maximum breaker size for #10 awg conductor - HVAC?

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winslowfam

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Hi...we have an HVAC unit with a minimum circuit amps (MCA) rating of 33A and a maximum overcurrent protective device (MOPD) size of 45A from the manufacturer. I would like to use #10 AWG conductors for the power feeder but just want to make sure I am not running against NEC 240.4 (D) for small conductors where 30A breaker is the max size allowed for #10 AWG. I think having it be an HVAC circuit exempts me from that restriction.
 
You are correct. If you read 240.4(D), it states unless specifically permitted in (E) or (G) and (G) is overcurrent protection for specific applications, including HVAC.
Since 10 AWG is rated 35 amps at 75 degrees, you'll be fine. Just be aware that many AHJs don't understand this rule.
 
I did not notice a wire type. Note that the 60? limitation on NM cable would prevent a #10 NM being used. Also, on the off chance your terminations are rated 60?,#10 could not be used per 110.14.
(I have seen a few HVAC units which noted 60? termination limitations)

In general, you are correct.
 
augie47 said:
I did not notice a wire type. Note that the 60? limitation on NM cable would prevent a #10 NM being used. Also, on the off chance your terminations are rated 60?,#10 could not be used per 110.14.
(I have seen a few HVAC units which noted 60? termination limitations)

In general, you are correct.
Click to expand...

Nevermind had the wrong thread.
 
tom baker said:
You are correct. If you read 240.4(D), it states unless specifically permitted in (E) or (G) and (G) is overcurrent protection for specific applications, including HVAC.
Since 10 AWG is rated 35 amps at 75 degrees, you'll be fine. Just be aware that many AHJs don't understand this rule.
Click to expand...

They will not get away with not knowing this if they are inspecting for me, this is basic conductor and overcurrent device sizing that they should see on 70% or more of the work they inspect.

Jim W in Tampa said:
Also be sure this unit is cool only. Any heat strips changes everything. And as said romex is 60 degree
Click to expand...

If heat strips are involved then the unit markings should still be correct for minimum circuit ampacity and maximum overcurrent protection. If it is a field installable heat strip there will be a list of possible combinations of what could be installed and the requirements for each possibility, the installer is supposed to mark what is installed - they never do and I have to figure it out myself. If they do mark it I usually double check it anyway, HVAC guys are not always on top of things when it comes to electrical aspects of their equipment. I have seen many that troubleshoot by replacing components until it works - especially on electric heat units, sad thing is they are actually simpler than the gas units - with those they just get on the phone with a tech support guy and have them walk thru everything while on the phone.
 
upsizing weather needed or not

upsizing weather needed or not

I have always had a problem understanding the logic behind putting #10 wire on 40 amp breaker for A/C's. Most Experience A/C guys have informed me that units run better on oversized supply wire. I also think that the overall power consumption is less. Most jobs I do are T/M the customer pays for the minor cost for larger wire. He generally wants reliability.

More amperage pushing through a wire causes it to heat up>>>which takes more amperage to push power thru. COSTING MORE MONEY

Gas engines wear out quickly when running lean.

Thin Ropes wear out when lifting heavy loads.

Water spinklers can't spray much with a 1/4 supply hose

Your car can go 120mph, I'm sure it will last longer if driven that way constantly?

Air Impact guns do not run well w/tiny air hoses despite pressure. (pressure)x(volume) is similiar to (volts)x(amps)

Why are shortchanging your customers????

Feedback from a completed job confirms my point.
(5) A/C's (3) hot watertanks and more, for a 8000sq ft comercial space. Average utility bill $300 I purposely upsized the service to 800amp 3P 208 and ran at least one size wire upsize to every A/C , and H.W. an other loads

The customer spreads the word on how good his electrician and others did on his historic remodel.
 
the logic is that the wire may be sized for the minimum circuit ampacity not the starting current which is what the breaker is sized for. A larger wire for a smaller than ampacity load for that wire will have absolutley no effect on the units efficienvy regardles of what the hvac guy says. If he can prove me wrong let him have at it!
 
stew said:
the logic is that the wire may be sized for the minimum circuit ampacity not the starting current which is what the breaker is sized for. A larger wire for a smaller than ampacity load for that wire will have absolutley no effect on the units efficienvy regardles of what the hvac guy says. If he can prove me wrong let him have at it!
Click to expand...
It could be true where the distance is an issue and VD comes into play.
 
stew said:
the logic is that the wire may be sized for the minimum circuit ampacity not the starting current which is what the breaker is sized for. A larger wire for a smaller than ampacity load for that wire will have absolutley no effect on the units efficienvy regardles of what the hvac guy says. If he can prove me wrong let him have at it!
Click to expand...

Dennis Alwon said:
It could be true where the distance is an issue and VD comes into play.
Click to expand...


The same compressor that is required to be supplied by 10 AWG will run just as efficiently on 14 AWG if the length of conductor doesn't create excessive voltage drop - but even if the voltage does drop the current will go up and total KVA used by the compressor will remain somewhat constant - heat given up by the conductor will be an inefficiency. The 10 AWG will give up some heat also, and so will a 8 AWG.

Larger sized breaker is only for allowing starting. You will find sometimes that the larger the conductor the more starting current is allowed to flow because of lower circuit impedance and you will need the larger breaker to hold on startup.
 
rt66electric said:
I have always had a problem understanding the logic behind putting #10 wire on 40 amp breaker for A/C's. Most Experience A/C guys have informed me that units run better on oversized supply wire. I also think that the overall power consumption is less. Most jobs I do are T/M the customer pays for the minor cost for larger wire. He generally wants reliability.

More amperage pushing through a wire causes it to heat up>>>which takes more amperage to push power thru. COSTING MORE MONEY

Gas engines wear out quickly when running lean.

Thin Ropes wear out when lifting heavy loads.

Water spinklers can't spray much with a 1/4 supply hose

Your car can go 120mph, I'm sure it will last longer if driven that way constantly?

Air Impact guns do not run well w/tiny air hoses despite pressure. (pressure)x(volume) is similiar to (volts)x(amps)

Why are shortchanging your customers????

Feedback from a completed job confirms my point.
(5) A/C's (3) hot watertanks and more, for a 8000sq ft comercial space. Average utility bill $300 I purposely upsized the service to 800amp 3P 208 and ran at least one size wire upsize to every A/C , and H.W. an other loads

The customer spreads the word on how good his electrician and others did on his historic remodel.
Click to expand...


Your unneeded upsizing had less than 1 % to effect this.
 
rt66electric said:
I have always had a problem understanding the logic behind putting #10 wire on 40 amp breaker for A/C's. Most Experience A/C guys have informed me that units run better on oversized supply wire. I also think that the overall power consumption is less. Most jobs I do are T/M the customer pays for the minor cost for larger wire. He generally wants reliability.

More amperage pushing through a wire causes it to heat up>>>which takes more amperage to push power thru. COSTING MORE MONEY
Click to expand...


If the voltage is also dropping then your total VA is not changing all that much.


rt66electric said:
Gas engines wear out quickly when running lean.

Thin Ropes wear out when lifting heavy loads.

Water spinklers can't spray much with a 1/4 supply hose
Click to expand...

That will depend on volume of water sprinkler needs water pressure, and length of supply hose (similar to sizing conductor for current in a way)


rt66electric said:
Your car can go 120mph, I'm sure it will last longer if driven that way constantly?

Air Impact guns do not run well w/tiny air hoses despite pressure. (pressure)x(volume) is similiar to (volts)x(amps)
Click to expand...

Same as with water sprinkler - length of hose will make a difference - the inlet to the impact gun is usually not very big but pressure drop is minimal compared to a 100 foot hose of the same size.

rt66electric said:
Why are shortchanging your customers????

Feedback from a completed job confirms my point.
(5) A/C's (3) hot watertanks and more, for a 8000sq ft comercial space. Average utility bill $300 I purposely upsized the service to 800amp 3P 208 and ran at least one size wire upsize to every A/C , and H.W. an other loads
Click to expand...

The same installation with minimum sized conductors likely would result in average utility bill being about the same. Long feeders and branch circuits could make some difference in efficiency - the appliances you mention will still use the same amount of energy to do the same amount of work. Larger conductors will give up less heat but it is not that much heat.


Take a 240 volt resistive load that draws 10 amps = 2400 watts.

Now lets add one ohm of resistance to the conductors supplying this load.

Total current will be reduced to 9.6 amps. 92 watts will be given up as heat by the additional 1 ohm of resistance the 2400 watt load @ 240 volts will now use 2212 watts @ 230.4 volts.

That kind of inefficiency will add up but it will take a pretty long circuit to get 1 ohm of resistance so this figure is larger than what you would likely run into for conductors that need not be upsized because of voltage drop.


rt66electric said:
The customer spreads the word on how good his electrician and others did on his historic remodel.
Click to expand...
__________________
 
Even if there is a slight energy savings, if it's a replacement unit on the opposite end of the house from the panel and you have to tear apart their finished basement to get a larger sized cable through to the unit, how much money are you saving your customer? When running new, I will maximize the conductor size according to the unit just to allow for future, but if it's a replacement, I have no problem with the minimums.
 
Once again, I replied to a thread after only reading the first page! Is there any way to change things so that the big red "POST REPLY" button appears only at the end of the last page? I don't know how many times I've done this!
 
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