10 wire on a 40 amp breaker

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Yes, #10 could possibly used with a 40A (or larger) c/b for specific types of loads. A motor could be an example. See 240.3 and 240.4.
 
Here's an example using #12 0n a 45 amp breaker


A Home Inspector inspects a home and cites a violation of #12 THHN conductors connected to a 50 amp breaker in the load center. This branch circuit is feeding a 3 HP 230 V single phase motor; all parts are listed for 75 deg C.

Is the Home Inspector correct in citing this as a violation?

(Hint; articles 240.6, 310.16, 430.22(A), tables 430.52, and 430.148)




Answer: Yes, he is right.
  • Table 430.148 gives a 3HP motor a full load current of 17 amps
  • 430.22(A) requires conductors be sized at 125% of full load current
  • 17 x 125% = 21.25 amps
  • Table 310.16, #12 THHN 75 deg C column = 25 amps
  • Table 430.52 allows up to 250% for the inverse time breaker
  • 17 x 250% = 42.5 amps
  • 240.4 states we can go UP to the next STANDARD size breaker
  • 240.6 would let us go up to a 45 amp breaker
  • Answer 45 amp breaker.
Click to expand...


Roger
 
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roger said:
Here's an example using #12 0n a 45 amp breaker




Roger
Click to expand...

A Home Inspector inspects a home and cites a violation of #12 THHN conductors connected to a 50 amp breaker in the load center. This branch circuit is feeding a 3 HP 230 V single phase motor; all parts are listed for 75 deg C.

Is the Home Inspector correct in citing this as a violation?

(Hint; articles 240.6, 310.16, 430.22(A), tables 430.52, and 430.148)




Answer: Yes, he is right.
  • Table 430.148 gives a 3HP motor a full load current of 17 amps

  • 430.22(A) requires conductors be sized at 125% of full load current
  • 17 x 125% = 21.25 amps
  • Table 310.16, #12 THHN 75 deg C column = 25 amps
  • Table 430.52 allows up to 250% for the inverse time breaker
  • 17 x 250% = 42.5 amps
  • 240.4 states we can go UP to the next STANDARD size breaker
  • 240.6 would let us go up to a 45 amp breaker
  • Answer 45 amp breaker.
Click to expand...

Not to slam HI's but I don't think that this would be in their standard SOP. If they did cite it and were right it would be just because of luck. :)
 
the motor is single phase 480 volt protected by a 40amp breaker. with #10 wire.

flc is 18 amps

why doesn't 240.4d come into play
 
i just thought that there would be a differant calulation for a single phase 480 volt feed to an ac unit for a idf room.

i guess the engineener is right.
 
grumpy11 said:
i just thought that there would be a differant calulation for a single phase 480 volt feed to an ac unit for a idf room.

i guess the engineener is right.
Click to expand...

The engineer is likely right for an a/c unit. 240.4(G) would direct you to Article 440 for Air Conditioning and Refrigeration Equipment. The conductor and OCPD sizing would need to comply with Article 440.
 
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