Wiring for A/C and Heat Pumps

[wildcat]

I have been wiring A/C and heat pumps using 440-6 authority. For instance a MCA of 26 Amps and Max Overcurrent of 40 Amps has been wired with #10 THHN with a 40 Amp breaker. The inspector says a 40A breaker requires #8 wire. The wire is rated 40A by 310-16 which is protected by the breaker and the protected rating of the wire is 30A per 310-16. I think my installation is correct and safe. Any comments?

 

[don_resqcapt19]

Re: Wiring for A/C and Heat Pumps

The inspector needs to go back to code school.
Don

 

[wildcat]

Re: Wiring for A/C and Heat Pumps

Don,
This is the second inspector out of this office I have had to "educate". The lead inspector knows this is correct, but apparently has passed it on.

Wildcat

 

[sparkmantoo]

Re: Wiring for A/C and Heat Pumps

my question is what is the temperature rating on the equipment? (110.14C) also what about 240.3D? i was under the impression that if your equipment is not rated for the 90 deg column, including the breaker, you can use the 90 deg column but you are supposed to go by the lowest temperature rating of the equipment in that circuit. and if the mca is 26, isn't that value all ready have the 125% factored in all ready? the 40 amp max overcurrent protection is the max you should put on that equipment (440.22a). i am curious to know.

 

[don_resqcapt19]

Re: Wiring for A/C and Heat Pumps

my question is what is the temperature rating on the equipment? (110.14C)

The 60?C ampacity of #10 is 30 amps. The required circuit ampacity is only 26 amps. 110.14(C) is not a problem.

also what about 240.3D

Look at 240.4(D) where it refers to 240.4(E) through (G). 240.4(G) says that for this equipment the rules in Article 440 cover the conductor overcurrent protection, not the rule in 240.4(D).
Yes, the 125% is already included in the nameplate circuit ampacity requirement, and yes, the maximum permitted OCPD is 40 amps.
Don

 

[sparkmantoo]

Re: Wiring for A/C and Heat Pumps

i understand. could it have been put on a 30 amp breaker and be code compliant, or increase the wire size to #8 and fuse it down to 30 amp at the disconnect?

 

[iwire]

Re: Wiring for A/C and Heat Pumps

You could not use a 30 amp overcurrent device as the tag on the unit asked for 40 amps.

This installation is code compliant and safe.

To get into all of this would take a while but look at article 440 and 430 as to overcurrent protction and conductor sizing for these kind of loads.

Here is a link that might help

AC wire size

 

[a.wayne3@verizon.net]

Re: Wiring for A/C and Heat Pumps

My rule of thumb is wire for min. and fuse for max as listed on unit never been tagged........

 

[tpwd]

Re: Wiring for A/C and Heat Pumps

wildcat, you are correct and the inspector was wrong when he told you, that was the reason he turned it down. I assume the unit stated that the MOCP of 40-amps was acceptable for a fuse and/or HVAC breaker. And the THHN wire you speak of was rated for wet locations such as [THHN/THWN]if the unit was outside. Good call.

 

[sparkmantoo]

Re: Wiring for A/C and Heat Pumps

but isn't the mca the rating the manufacturer says is sufficient enough to size your conductors. if the mca has the 125% all ready factored in, how can i be wrong in sizing the ocp for that rating. and with the mocp it says that this is what they recommend NOT to exceed. so say if i have a a/c unit that fully loaded comes out to 20.5 amps, i add the 125% to that, which roughly comes out to 26 amps, i can't protect it with a 30 ocp? i have to use a 40 amp ocp but use #10 wire because #10 is good for 40 amps.

 

[iwire]

Re: Wiring for A/C and Heat Pumps

Hi Sparkmantoo, The reason to go with the 40 amp breaker is not for the running load current, it is for the startup current of the motor loads. 2002 NEC 440.22(A) and (B)

Refrigeration compressors take a large amount of current to get them moving.

If you look at 2002 NEC 240.4(G) you will see a list of items that overcurrent protection is treated differently for.

Items in 430 Motors and 440 Air Conditioning and Refrigeration, have much different rules for conductor and short circuit protection.

In a typical lighting circuit the breaker at the panel provides both overcurrent and short circuit, ground fault protection.

For article 430 and 440 loads the breaker at the panel provides only short circuit, ground fault protection (phase to phase or phase to ground)

The overcurrent protection of the conductors is provided by the motor overloads that are in the equipment or that you install. Look at 2002 NEC 440.52(B)

There are instances you could have a much larger OCPD at the panel than the conductors are rated for, and still be code compliant.

The key for HVAC loads is to follow the name plate on the unit, I may have been wrong to say you could not use the 30 amp breaker, as the unit said maximum breaker 40, so anything less then that would be code compliant but it may not handle the start up loads.

Where I work we would use the size OCPD they say but we may run larger wire than needed by code for voltage drop reasons.

(Edit in bold, due to wrong info given)

As a side note if the unit tag only shows fuses, you must have fuses in the circuit.

Here is a link about this.

Topic: fusible disconnects

[ April 18, 2003, 05:47 PM: Message edited by: iwire ]

 

[don_resqcapt19]

Re: Wiring for A/C and Heat Pumps

Bob,

As a side note if the unit tag does not show breakers, you must have fuses in the cicuit.

I don't agree. If the nameplate does not specify, you can use either. If it specifies fuses, then fuses are required. Many specify fuses or HCAR breakers.
Don

 

[gwz2]

Re: Wiring for A/C and Heat Pumps

I have seen labels which noted similaar to:

OCPD 40Amps

Which means any type of Over-Current Protection Device, time-delay, non time delay and if a CB is used, it doesn't need to be a HACR type.

 

[iwire]

Re: Wiring for A/C and Heat Pumps

Don, Thanks you are right, I do not know where that came from. Typing faster than I was thinking I guess.

[ April 18, 2003, 04:57 PM: Message edited by: iwire ]

 

[sparkmantoo]

Re: Wiring for A/C and Heat Pumps

i still think that when the manufacturer gives the minimun circuit ampacity they are telling you that with the motor and the 125% added for the startup, that this is what they determined to be a safe value for sizing the conductors and ocp. the maximum overcurrent protection is the max you can protect that unit at. a 30 amp breaker on a 20.5 amp load is about 150% ( a little more) of that load. don' you think that that is sufficient to allow for start up? sure a 26 amp load can be put on a #10, i agree, but sticking it on a 40 amp breaker i don't agree. i think that when i size a unit and i use a 40 amp breaker then i'm going to use #8 wire. i feel better about myself knowing that i pulled a larger conductor to the motor, even when the code says i can go smaller. better to be to big than not big enough.

 

[iwire]

Re: Wiring for A/C and Heat Pumps

Originally posted by sparkmantoo:
30 amp breaker on a 20.5 amp load is about 150% ( a little more) of that load. don' you think that is sufficient to allow for start up?

Maybe enough, maybe not, but the way they labeled the unit they left options if that 30 amp breaker did not hold.


If a refrigerant compressor tries to short cycle, it usually will not start up and draws locked rotor current until the thermal protector on the unit opens. I think the idea is for the branch circuit breaker to hold through this, which may happen a couple of times in a row until the unit actually starts.

Originally posted by sparkmantoo:
sure a 26 amp load can be put on a #10, I agree, but sticking it on a 40 amp breaker I don't agree.

Don't agree personally or don't agree the code allows this?

Originally posted by sparkmantoo:
I think that when I size a unit and I use a 40 amp breaker then I'm going to use #8 wire.

That's great, I was not trying to say you should not, the code is just the minimum, the jobs I work on have engineered drawings and most times the conductors are sized to the OCPD or larger for voltage drop, but I also have had plans that had 3 AWG copper on 225 amp breakers and that was within code.

 

[sparkmantoo]

Re: Wiring for A/C and Heat Pumps

i don't agree with it personnally. i will still pull #8 to a 40 amp brkr even though the code say i can pull #10. why i do this is to give a little bit more conductor for the equipment to run on. i guess i'm just not a bare minimum kind of person. sorry , it just the marine in me. ooh rah!

 

[roger]

Re: Wiring for A/C and Heat Pumps

Sparkmantoo, as iwire, others, and yourself have pointed out, the code is the minimum.

Using larger conductors is a possitve if competiton doesn't come in to play in bidding on large projects.

Besides the confussion associated with which code article applies, larger conductors can pay for themselfs in a period of time (I2 R) and save money for the customer.

Roger

[ April 23, 2003, 06:50 PM: Message edited by: roger ]