20a 120v (2) networks #12 THHN derating

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Pipeandwire

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Hello, I’ve read multiple threads regarding this and haven’t come up with a clear answer. I would like to pull 2 #12 networks in a single 3/4 conduit each on a 20a breaker. Regular ole Thqb ge bolt on breaker. Length for discussion 50’. Derating gets me to 70% - 21 amps , but is this acceptable?
 
Pipeandwire said:
Neutral ccc
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You said two circuits. So that would be 4 Ccc. 2 hots, 2 neutrals.?????

For derating purposes use the 90*c column and get your answer. Then you compare that number to the 75*c column for wire ampacity and use which ever number is lower.
 
Pipeandwire said:
But my conundrum is the breaker at 75c does that kill the whole theory and require me to go to 10s?
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No, because there are 2 separate checks: one starts with the ampacity based on the full insulation temperature rating of the conductor and applies adjustment (for CCCs) and correction (for ambient temperature) factors. The other just looks at the ampacity based based on the termination temperature limit, no correction or adjustment.

So you don't have to do both 75C and ampacity adjustment at the same time; you just check separately (a) 75C without ampacity adjustment, and (b) 90C with ampacity adjustment.

Cheers, Wayne
 
As Wayne says in post 10, you do separate checks for derating in the conduit and ampacity at the circuit breakers.

The wire itself has a 90C temperature rating. So in the conduit you can derate from the 90C amapcity.

At the termination the circuit breaker has a 75C rating. So at the termination you are restricted to the 75C amapcity. But at the termination the wires are not bundled together, so you don't have to worry about derating for total CCC.

If the conduit were to travel through a hot roof area, in that location (but only in that location) you would need to evaluate for high ambient conditions and in that location you would have yet another calculated ampacity.

In essence you have a chain of different ampacities, each calculated for the specific conditions at different locations on the wire. You are allowed to base your operating current on the weakest link of that chain.

-Jon
 
roger said:
Propper terminology helps, you should have said two multi-wire branch circuits.
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Funny I never heard these being called "networks" until I posted here. The slang term that we use is "full sets" or to a lesser degree "full boat". 🤔
 
Honestly, 30 years and I've never heard it called anything other then a network if it has all 3 phases and a neutral. Yes, different regions different terminologies.
 
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