conductor sizing vs. breaker sizing

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Donald Sherman

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I've always been taught that my conductor size couldn't be smaller than what the breaker is sized for. We have an HVAC distributer who is telling my employees differant.

Example: Heat pump outdoor condensor section
min. brch. cir. ampacity 24
min. and max circuit breaker protection 40 amps. per nameplate.

They say we can install the 40 amp breaker on existing circuit that had a #10 NM-B romex wire. I was always thought you couldn't put larger than a 30 amp breaker on this wire.

Which way is correct?
 
He is correct. 240.4(G). The #10 on a 30A breaker rule 240.4(D) applies to branch circuits, and other circuits not listed in 240.4(G). In Air Conditioning and Motors etc etc. the 'standard' sizing rules don't apply.
See Art. 440 for more specifics on conductor and OCPD for A/C.
 
Donald Sherman said:
I've always been taught that my conductor size couldn't be smaller than what the breaker is sized for. We have an HVAC distributer who is telling my employees differant.

Example: Heat pump outdoor condensor section
min. brch. cir. ampacity 24
min. and max circuit breaker protection 40 amps. per nameplate.

They say we can install the 40 amp breaker on existing circuit that had a #10 NM-B romex wire. I was always thought you couldn't put larger than a 30 amp breaker on this wire.

Which way is correct?
Click to expand...

The HVAC guy.

210.2 directs the user to 440.6, 440.31, 440.32

Give those 3 sections an eyeballing....after you've read them, we can discuss this further.
 
He is right, the breaker is providing short circuit and ground fault protection.

Test your knowledge with this question: What size conductor and protection device must you use for a 24A motor-compressor on a 240V circuit (Fig. 3)


Step 1: Size the branch-circuit conductor per Table 310.16 and 440.32.
24A x 1.25 = 30A Per 110.14(C)(1)(a) and Table 310.16, you would select a 10 AWG conductor, rated 30A at 60?C.


Step 2: Size the branch-circuit protection device per 240.6(A) and 440.22(A).
24A x 1.75 = 42A Selecting the next size down protection device yields a 40A device.

If a 40A protection device isn?t capable of carrying the starting current, you can size the protection device up to 225% of the equipment load current rating (24A x 2.25 = 54A, next size down 50A).

Conductor sizing. Size these branch-circuit conductors no smaller than the spec on the equipment nameplate. If the equipment doesn?t have a nameplate that specifies the branch-circuit conductors, size the conductors per 440.32.
Click to expand...

http://ecmweb.com/ar/keep_your_cool/
 
Donald Sherman said:
I've always been taught that my conductor size couldn't be smaller than what the breaker is sized for. We have an HVAC distributer who is telling my employees differant.
Click to expand...

The HVAC guy is correct. You can have a number 10 wire and still use a 40 amp breaker on it.

If you look on the nameplate of the heat pump it will tell you what the minimum circuit ampacity can be. It also states the max overcurrect protection. Usually it is about 175% (I believe) of the minimum circuit ampacity.

Thus 24 amps times 175% will give you 42 amps. So you can put a 40 amp breaker on it.
 
And he could really scare you by using #12 wire. Even at 60C, #12 copper has an ampacity of 25 which is higher than the compressor 24MCA value. The 20A breaker on a #12 goes out the window on motor, HVAC, welder, and a few other types of circuits.
 
With the Articles you all have had me look up I see where breaker can be sized differently than the conductor for the condensor. I see why I'm just an hvac guy. One question though. The table refered to for conductor sizing it was stated to use the 60deg C. column. The table says this is based on 30deg C. with a correction table below it for other temperatures. Alot of our applications have this cable running across an attic to get to where the condensor is. In southside Virginia where we are these attics can get upward of 120deg F. How much bearing does this have on us being able to use this same #10 conductor in this application?
I think all of you for the help here. I don't want to arm my employees with bad information.
 
M. D. said:
24A x 1.25 = 30A
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suemarkp said:
And he could really scare you by using #12 wire. Even at 60C, #12 copper has an ampacity of 25 which is higher than the compressor 24MCA value.
Click to expand...
I agree with Sue. From what I understand, you needn't add the 25%. #12 would suffice for a 24a MCA. The 20a number doesn't apply in Art. 440 at all.
 
Larry , I think in the example I posted it was ,if the nameplate did not contain the circuit information.
You are correct when the info is there they have already done the calculation , no need to do it again.
Here is a visual from the link.



409ecwb15fig2.jpg
 
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What if there is a maximum fuse size but not a minimum fuse size. I always put the max. Is it up to the AHJ or is there something in the code that states so?:confused: I have read your comments and they have been helpful.
 
jarilu422 said:
What if there is a maximum fuse size but not a minimum fuse size. I always put the max. Is it up to the AHJ or is there something in the code that states so?:confused: I have read your comments and they have been helpful.
Click to expand...


The OCPD is sized anywhere between the MCA and the Maximum OCPD listed on the unit. The unit may not start when the OCPD is sized for the MCA.
 
jarilu422,

Reminder, when the name plate states max fuse size, a fuse must be used

somewhere on the circuit. fuse does not = circuit breaker.
 
Your esxplaination was well appreciated and consise- We are having and issue in our area of California- Our local inspector says there is an exception to section 440 that does not allow this formula on residental HVAC systems- I cannot find od interperat that exception- They read it as wire ampacity must match overcurrent period- in our market we are in- the HVAC units we are replacing will now require new wire run IE #8 to a #6- tough to explain this to a home owner when the wire sizing and breaker szing was excepted in the past-

Any input would be greatly appreciated
 
lkapigian said:
Your esxplaination was well appreciated and consise- We are having and issue in our area of California- Our local inspector says there is an exception to section 440 that does not allow this formula on residental HVAC systems- I cannot find od interperat that exception- They read it as wire ampacity must match overcurrent period- in our market we are in- the HVAC units we are replacing will now require new wire run IE #8 to a #6- tough to explain this to a home owner when the wire sizing and breaker szing was excepted in the past-

Any input would be greatly appreciated
Click to expand...

Ask for an article number.
I do know that California has its own code. Is there an amendment to the NEC regarding this? Is there a local amendment to the CA NEC regarding this?
And why just residential?
 
lkapigian said:
Your esxplaination was well appreciated and consise- We are having and issue in our area of California- Our local inspector says there is an exception to section 440 that does not allow this formula on residental HVAC systems
Click to expand...


He must have a different code book than the rest of us, unless it's a local code. This is not in the NEC.
 
I am still waiting for there official ruling- as of yet, I am dealing with "because I said so"- I would just like to see it in writing so I can give my customers the reason I am having to run new wire to their new A/C unit when the wire has been fine all these years
 
Ok, they faxed me Article 240.4 item 2 as the reason we have to match breaker with wire size- I don't understad how this comes into play with 440

Thanks

Larry!!
 
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