Min Ckt Amp also min breaker size?

[cmiller425]

I recently installed an A/C system with nameplate rating of
Max Fuse/Bkr 60
Min Ckt Amp 52.1

Breaker panel has 50 amp circuit breaker.

Inspector failed inspection, saying the breaker was too small. Is the "Min Ckt Amp" rating only for wire sizing or does it also serve as a "Min Bkr" rating?

Where does the NEC address this?

 

[George Stolz]

Re: Min Ckt Amp also min breaker size?

By installing a 50, you're over-protecting the wire. Why would be red-tagged for installing a lower-rating OCPD on a conductor?

Now, given that it's calling for a 60, you could run the risk of the breaker kicking at startup. But that's a warranty issue, not an NEC issue.

I believe he is wrong. I have (as I wander out on a ledge) often installed breakers 5 or 10 amps lower than the max allowed, just due to the fact that I don't have room on my truck for 35 amp breakers, and can't justify carrying more than one.

Would he cite you for installing a 15-amp breaker on a #10 wire? Would section would he cite?

Stick it to the man. (After consulting with the boss, of course. )

 

[dillon3c]

Re: Min Ckt Amp also min breaker size?

Originally posted by cmiller425:
I recently installed an A/C system with nameplate rating of
Max Fuse/Bkr 60
Min Ckt Amp 52.1

Breaker panel has 50 amp circuit breaker.


Where does the NEC address this?

*Branch select. current= 52.1 amperes
Max fuse/breaker 60 amperes

52.1 x *1.15 (115%)percent=*59.915
next size up. art 240.6 is *60amperes

see 440.12(A) & 440.12(B)-ampere rating

*same a manufacturer suggest..

 

[steve66]

Re: Min Ckt Amp also min breaker size?

If the unit says the minimum circuit ampacity is 52 amps, you must provide a branch circuit (wire and breaker) that can provide at least that much current. So the 60A breaker is required, and your inspector is right.

George: it may seem backwards, but that is the way it is. I think there are concerns that a smaller breaker may overheat and malfunction before it trips. A 50 amp breaker will probably never trip at 60 amps, and any overloads on the AC unit would be sized for the full 60A.

Steve

[ July 01, 2005, 08:53 AM: Message edited by: steve66 ]

 

[jwelectric]

Re: Min Ckt Amp also min breaker size?

What is the purpose of the breaker in the first place?

440.21 General.
The provisions of Part III specify devices intended to protect the branch-circuit conductors, control apparatus, and motors in circuits supplying hermetic refrigerant motor-compressors against overcurrent due to short circuits and grounds. They are in addition to or amendatory of the provisions of Article 240.

What must this short circuits and ground fault device be able to do?

440.21 (B) Rating or Setting for Equipment. The equipment branch-circuit short-circuit and ground-fault protective device shall be capable of carrying the starting current of the equipment. Where the hermetic refrigerant motor-compressor is the only load on the circuit, the protection shall conform with 440.22(A).

The original post stated the branch-circuit short-circuit and ground-fault protective device was lower that what the minimum called for thus the red tag.

 

[allenwayne]

Re: Min Ckt Amp also min breaker size?

We have to fuse to the max breaker size listed on the data plate but we can wire to the minimum ampere rated.our inspectors say that even though the lower rated breaker is protecting the branch circuit the manufacturer rates what is the max breaker allowed and that the OCPD has to match.
With all the different SEER ratings wire size and breaker size will change from model/manufacturer .One might have a min amp listed as 18 amp and max breaker is a 35 amp breaker This unit can be wired in #12 and have an OCPD of 35 amp

 

[infinity]

Re: Min Ckt Amp also min breaker size?

One might have a min amp listed as 18 amp and max breaker is a 35 amp breaker This unit can be wired in #12 and have an OCPD of 35 amp
--------------------------------------------------------------------------------

You could even use #14 THHN for a unit with a minimum ampacity of 18 amps.

I agree with Steve, the minimum circuit ampacity would be for sizing the conductors and the OCPD. It can't be smaller than the listed minimum since the unit is including an extra 25% of ampacity by the manufacturer. Also I would add that it certainly doesn't have to be the maximum.

 

[milwaukeesteve]

Re: Min Ckt Amp also min breaker size?

Dillon is correct.
When wiring A/C units (and refrigeration equipment) we use a different code section than normal loads, even motor loads. THis is Art 440.
For breaker sizing, the breaker is sized to allow for startup (HACR rated breakers are special breakers for this too) and to protect the conductors.
Minimum ckt amps is the minimum amps that will the unit will be pulling with the compressor and the evaporator motors both running. (based on nameplates for both). This is not the maximum running amps because startup draws more (hence the HACR breaker).
Conductor sizing is based on AMPERAGE only, and has nothing to do with the breaker size, and we are not subject to standard sizes for 15,20 and 30 Amp circuits. You could use a # 14 for its full rating (watch ambient temps), and not be limited to 15 Amps only.

Edited: I forgot to reference art 440

[ July 01, 2005, 05:21 PM: Message edited by: milwaukeesteve ]

 

[infinity]

Re: Min Ckt Amp also min breaker size?

Minimum ckt amps is the minimum amps that will the unit will be pulling with the compressor and the evaporator motors both running.

The minimum circuit ampacity is the compressor FLA X 125% plus the evaporator FLA. It is not the running FLA sum of the two.

 

[infinity]

Re: Min Ckt Amp also min breaker size?

52.1 x *1.15 (115%)percent=*59.915
next size up. art 240.6 is *60amperes

Just curious, where did you get the 115% from?

 

[ryan_618]

Re: Min Ckt Amp also min breaker size?

The motor has to be able to start.

 

[infinity]

Re: Min Ckt Amp also min breaker size?

The motor has to be able to start.

OK, but why 115%? The compressor already has an extra 25% built into the minimum circuit ampacity figure so where do we come up with 115%?

 

[ryan_618]

Re: Min Ckt Amp also min breaker size?

I don't have my NEC here, as I am not at home. I think the reference is 440.23. The same thing applies to overload protection for motors, unless they have a 40 degree or less temperature rise or a service factor of greater that 1.15

 

[George Stolz]

Re: Min Ckt Amp also min breaker size?

Originally posted by ryan_618:
I don't have my NEC here, as I am not at home.

 

[allenwayne]

Re: Min Ckt Amp also min breaker size?

Now wiring for min amps. and fusimg for max,has bit us a couple of times.We have done one area of SFR That has a granny flat they called it which was an additional studio over the garage and the building got a two meter/main system.
About 1/2 way through the complex .HVACcontractor was changed and they changed brands of hvac equiptment.We had wired about 150 of these at this time and the main house had 2 5 KW A/H/U`S
and the granny flat also had a 5 kw A/H/U.
Here comes trim out only to find that the 2nd floor a/h/u on the 2nd floor of main house was still a 5 kw unit but the model they used on these about 30 units had a min of 32 amps

 

[ryan_618]

Re: Min Ckt Amp also min breaker size?

Hmmm....wait a minute, maybe it is 125%

440.32 Single Motor-Compressor.
Branch-circuit conductors supplying a single motor-compressor shall have an ampacity not less than 125 percent of either the motor-compressor rated-load current or the branch-circuit selection current, whichever is greater.
For a wye-start, delta-run connected motor-compressor, the selection of branch-circuit conductors between the controller and the motor-compressor shall be permitted to be based on 72 percent of either the motor-compressor rated-load current or the branch-circuit selection current, whichever is greater.
FPN: The individual motor circuit conductors of wye-start, delta-run connected motor-compressors carry 58 percent of the rated load current. The multiplier of 72 percent is obtained by multiplying 58 percent by 1.25.

 

[allenwayne]

Re: Min Ckt Amp also min breaker size?

i thought that this thread was about SFR single phase installations.Not 3 phase eiter delta/wye connections.Wire fpr min. amp/ fuse to max breaker size and permit gets signed off
You can argue that the min. is 30 amps and the max is 50 but you have a 40 installed wrong !!!!!! Fuse to max breaker listed by manufacturer and supply within guidelines of min no inspector can tag you on this install.
Go lower than max breaker size and you are begging to get tagged

 

[iwire]

Re: Min Ckt Amp also min breaker size?

Originally posted by ryan_618:
Hmmm....wait a minute, maybe it is 125%

440.32 Single Motor-Compressor.
Branch-circuit conductors supplying a single motor-compressor ..............

Ryan I highly doubt that section is relevant to the opening post of this thread.

I recently installed an A/C system with nameplate rating of
Max Fuse/Bkr 60
Min Ckt Amp 52.1

Breaker panel has 50 amp circuit breaker.

Inspector failed inspection, saying the breaker was too small. Is the "Min Ckt Amp" rating only for wire sizing or does it also serve as a "Min Bkr" rating?

Where does the NEC address this?

We do not have to add 15% or 25% to the MCA on a multi motor unit labeled as shown above.

I would be willing to bet very few of wire Single Motor-Compressors.

 

[infinity]

Re: Min Ckt Amp also min breaker size?

Go lower than max breaker size and you are begging to get tagged

Where in the NEC does it say that you must protect at the maximum listed OCP on an AC unit?

 

[jwelectric]

Re: Min Ckt Amp also min breaker size?

Originally posted by infinity:

Go lower than max breaker size and you are begging to get tagged

Where in the NEC does it say that you must protect at the maximum listed OCP on an AC unit?

I don?t think that anyone here can find that statement in any code cycle.

The only section that I can find that would remotely come close would be

440.21 (B) Rating or Setting for Equipment. The equipment branch-circuit short-circuit and ground-fault protective device shall be capable of carrying the starting current of the equipment. Where the hermetic refrigerant motor-compressor is the only load on the circuit, the protection shall conform with 440.22(A).

If power hasn?t been turned on yet I don?t see this section being used either.