a). THHN is 90C thermal, so you use the 90C column in Table 310.15(B)(16).

12 AWG = 30A x 70% = 21A. (but 240.4(D) limits you to 20A.)
6 AWG aluminum = 55A x 70% = 38.5A.
6 AWG copper = 75A x 70% = 52.5A

Note that this is not to be confused with the continuous-load derate. Suppose you have a Siemens EVSE which is programmed to charge at 30A actual and instructions state a 40A circuit. You must derate the 30A by 125% giving 37.5A. That fits in the 38.5A of your #6 aluminum. Since they do not sell 37.5A breakers, you round up to the next size, 40A.



Now if this is inconvenient, merge two 120V circuits into a multi-wire branch circuit or MWBC. Because for this thermal derate, a MWBC counts as 1 circuit and you have 3 circuits. Which gets you an 80% derate.

12 AWG is still 20A lol
6 AWG AL = 55A x 80% = 44A
6 AWG Cu = 75A x 80% = 60A
I appreciate the advice here. I won't be doing a mwbc because I don't need it, and my skill level doesn't support it.

So to elaborate on what you said. Lets say I did the one 50amp breaker 6awg circuit and I wanted it to be good for a continuous load. This is in the same conduit as two 12awg 20amp breaker circuits that are non continuous. How does that work out? copper wire.

I won't be using that 50amp for a continuous load now. I'd just like the possibility later and to know what it can support.
 

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The continuous load needs to have a 125% derate. So the 50A continuous load needs to be on a 62.5A circuit.

A 40A continuous load needs a 50A circuit. So the maximum continuous load that is supportable on a 50A circuit is 40A.
 
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