Sizing Wire for motors

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Jimmy4645

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I have a situation where the condensing unit manufacturer is stating that the MOCP is 100 amp breaker and the MCA is 62 amps. What is the correct wire size?

I understand you take the FLA and mulitple it by 1.25% which gives you the MCA (minimum circuit ampacity) = #6 wire. But is there something in the code that states you can only be within one wire size of your protection and that #4 wire will be required and not #6.

Thanks guys for the help
 
Forget any calculations. The conductor size is based solely on the MCA which would mean a #6 THHN (65 amps @ 75 degrees C) in a raceway would be adequate.
 
Jimmy4645 said:
I have a situation where the condensing unit manufacturer is stating that the MOCP is 100 amp breaker and the MCA is 62 amps. What is the correct wire size?

I understand you take the FLA and mulitple it by 1.25% which gives you the MCA (minimum circuit ampacity) = #6 wire. But is there something in the code that states you can only be within one wire size of your protection and that #4 wire will be required and not #6.

Thanks guys for the help
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You can have OCP well over the rating of the wire size since the overload protection is buit into the motor. The OCPD is giving you ground fault and short circuit protection. The motor has the overload protection.
 
Jimmy, depending on your wiring method, there can be an all important almost hidden footnote in infinity's correct answer.
If you are using NM cable (and SE under the 2008 Code) keep it mind that it (they) must be terminated at the 60o amapcity.
So that #6 he mentioned in a raceway, becomes a #4 if it is NM or SE cable.

and welcome to the Forum.
 
In answer to your question no there is no NEC rule that limits the OCPD to be within one size of the wire in this situation. You can install a 100 amp breaker on the #6-- providing #6 is the appropriate wire size (see Augie's note).

Usually the OCPD is based on the MCA times 175% (art. 440.22) 2005. In this case it would be 108 amps thus 100 amps is code compliant.
 
Don't forget about voltage drop... you may have to upsize the wire if a long run. Limiting the voltage drop is not an NEC requirement but you have to deliver the correct voltage to the unit for it to run properly and not have the mfr dishonor the warranty.
 
Dennis Alwon said:
Usually the OCPD is based on the MCA times 175% (art. 440.22) 2005. In this case it would be 108 amps thus 100 amps is code compliant.
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Dennis, I think that you meant the FLA? The MCA already contains an additional 25% for the largest load plus all of the smaller loads added together at 100%.
 
infinity said:
Dennis, I think that you meant the FLA? The MCA already contains an additional 25% for the largest load plus all of the smaller loads added together at 100%.
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Well I am not sure about that. Art. 440.22 states

440.22 Application and Selection.
(A) Rating or Setting for Individual Motor-Compressor. The motor-compressor branch-circuit short-circuit and ground-fault protective device shall be capable of carrying the starting current of the motor. A protective device having a rating or setting not exceeding 175 percent of the motor-compressor rated-load current or branch-circuit selection current, whichever is greater, shall be permitted, provided that, where the protection specified is not sufficient for the starting current of the motor, the rating or setting shall be permitted to be increased but shall not exceed 225 percent of the motor rated-load current or branch-circuit selection current, whichever is greater.
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The 175 amp is usually based on the MCA if I am not mistaken. Please correct me if I am wrong.

If the mca is 62 in the given example then the FLA would appear to be about 50 amps. 175% of 50 = 87.5amps. That will not necessarly allow a 100 amp OCPD.
 
Dennis Alwon said:
Well I am not sure about that. Art. 440.22 states



The 175 amp is usually based on the MCA if I am not mistaken. Please correct me if I am wrong.

If the mca is 62 in the given example then the FLA would appear to be about 50 amps. 175% of 50 = 87.5amps. That will not necessarly allow a 100 amp OCPD.
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It would seem that you could calculate it either way if the words branch-circuit selection current refers to the conductors themselves.
 
yeah the FLA is 49.6 amps.. MCA is at 62 amps, which is (49.6 X 1.25% = 62 amps).


I found a section of the code, which states my original intent saying that conductor can be within one size of the over current protection...

NEC 2005 240.4(B) Protection of Conductors
The next higher standard over current device rating (above the ampacity of the conductors being protected) shall be permitted to be used, providing all of the following conditions are met: (1)(2)(3) Which are all met. Please look in the NEC book. Too much to write.

But the way I interpret this is that if the manufacturer is stating that a MOCP of 100 amps is required and per 240.4(B) the next higher standard size within 100 amps is #4 (85 amps) is required even though the MCA is at 62 amps and #6 wire (65amps) sounds like the wire to use...

Sorry if I confused anymore.. I elaborate more if my thoughts are confusing
 
Jimmy4645 said:
yeah the FLA is 49.6 amps.. MCA is at 62 amps, which is (49.6 X 1.25% = 62 amps).


I found a section of the code, which states my original intent saying that conductor can be within one size of the over current protection...

NEC 2005 240.4(B) Protection of Conductors
The next higher standard over current device rating (above the ampacity of the conductors being protected) shall be permitted to be used, providing all of the following conditions are met: (1)(2)(3) Which are all met. Please look in the NEC book. Too much to write.

But the way I interpret this is that if the manufacturer is stating that a MOCP of 100 amps is required and per 240.4(B) the next higher standard size within 100 amps is #4 (85 amps) is required even though the MCA is at 62 amps and #6 wire (65amps) sounds like the wire to use...

Sorry if I confused anymore.. I elaborate more if my thoughts are confusing
Click to expand...


Sorry to go off on a tangent. Forget any calculations at this point. The equipment contains a nameplate with all the information required for your use. Take the MCA and use that to size the conductors. That's all that's required.
 
Jimmy4645 said:
yeah the FLA is 49.6 amps.. MCA is at 62 amps, which is (49.6 X 1.25% = 62 amps).


I found a section of the code, which states my original intent saying that conductor can be within one size of the over current protection...

NEC 2005 240.4(B) Protection of Conductors
The next higher standard over current device rating (above the ampacity of the conductors being protected) shall be permitted to be used, providing all of the following conditions are met: (1)(2)(3) Which are all met. Please look in the NEC book. Too much to write.

But the way I interpret this is that if the manufacturer is stating that a MOCP of 100 amps is required and per 240.4(B) the next higher standard size within 100 amps is #4 (85 amps) is required even though the MCA is at 62 amps and #6 wire (65amps) sounds like the wire to use...

Sorry if I confused anymore.. I elaborate more if my thoughts are confusing
Click to expand...

I don't believe 240.4 really applies in this situation. For instance, I would use 240.4 if I was running #6 copper (lets say in conduit thhn) and the load was say 62 amps. I could then go the next higher size breaker of 70 amps.

I don't think that is the case with motors. Since you are allow 175% over the OCPD already, to go to the next size would be going over the 175% limit. There are situation where you can go higher but as Trevor stated if the unit specifies the max. ocpd as 100 amps, then it is allowable with #6 thhn as long as it is not part of NM cable or SE cable.
 
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