breaker size/wire size after ampacity adjustments and corrections.

[Larry Bohn]

Given a 45 amp continuous load, 240V, single phase, (two current carrying conductors). This circuit will be pulled through a rigid metal conduit which already has eight current carrying conductors. THHN will be used for wire. Total of 10 current carrying conductors will be in the conduit. Here's my calculation.

210.19(A)1, 210.20(A), and 240.4(B) -- 45 X 1.25 =56.25 amps , choose 60 amp breaker and #4 THHN to comply with 110.14(C) for the breaker terminations.

210.19(A)2 and T.310.15(C)1 -- 45 / .5 = 90 amps, choose #4 THHN for wires when in the raceway.

Now we have a 60 amp breaker with #4 THHN wires. However, 240.4(B) also says the wires shall be protected at their ampacity, with the option of going up to the next breaker size above the ampacity of the wires.

#4 THHN in the conduit, after derating, has an ampacity of 47.5 amps, meaning the maximum overcurrent device cannot be any greater than 50 amps (240.4(B).

Upsizing the wire to a #3 THHN we have 115A x 50% = 57.5 amps, choosing the 60 amp breaker, (which is required by 210.20), is OK.

Final answer, 60 amp breaker (with 60 degree terminations), and #3 THHN. Am I correct? Nobody that I've been around ever checks, after calculating the ampacity of the wire required in the conduit, to see if the breaker size is still OK. They'll stay with the 60 amp breaker and #4 THHN.

What do you think?

 

[wwhitney]

Given a 45 amp continuous load, 240V, single phase, (two current carrying conductors). This circuit will be pulled through a rigid metal conduit which already has eight current carrying conductors. THHN will be used for wire. Total of 10 current carrying conductors will be in the conduit. Here's my calculation.

210.19(A)1, 210.20(A), and 240.4(B) -- 45 X 1.25 =56.25 amps , choose 60 amp breaker and #4 THHN to comply with 110.14(C) for the breaker terminations.

I believe that any modern breakers and panelboards will be marked for 75C terminations. So if the load is marked for 75C terminations as well, #6 Cu would comply for this step (75C ampacity is 65A). If not, #4 is correct. Doesn't actually matter, as the 240.4 check ultimately controls.

Final answer, 60 amp breaker (with 60 degree terminations), and #3 THHN. Am I correct?

Yes to this and everything else leading up to it.

Nobody that I've been around ever checks, after calculating the ampacity of the wire required in the conduit, to see if the breaker size is still OK. They'll stay with the 60 amp breaker and #4 THHN.

They are violating 240.4, good that you did the last check.

Cheers, Wayne

 

[Engser18]

By code, you are correct for your new circuit. Are you worry the existing circuits?

 

[Larry Bohn]

By code, you are correct for your new circuit. Are you worry the existing circuits?

The other current carrying conductors in the conduit are two three phase, 480V motor circuits to 5 HP motors and one single phase 480V, 1500W hydraulic tank heater. Those circuits will be OK.