240.4 D vs 430

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thepij

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I understand that motor MCA and MOCP will allow a wire to be fed from a breaker that has higher ampacity than the wire. Does this also allow it to be in conflict or override 240.4 D? So could a motor with 21 MCA (#12 conductor) be protected with a 25 amp circuit breaker.

I am using the 2005 NEC.
 
thepij said:
I understand that motor MCA and MOCP will allow a wire to be fed from a breaker that has higher ampacity than the wire. Does this also allow it to be in conflict or override 240.4 D? So could a motor with 21 MCA (#12 conductor) be protected with a 25 amp circuit breaker.

I am using the 2005 NEC.
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Yes. If you look at the second page of Art 240, You see table 240.3 that refers you to other articles for different equipment. If you look at motors, you are referred to 430. In table 430.52 you will find that you may use a breaker sized at 250% of the motor full load amps. In your case with a motors FLA at 17 amps you could use a 40 amp breaker.
However you would be limited by the MOCP for the equipment.
 
I agree with Bob.

240.4(D) starts by saying " Unless specifically permitted in 240.4(E) or (G).......

Table 240.4(G) specifically permits conductors for motors to comply with Article 430.

Chris
 
thepij said:
I understand that motor MCA and MOCP will allow a wire to be fed from a breaker that has higher ampacity than the wire. Does this also allow it to be in conflict or override 240.4 D? So could a motor with 21 MCA (#12 conductor) be protected with a 25 amp circuit breaker.

I am using the 2005 NEC.
Click to expand...

What is the voltage and HP of the motor?

Your circuit conductors are based on 125% of the table FLA. If your FLA are 21A according to 240.4D you would have to up size your wire to a #10.

bob said:
In your case with a motors FLA at 17 amps you could use a 40 amp breaker.
However you would be limited by the MOCP for the equipment.
Click to expand...

I would agree. Your branch circuit protection is also sized per table FLA using 430.52
 
TT009 said:
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Your circuit conductors are based on 125% of the table FLA. If your FLA are 21A according to 240.4D you would have to up size your wire to a #10. ...
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The rule in 240.4(D), when it applies, does not change the maximum permitted ampacity of the conductor...it just sets a maximum size for the OCPD. You are correct that if you have a motor with a "table" full load current of 21 amps, you will have to use a #10, but that has nothing to do with 240.4(D).
 
don_resqcapt19 said:
The rule in 240.4(D), when it applies, does not change the maximum permitted ampacity of the conductor...it just sets a maximum size for the OCPD. You are correct that if you have a motor with a "table" full load current of 21 amps, you will have to use a #10, but that has nothing to do with 240.4(D).
Click to expand...

When I posted this I guess I wasn't really thinking about it...Because it is a motor 240.4(D) does not apply. I think it is (E) or (G) that tells you motors are exempt.

In this case, I posted you would have to up size your wire...I was wrong, it could be a #12 with a calculated FLA of 21A.
 
don_resqcapt19 said:
The rule in 240.4(D), when it applies, does not change the maximum permitted ampacity of the conductor...it just sets a maximum size for the OCPD. You are correct that if you have a motor with a "table" full load current of 21 amps, you will have to use a #10, but that has nothing to do with 240.4(D).
Click to expand...

Don the OP said "So could a motor with 21 MCA (#12 conductor) be protected with a 25 amp circuit breaker." Since MCA rating has the 125% built in, the FLA would be 17 amps. Would you still think the #12 would not be allowed?
 
I think was a little unclear. If the FLC is 9.8, then 125% is the MCA which is 12.25 amps. According to the 75 deg. column, #12 awg is rated up to 25 amps. Then 250% is the MOCP which is 24.5 amps. Does 240 D prohibit the 25 amp c/b with the #12 conductors for a motor?
 
thepij said:
I think was a little unclear. If the FLC is 9.8, then 125% is the MCA which is 12.25 amps. According to the 75 deg. column, #12 awg is rated up to 25 amps. Then 250% is the MOCP which is 24.5 amps. Does 240 D prohibit the 25 amp c/b with the #12 conductors for a motor?
Click to expand...
No it does not.


Your origional post said "So could a motor with 21 MCA (#12 conductor) be protected with a 25 amp circuit breaker."
 
bob said:
Don the OP said "So could a motor with 21 MCA (#12 conductor) be protected with a 25 amp circuit breaker." Since MCA rating has the 125% built in, the FLA would be 17 amps. Would you still think the #12 would not be allowed?
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The post I replied to did not say MCA so I did not take that into account. A #12 can be used for motor and AC circuits that require conductors with an ampacity of 25 amps or less.
 
don_resqcapt19 said:
The post I replied to did not say MCA so I did not take that into account. A #12 can be used for motor and AC circuits that require conductors with an ampacity of 25 amps or less.
Click to expand...

Thanks. This is as I understood the rule. I think there is much misunderstanding regarding #12 overcurrent protection when used with motors.
 
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