wire size for 25a curcuit

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rjhavens1 said:
I am installing a peice of equipment on and existing curcuit that has a 25a breaker and 12g wire. Is 12g sufficient for a 25a curcuit, it is 3 phase commercial.
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Usually not unless you are feeding an a/c or something that allows you to fuse over the wire size. #12 THHN is good for a 20 amp breaker not 25. You would need 10 gauge. Although table 310.15 has 25 amps check the asterik and go to article 240.4(D)
 
You can use #12 THHN for the load you described with 25 amp fuses. If the fuses only provide ground fault and short circuit protection they may be larger than the values listed in 310.16.

Also 240.4(D) doesn't apply to this type of load.
 
rjhavens1 said:
It is a Refrigeration condensing unit with a fused disconnect with 25a fuses
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I believe article 440.22 should apply--A protective device of not more than 175 percent of the motor-compressor rated load current....
 
:smile: Thanks for the reply
We were trying to use the existing curcuit (25a 3pole breaker & 12g) and just change the equipment and still be in compliance with the NEC. If we were starting from scratch I would indeed use # 10. We are unsure weather or not to return and bid a new curcuit.
 
Okay, so

  • 25A OCPD
  • 12 AWG Copper conductors in conduit
  • 50 ft from panel
  • Minimum Circuit Ampacity on condensing unit is 16.4A
  • Max OCPD 30A
I'd say with that short of run and 16.4 MCA you'd be perfectly fine with 12 AWG conductors.
 
I agree with Don, an MCA of 16.4 amps will be fine with #14 AWG conductors. I would simply use what is already there. #12 with a 25 amp OCPD is fine.
 
don_resqcapt19 said:
Actually in that case the code would permit the use of #14 with the 30 amp OCPD.
Don
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Don I have an inkling but I am not sure how one figures the motor load based on the min and max ocp. Can you share that with me. I would normally read it off the motor.
 
This type of equipment contains a nameplate that has the MCA which is calculated by the manufacturer. Typically the MCA is the FLA of the motor X 125% plus any additional smaller loads
(if applicable). This is used to size the conductors. The provisions of 240.4(D) do not apply to this equipment so the ampacities listed in 310.16 can be used. 310.16 lists #14 THHN with an ampacity of 20 amps, therefore this is permitted to be used to supply a load with a 16.4 amp MCA.

Since the unit must be able to start the manufacturer lists a maximum overcurrent protection value that can be used to size the OCPD. The OCPD is permitted to be any size between the MCA and the maximum listed on the nameplate.
 
Thanks Trevor-- what I was missing was that 14 could be used for a 16 amp circuit. I see now that 240.4(D) says unless permitted in 240.4(E)or(G). 240.4(G)permits 14 gauge with a/c, refrig. etc-- live and learn. I knew you could fuse higher but didn't realize this so called exception in 240.4(D). Thanks again
 
:smile: Thanks for all the help guys, but my boss wants proof in the code book on where it says it is ok to run 12awg on a 25a OCPD for the load that I have described. On Table 310-16 , 12thhn has a 90degree rating. The table shows 30amp capacity with a obelisk next to the rating. According to the bottom of the table it states the OCPD with conductors marked with obelisk shall not excedd 15a for 14awg and 20a for 12awg. Is there something we are missing?:-?
 
rjhavens1 said:
:smile: Thanks for all the help guys, but my boss wants proof in the code book on where it says it is ok to run 12awg on a 25a OCPD for the load that I have described. On Table 310-16 , 12thhn has a 90degree rating. The table shows 30amp capacity with a obelisk next to the rating. According to the bottom of the table it states the OCPD with conductors marked with obelisk shall not excedd 15a for 14awg and 20a for 12awg. Is there something we are missing?:-?
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Here is the problem or solution---article 310.16 has an asterik that relates to article 240.4(D). This article, in the the first sentence, says Unless specifically permitted in article 240.4(E) or (G) the overcurrent protection shall not exceed 15 amps for 14, etc. However, 240.4(G) states the permission to disregard (D). Remember (D) states unless otherwise permitted--(G) is where it is specifiically permitted. In your case it is refrigeration. Since 14 gauge is good for 20 amps (Table 310.16) 75 DEGREE, you can then use 14 gauge wire for the 16.4 amp load. Now I would run 12 guage and according to article 440.22 (A) 2nd sentence you can use an OCP rated at 175% of the motor compressor load--- 16.4 times 175%= 28.7 AMP ocp-- THEREFORE 25 AMP IS FINE WITH #12-- yOU HAVE TO USE 75 DEGREE RATINGS BECAUSE THE BREAKERS ARE ONLY RATED AT 75 DEGREE
 
rjhavens1 said:
:smile: Thanks for all the help guys, but my boss wants proof in the code book on where it says it is ok to run 12awg on a 25a OCPD for the load that I have described.
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It's a little bit tricky to convey - because there's not a clear sentence that flat out says the practice is okay. We're kind of trying to describe a vacuum, in a way. I agree with Dennis' read on this, but I want to put it a little differently.

On Table 310-16 , 12thhn has a 90degree rating.
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Right, 310.16 is the first step. Terminations are generally rated at 75 degrees, but anything under 100A could have 60 degree terminations, too. See 110.14(C)(1) for more information, too.

The table shows 30amp capacity with a obelisk next to the rating.
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It does?

obelisk.gif
Sorry, smart aleck moment. Back on topic: :D

According to the bottom of the table it states the OCPD with conductors marked with an asterisk shall not exceed 15a for 14awg and 20a for 12awg. Is there something we are missing?:-?
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Yes, you are. Think of it this way: Why would they bother giving us ampacities if they always had to be breakered #12 to 20A, #14 to 15A, etc?

It tells you to see 240.4(D), but that doesn't mean that 240.4(D) always applies. That table on the next page from 240.4(D), Table 240.4(G) removes every single one of those list items from the rule given to us in 240.4(D).

240.4(D) says that any item in Table 240.4(G) is not governed by 240.4(D).
 
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