Oven Wiring Sharing Conduit with another Circuit

[Paulski]

I'm installing a new double convection wall oven that requires a bigger breaker/wiring. Manual calls for a 40 amp breaker.

Current set up has the oven wiring (30 amp) and the central air fan (20 amp) running in the same 3/4" FMC that runs 10' into a j box in the attic, is that allowed?


Here is my plan If an oven circuit is allowed to share conduits:

From main panel:
40 amp breaker - 3 6g Thhn wires
20 amp Breaker - 2 12g Thhn wires
1 8g ground wire
All feed into 3/4" FMC to a J box 10' away in attic. (39% conduit fill)

In the attic J box:
the 8g ground will connect to a bar with 3 slots
the 2 12g will wire nut splice to the existing 12/2 NM and run out of the box to the outlet in the attic for the central air fan (NM ground will go to ground bar)
the 3 6g wires will pass through box into 3/4" FMC to the oven Jbox ~40' away
Another 8g ground will go from the ground bar and run along the 3 6g wires to the oven

 

[Sinatra]

I don’t think there’s and issue with the two circuits in the same raceway, but you will need to increase the FMC from the panel to 1”. You are over the fill capacity.

 

[Paulski]

I don’t think there’s and issue with the two circuits in the same raceway, but you will need to increase the FMC from the panel to 1”. You are over the fill capacity.

I thought max fill was 40%? I am at 39%

 

[sparky 1971]

Use #8 THHN for the oven and a #10 ground. Thank me later.

 

[wyliesdiesels]

you dont need #6 for 40a. use #8 as said above, which will also work for a 50a circuit.

 

[alfredeneuman]

You don't need a #8 ground wire.
30-60A circuits only require a #10.
Edit: Are using copper or aluminum wire for the ground?
If aluminum you're right, not so with copper.

 

[Paulski]

Thanks everyone.
I am using copper, I'll go with #8 for all 4 wires, I was going with #8 ground because I already have 50' of green Thhn. The run is straight with only 1 90* bend on each side of the box so I'll see how pulling goes.

 

[Norcal]

Thanks everyone.
I am using copper, I'll go with #8 for all 4 wires, I was going with #8 ground because I already have 50' of green Thhn. The run is straight with only 1 90* bend on each side of the box so I'll see how pulling goes.

I hope your not planning to use the green for all four conductors, only permitted for grounding, the neutral is required to have white or gray insulation, phase tape is not permitted for that size conductor.

 

[wyliesdiesels]

the run is 40' at least so a 50' roll of green wouldnt be long enough for 4 conductors

 

[exranger06]

Since you have 4 current carrying conductors in the same conduit, the ampacity needs to be decreased to 80%. (310.15(C)(1)). In addition to that, you also need to derate further for ambient temperature (310.15(B)). An attic is likely to get really hot in the summer where this can be an issue. If the ambient temp gets up to 120 degrees F, the ampacity of #8 THWN just dropped to 36A (including derating for 4 current carrying conductors) and the #12 dropped to 19.68A. You'd have to upsize to #6 and #10 to keep them on a 40A and 20A breaker, respectively. Which means you may have to increase the conduit size to fit them (I didn't do a conduit fill calculation).