A Couple of Electrical Questions

[Human Joe]

Heya Everyone,

I have a couple of questions about an upcoming electrical install I have coming up. First I plan on having 2 240v circuits (30a and 40a with 8g wire) and 2 120v circuits (both 20A with 12g wire) running in 1 in. EMT, the wire will be THHN. All of these circuits will be using a common ground (8 g wire). My question is do I have to keep the ground wire 8 gauge all the way through the conduit. I wonder this because once the 220v circuit peal off into their own receptacles does the common ground still need to be 8g or can I down size the ground to the remaining conductors that are size 12g.?

The second question is derating the wires running in the conduit. 7-9 current carrying conductors in one conduit means the Amperage gets dropped to 70%. My question is the amperage that gets modified is the maximum amperage capacity of wire? For Example I'm using 105 degree 12g THHN. following NEC, 90 degree THHN has a max amperage of 30a. 70% would be 21a which means my 20a breaker is fine. Is that how I run those calculations?

Thanks,
Adam

[Howardly]

I haven't seen a code book since the 60's in H.S.

I'd run 10's for the 30 amp circuits w/ 30 amp breakers and 8's for the 40's w/ 40 amp breakers.

A 12g. ground for those circuits.

The conduit de-rating I believe is not for the current capacity of the wires, it's a lessening of the conduit capacity due to the number of conductors and heat they will create.

[GHR]

Human Joe said:

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Heya Everyone,

I have a couple of questions about an upcoming electrical install I have coming up. First I plan on having 2 240v circuits (30a and 40a with 8g wire) and 2 120v circuits (both 20A with 12g wire) running in 1 in. EMT, the wire will be THHN.

The second question is derating the wires running in the conduit. 7-9 current carrying conductors in one conduit means the Amperage gets dropped to 70%. My question is the amperage that gets modified is the maximum amperage capacity of wire? For Example I'm using 105 degree 12g THHN. following NEC, 90 degree THHN has a max amperage of 30a. 70% would be 21a which means my 20a breaker is fine. Is that how I run those calculations?

Thanks,
Adam

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If you look at the tables, the derating of 7-9 current carrying wires from the maximum allows use to use the "standard" ampacity.

So you are doing it right, there is no need to worry about derating for 7-9 wires.

I would run 2 ground wires. One for the 30a and 40a circuits and one for the 20a circuits. It makes connections to the devices easier. (I don't know the answer to your question, but i do know this will work.)

[Bob10]

Could he just run all the wires to a junction box with a ground bar and run just the large ground to reduce the fill of the conduit?

PS it would probably make adding the circuits easier in the long run even if he has to run the 12g. I don't remember how I handled the grounds for my kitchen but I did run it all into a large box once near the kitchen to make the pulls all over the kitchen considerably shorter .

[TDKPE]

Human Joe said:

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First I plan on having 2 240v circuits (30a and 40a with 8g wire) and 2 120v circuits (both 20A with 12g wire) running in 1 in. EMT, the wire will be THHN. All of these circuits will be using a common ground (8 g wire). My question is do I have to keep the ground wire 8 gauge all the way through the conduit. I wonder this because once the 220v circuit peal off into their own receptacles does the common ground still need to be 8g or can I down size the ground to the remaining conductors that are size 12g.?

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First, a 10 gauge copper equipment grounding conductor is good from 30A through 60A. 15A and 20A need 14 gauge and 12 gauge copper, respectively. These are minimums [NEC Table 250.122]. So you only need 10 gauge copper for the equipment grounding conductor of your 30 and 40A circuits.

As to continuity, the way the NEC is written, you would need 10 gauge minimum for the entire run, as it's sized to the largest overcurrent device protecting conductors in the raceway or conduit.

NEC 250.122© Multiple Circuits. Where a single equipment grounding
conductor is run with multiple circuits in the same
raceway, cable, or cable tray, it shall be sized for the largest
overcurrent device protecting conductors in the raceway,
cable, or cable tray. Equipment grounding conductors installed
in cable trays shall meet the minimum requirements
of 392.10(B)(1)©.

Human Joe said:

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The second question is derating the wires running in the conduit. 7-9 current carrying conductors in one conduit means the Amperage gets dropped to 70%. My question is the amperage that gets modified is the maximum amperage capacity of wire? For Example I'm using 105 degree 12g THHN. following NEC, 90 degree THHN has a max amperage of 30a. 70% would be 21a which means my 20a breaker is fine. Is that how I run those calculations?

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Yes, that's how it's done. Derating is based on the actual ampacity of conductors, and not on the additional restrictions that may apply to small conductors most (but not all) of the time.