Sizing AC CB

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starsandstripes

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I was told to run pipe and wire for a 3 phase ac unit with a MCA of 23 amps, so I used #10 THHN. When the unit arrived the nameplate indicated MCA 23 amps, with a max fuse or cb size of 40 amps. I don't believe I have 90 degree terminations. Should I use a 30 or 40 amp cb?
 
You could use a 25, 30 or 40 amp OCPD. The 40 amp may prevent startup issues that may occur with a smaller OCPD. As Bob said you could use #12 THHN since it's ampacity is 25 amps and is greater than the MCA which is used to size the conductors.
 
starsandstripes said:
I was told to run pipe and wire for a 3 phase ac unit with a MCA of 23 amps, so I used #10 THHN. When the unit arrived the nameplate indicated MCA 23 amps, with a max fuse or cb size of 40 amps. I don't believe I have 90 degree terminations. Should I use a 30 or 40 amp cb?
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This question comes up all the time. It is very confusing at first but you must remember the a/c unit has overload protection built in. The breaker is strictly for ground fault and short circuit protection. The protection in the unit will protect the wire from becoming overloaded.
 
240.4(G) refers us to 440 Parts 3 & 4 for AC equipment. Where does it allow us to use the 25A value stated in 310.16? I thought that those ampacities were to be used for derating purposes only:-?
 
A/A Fuel GTX said:
240.4(G) refers us to 440 Parts 3 & 4 for AC equipment. Where does it allow us to use the 25A value stated in 310.16? I thought that those ampacities were to be used for derating purposes only:-?
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The reason we generally use #12 THHN at 20 amps is because of 240.4(D)(5). 240.4(G) allows us to use the ampacities listed in 310.16 without the restrictions of 240.4(D).

240.4(D) Small Conductors. Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed that required by (D)(1) through (D)(7) after any correction factors for ambient temperature and number of conductors have been applied.
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infinity said:
The reason we generally use #12 THHN at 20 amps is because of 240.4(D)(5). 240.4(G) allows us to use the ampacities listed in 310.16 without the restrictions of 240.4(D).
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Ok but where in 240.4(G) does it say we can use #12 to carry 25A:-?
 
A/A Fuel GTX said:
Ok but where in 240.4(G) does it say we can use #12 to carry 25A:-?
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240.4(G) deals with overcurrent device sizing not ampacity of conductors.

Table 310.16 gives us the ampacity of conductors. Table 310.16 shows that the ampacity of #12 THHN from the 75 degree column is 25 amps. Because 240.4(G) says that we can size the OCPD for an AC unit in accordance with Part III of Article 440 we are not limited by the small conductor rules of 240.4(D) and can install a larger OCPD.

Chris
 
raider1 said:
240.4(G) deals with overcurrent device sizing not ampacity of conductors.

Table 310.16 gives us the ampacity of conductors. Table 310.16 shows that the ampacity of #12 THHN from the 75 degree column is 25 amps. Because 240.4(G) says that we can size the OCPD for an AC unit in accordance with Part III of Article 440 we are not limited by the small conductor rules of 240.4(D) and can install a larger OCPD.

Chris
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This is also where it gets tricky. For AC units were actually sizing the conductor to the MCA not the OCPD. The MAX OCPD will typically exceed the ampacity listed in 310.16 since it's only providing short circuit and ground fault protection as Dennis mentioned.
 
This is one thing in the code book among many I disagree with, and ignore. I use the wire size needed to supply the current necessary for the load and use an OCPD that will protect the wire used. End of story.
 
infinity said:
This is also where it gets tricky. For AC units were actually sizing the conductor to the MCA not the OCPD. The MAX OCPD will typically exceed the ampacity listed in 310.16 since it's only providing short circuit and ground fault protection as Dennis mentioned.
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Yes, I agree. For AC units you would use the minimum circuit ampacity marked on the nameplate to size the conductors and the maximum OCPD would also be marked on the nameplate. For AC units the maximum OCPD is typically 175% of the FLA's.

Chris
 
neutral said:
T I use the wire size needed to supply the current necessary for the load and use an OCPD that will protect the wire used. End of story.
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Do what you want but the wire is protected by the units overload protection. If you truly understood this you would not make statements like this. #12 is rated 25 amps, look at Table 310.16. I am not saying you should not use a larger size just that you don't have to. I tend to oversize because in years past people would add an addition and they would install a larger unit. Today 16 seer and larger units are so efficient and they keep getting better that the wire sizes from years ago are larger than the new units need.
 
neutral said:
This is one thing in the code book among many I disagree with, and ignore. I use the wire size needed to supply the current necessary for the load and use an OCPD that will protect the wire used. End of story.
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That's OK too. It's just that you're wasting money on using larger conductors than the code or the load requires.
 
neutral said:
This is one thing in the code book among many I disagree with, and ignore. I use the wire size needed to supply the current necessary for the load and use an OCPD that will protect the wire used. End of story.
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You can certainly do that, but it does not make it any safer. The conductors are protected from over current.

It is imposable to overload the conductors if you follow the NEC and the nameplate.
 
raider1 said:
For AC units the maximum OCPD is typically 175% of the FLA's.

Chris
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I have got a question on that. I agree with your statement but if the unit says max OCPD and it is 175% of the FLA then how does 440.22 come into play. Would it ever be legal to use the 225%. Any thoughts

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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Dennis Alwon said:
I have got a question on that. I agree with your statement but if the unit says max OCPD and it is 175% of the FLA then how does 440.22 come into play. Would it ever be legal to use the 225%. Any thoughts
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That is a good question. If you exceed the MAX OCPD marking on the nameplate is that a violation of 110.3(B)?

For what it is worth I have never had a issue where the MAX OCPD marked on the AC unit did not allow the unit to work properly, and would have needed to upsize the OCPD.

Chris
 
Money is never an issue. If everything in the code book was absolute there wouldn't be so many changes every code cycle. After 50 years doing Electrical work I think I understand as much as most Electricians, but I still learn something every day. Just knowing how to follow an instruction in a book don't make you an Electrician, Just a good follower and worker maybe.
 
iwire said:
You can certainly do that, but it does not make it any safer. The conductors are protected from over current.

It is imposable to overload the conductors if you follow the NEC and the nameplate.
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suppose the overload device in the unit fails?
 
raider1 said:
That is a good question. If you exceed the MAX OCPD marking on the nameplate is that a violation of 110.3(B)?

For what it is worth I have never had a issue where the MAX OCPD marked on the AC unit did not allow the unit to work properly, and would have needed to upsize the OCPD.

Chris
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Nor have I but I wondered about larger commercial units. Seems like it is in the NEC for some reason. :smile:
 
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