hvac conductor ampacity selection

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stew

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440,6 says that the ampacities of conductors SHALL be selected from tables 310,16 or calculated in accordance with 310,15 as applicable. My question is can I use 10 awg cu for a unit rated at 35 amps or must I fall back to 240.4(D)? 440,6 indicates I can use the tables but does 240.4(D) overide this for # 10 awg and bump me down to 30 amps thus requiring a # 8 for this application?
 
NO, 240.4(D) does not apply to installations that fall under article 440. Simply use the values in 310.16. If all of your terminations are rated for 75 degrees C then you may use the ampacity in the 75 degree C column which for #10 THHN is 35 amps. I'm assuming that the 35 amp ampacity is the minimum circuit ampacity (MCA) listed on the unit.
 
(D) Small Conductors Unless specifically permitted in 240.4(E) or 240.4(G), the overcurrent protection shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper; or 15 amperes for 12 AWG and 25 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied.
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240.4(G) says that the rules in 440 apply to HVAC conductor overcurrent protection.
Don
 
don Are you saying that I may NOT use the values in 310.16 then for conductor ampacitiy and that 240(G) only pertains to overcurrent protection not ampacity. if so then it seems that 310,16 would not apply and I would have to use the value in 240.4(D) of 30 and not 35 .
 
Stew,
don Are you saying that I may NOT use the values in 310.16 then for conductor ampacitiy and that 240(G) only pertains to overcurrent protection not ampacity. if so then it seems that 310,16 would not apply and I would have to use the value in 240.4(D) of 30 and not 35 .
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I am saying that the ampacities in 310.16 apply and that the overcurrent protection rules in 240 do not apply for this application.
Don
 
Why does the code allow you to "cheat" on your wire size for these paticular applications. It seems to me we should be going towards the overkill side with this type of equipment. Most of this equipment is running in hot environments and when the equipment doesnt get cleaned on a regular basis,the head pressure jumps up causing even more load.
 
tavis,
Why does the code allow you to "cheat" on your wire size for these paticular applications. It seems to me we should be going towards the overkill side with this type of equipment. Most of this equipment is running in hot environments and when the equipment doesnt get cleaned on a regular basis,the head pressure jumps up causing even more load.
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This isn't cheating...it is just placing the overload protection at the load end of the conductors. The branch circuit protective device is only providing the short circuit and ground fault protection. If the head pressure is too high and the load exceeds the overload trip point, then the equipment overload will take the unit out of service.
Don
 
I say that if the minimum circuit ampacity is 35 amps you can not use 10 AWG. However if the minimum circuit ampacity was 30 and maximum over-current device was listed at 35 amps you could protect the 10 AWG with a 35 amp over-current device 440.22
 
the min circuit ampacity is 35 amps and the maximum breaker size is 50. I got called for using 10 awg on this and I need to be sure what to tell the inspector if i am going to argue with him on ampacity. specifically whre does it actually say that i can use the vaulues listed in 310.6 and not have him refer me back to the 15/20/30 rule for small conductors. And it certainly is not cheating for this or any other electric motor driven load to use a larger breaker size than normally occurs because you must use a breaker size that will start the load which is allowable by code. The ampacity of the wire is also determined by 125% of the code table current and in this case the manufacturer of the hermetic compressor on the hvac has done the ampacity calculation, They are required by code to list this calculated ampacity on the nameplate. A lot of inspectors we deal with do not know how to apply the proper code for these applications sadly to say.
 
I would start by asking what is the ampacity of a #10 THHN conductor? It's 40 amps. Look at the 90 degree column in 310.16. We know that almost all terminations are only listed for 75 degrees C so we are forced to used the 75 degree C column. If the equipment falls under Article 440 then the provisions of 240.4(D), (the 15,20,30 small conductor rule for #14,#12 and #10) conductors do not apply. The conductors are sized according to the ampacity listed in 310.16 which for 75 degrees is 35 amps.

One other possibility is if you used NM cable than you would be required to use the 60 degree C rating.
 
trevor actually we used # 8/3 nm from the panel to a J box a few feet down the outside wall. the hvac contractor decided to locate the unit 20 feet further down thw outside wall than where it originally was supposed to go . we could only find 8/3 nm locally where we did this job. when the cut length was too short I went to the local hardware store instead of the 25 miles one way to the big box and purchased an outdoor splice box and # 10 thhn to complete the job. got called on the use of # 10 thhn on 25 amp nameplate ampacity. says the conductor is too small. says it must be as specified in 310,16 which seems to be 35 amps in my code book anyhow. If you cant use the 35 amp rating of a # 10 wire in any case then why publish the ampacity at 35? same thing applies to # 12 And # 14 as far as I can see as long as we are talking thhn.
 
stew said:
trevor actually we used # 8/3 nm from the panel to a J box a few feet down the outside wall. the hvac contractor decided to locate the unit 20 feet further down thw outside wall than where it originally was supposed to go . we could only find 8/3 nm locally where we did this job. when the cut length was too short I went to the local hardware store instead of the 25 miles one way to the big box and purchased an outdoor splice box and # 10 thhn to complete the job. got called on the use of # 10 thhn on 25 amp nameplate ampacity. says the conductor is too small. says it must be as specified in 310,16 which seems to be 35 amps in my code book anyhow. If you cant use the 35 amp rating of a # 10 wire in any case then why publish the ampacity at 35? same thing applies to # 12 And # 14 as far as I can see as long as we are talking thhn.
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As long as you're using an insulation rated for 75 degrees C you're OK with # 10 and an MCA of 35 amps. That is unless you need to derate for ambient temperature. That may or may not effect your final ampacity.

If the MCA on the nameplate is 25 amps than #12 THHN would be compliant.
 
Stew, so which is it minimum circuit ampacity 25 or 35 as previously stated and the maximum over-current 50 amps?
 
the minimum circuit ampacity is 35 and the maximum fuse or breaker is 50 as stated on the hvac nameplate. thus #10 thhn cu per 440.6 which states the ampacity MUST be selected from 310.16. THHN 10 is of course rated at 35 amps and thats what i used to complete the run . If I had been able to obtain 8 quickly at the local store i would have just continued with it bit could not so just used the 10 that was avail.
 
thought i would let you guys know we won one for the good guys. after inspector reread the 440 section he"'ALLOWED" THE 10 AWG CU. I pointed out to him that the code allowed me to do it not him.He says well ya thats what i meant, ha ha one for the good guys and I didnt have to travel 40 miles to fix something that didnt need to be fixed. He fired the last shot howver by saying that "this time" he did not cite me for no expansion fitting on the pvc. This run is on the back shaded side of the house. there is 8 feet of pipe on the wall down near the ground and then it goes under a bay window for about 8 more feet and the six feet to the unit. totally in the shade and in our area the average yearly temp according to the weather bureau is 56 degrees!!! hardly a need for an expansion joint in my opinion . but he "let me get by with it this time"
 
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