Sizing Breaker for Air Handling Unit

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new_ee

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For sizing breaker for an air handling unit I was using the HP to find FLA and then using FLA*2.5 as stated in 430.52 for inverse time circuit breaker. Is this correct?

Now for A/C equipment with a compressor motor I take the largest motor FLA * 1.75 and add remaining motor FLA to get the breaker size. This was based on section 440.22 Is this correct?

I'm not sure what to do about an air handling unit that also has electric heat. The electric heat load is much larger than the fan motor load. Any hints?
 
Re: Sizing Breaker for Air Handling Unit

This equipment should have on the name plate the minimum circuit capacity and the max overcurrent device as listed by the mfg. There figures are what you are to use.
 
Re: Sizing Breaker for Air Handling Unit

Air Handling Unit has a supply & a return fan. first, size breaker for largest motor, then add the rest, NEC 430.62(A). Example: supply fan is 7-1/2 hp (11 FLA, 480V), return fan is 5hp (7.6 FLA), so breaker size for both:

11 x 2.5 = 27.5 next larger size is 30 amp breaker. Now Add 7.6 FLA + 30 amp = 37.6 amp, select next lower rating, then you end up with 35 amp breaker. For practical purposes, 30 breaker will also work.
 
Re: Sizing Breaker for Air Handling Unit

I also agree with Bob, you can use whatever the manufacturer has sugeested.
 
Re: Sizing Breaker for Air Handling Unit

what section of "the" codes says you must use manufacturers rating?
 
Re: Sizing Breaker for Air Handling Unit

440.6 Ampacity and Rating.
The size of conductors for equipment covered by this article shall be selected from Table 310.16 through Table 310.19 or calculated in accordance with 310.15 as applicable. The required ampacity of conductors and rating of equipment shall be determined according to 440.6(A) and 440.6(B).
(A) Hermetic Refrigerant Motor-Compressor. For a hermetic refrigerant motor-compressor, the rated-load current marked on the nameplate of the equipment in which the motor-compressor is employed shall be used in determining the rating or ampacity of the disconnecting means, the branch-circuit conductors, the controller, the branch-circuit short-circuit and ground-fault protection, and the separate motor overload protection. Where no rated-load current is shown on the equipment nameplate, the rated-load current shown on the compressor nameplate shall be used.
:)
 
Re: Sizing Breaker for Air Handling Unit

got it, thanks! that takes care of the a/c units. but what about the air handling unit?
 
Re: Sizing Breaker for Air Handling Unit

440.6(B) refers you to 440 Part III for combination load equipment branch circuit sizing. It does not restrict you to the manufacturer's nameplate information.

440.22(B)(1) or (2) is applicable depending on the size of the compressor versus the heater. 440.22(C) is only applicable if overload relay tables have been provided.
 
Re: Sizing Breaker for Air Handling Unit

The reason thier is no nameplate info for the heating unit is because the determination of the number and size of the grids is detemined in the field. Thats why you need to have field info to determine your load charictaristics. Usually the fan on the handler is very small of course in comparison to the heat strips. We often install circuits for thes combo units and most of the manufacturers have a chart in thier infor that will give you wire size for various grid configurations.We run in toi split systems that you can either use on large supply wire to feed the internal breakers ( typically a 30 and a 60) or use 2 separate circuits to feed which is what we do. We just feed each beraker with the corresponding table requirement.. Hope this helps somewhat Joe
 
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