Breaker Sizing and HVAC MCA

[kyleapec]

I am working on a job where the submittals on multiple HVAC equipment is different than the HVAC equipment schedule. As an example on a VAV we have an MCA of 36.90A and an MOP of 40A, the breaker shown by the engineer on the panel schedules are 35A breakers. I am worried about nuisance tripping, my understanding is that it is not a code violation. Though I do think it is a suboptimal design. The mechanical engineer is saying it is fine and to proceed, should I drop it or am I correct in my worries, thanks.

 

[LarryFine]

Create a paper trail of your mention and the response, then proceed as directed.

 

[infinity]

I am working on a job where the submittals on multiple HVAC equipment is different than the HVAC equipment schedule. As an example on a VAV we have an MCA of 36.90A and an MOP of 40A, the breaker shown by the engineer on the panel schedules are 35A breakers. I am worried about nuisance tripping, my understanding is that it is not a code violation. Though I do think it is a suboptimal design. The mechanical engineer is saying it is fine and to proceed, should I drop it or am I correct in my worries, thanks.

The minimum circuit ampacity is 36.9 amps so how is a 35 amp circuit code compliant?

 

[kyleapec]

My understanding was that the MCA deals with wire size, but that is why I came here. It seems very hokey to me, but I couldnt figure out if it was a code violation. I really don't like it, but I came here to see if I can get some ammo. Instead of coming back after the job is finished and costing everyone twice as much.

 

[LarryFine]

I really don't like it, but I came here to see if I can get some ammo. Instead of coming back after the job is finished and costing everyone twice as much.

That's why I responded as I did. If anything, it should make money for you, not cost you.

 

[ActionDave]

MOCP is approved by the NEC and is calculated by the manufacturer so there is no reason not to use it. There is also nothing in the code that says you have to use it.

How much does it matter to you? I'd tell whoever is in charge that it is a lousy design and let it go.

 

[infinity]

You do not need to use the MaxOCPD but you do need to provide a circuit that is at least as large as the MCA. Even though a 35 amp OCPD will most probably work it still isn't code compliant. What size conductors are you planning on using?

 

[Eddie702]

My guess would be that the HVAC company submitted on different equipment than that which was originally specd. I would install the breakers that the engineer sized and go for a change order. Their mistake shouldn't cost you a penny. Just document and record your objection. It's up to the engineer to fix this. Could be a wire change as well as far as you know.

Those vavs must have electric heat in them.

 

[StarCat]

I am working on a job where the submittals on multiple HVAC equipment is different than the HVAC equipment schedule. As an example on a VAV we have an MCA of 36.90A and an MOP of 40A, the breaker shown by the engineer on the panel schedules are 35A breakers. I am worried about nuisance tripping, my understanding is that it is not a code violation. Though I do think it is a suboptimal design. The mechanical engineer is saying it is fine and to proceed, should I drop it or am I correct in my worries, thanks.

VAV applications are generally what are also know as Fan powered boxes. They come in several general types. By your description I am assuming Electric Heat packs are standard. While Compressor loads are always best sized to MOCP, your VAV boxes may be fine at 35A.You are powering a heat pack, and a fan drive typically. The exact specs on the heat pack may be helpful to your decision.
While I have seen at least one 2 element heat pack trip on startup, its quite rare. I solved this one by seuqencing the second stage. This is with a factory breaker on the heat package. The exact starting characteristics of the Box in heat are at issue with respect to inrush. Often times VAVs have to prove airflow before the heat will start. This is to your advantage. Look for a static pressure switch in the control panel.

 

[powerpete69]

Tricky question. The good thing is that you are most likely using #8 wire in either case. The manufacturer is correct in their spec. The engineer is too lazy to update his drawings. I would confront the engineer one more time or talk to his boss. The Manufacturer recommends 40A breaker for this piece of equipment. The engineer likely has no idea of the internals of the piece of equipment. Or it's possible he specs this particular unit all the time with a 35A breaker and it works great, who knows. If for some reason the engineer doesn't bend, go ahead and install the 35A breaker. I would keep a 40A spare breaker handy or at least keep a distributor on standby nearby that can get you that 40A breaker in a hurry. You may not need it, but there is certainly a chance you may. In regards to your question, should you be concerned about this?....."maybe" is a pretty good answer.

 

[JoeStillman]

Just to muddy the waters a bit; VAV boxes are mainly electric heaters. If it's a 240V heater operating at 208V, it will only draw 75% of nameplate current (and only give 75% of the heat output). MOP stands for Maximum Overcurrent Protection, not minimum.

 

[infinity]

Am I missing something, from a code perspective how does the nameplate requirement of a circuit ampacity of 36.9 amp work on a 35 amp circuit?

 

[LarryFine]

Am I missing something, from a code perspective how does the nameplate requirement of a circuit ampacity of 36.9 amp work on a 35 amp circuit?

My guess is it's a mistake, and someone thought the breaker couldn't exceed the 36.9a, and that the MOC should be 50a.

 

[Little Bill]

All though probably not a good design, I think the only thing the code would be concerned with is:
Is the wire sized to the MCA or larger
Is the Max OCP of 40A being exceeded

 

[LarryFine]

Is the Max OCP of 40A being exceeded

No, just the opposite: they're spec'ing a 35a breaker.

 

[Little Bill]

No, just the opposite: they're spec'ing a 35a breaker.

The nameplate said Max 40A, that's what I meant by not exceeding the MOCP of 40A. What the engineer specs is not the nameplate rating.

 

[infinity]

The nameplate said Max 40A, that's what I meant by not exceeding the MOCP of 40A. What the engineer specs is not the nameplate rating.

The nameplate calls for a 36.9 MINIMUM circuit size that's is not possible with a 35 amp OCPD.

 

[Little Bill]

The nameplate calls for a 36.9 MINIMUM circuit size that's is not possible with a 35 amp OCPD.

See post #14

 

[infinity]

See post #14

Bill I don't follow. If a dishwasher nameplate says requires a 20 amp circuit how can you use a 15 amp circuit and be code compliant? Same applies here, a 35 amp circuit does not meet the nameplate minimum required by the manufacturer.

 

[Little Bill]

While we don't know the wire size the OP will use, the nameplate of "max" 40A is complied with using a 35A breaker as it does not exceed the 40A max. As long as the wire is sized to the MCA, it would also be compliant. It doesn't look right that the Engineer speced a 35A breaker but if you go by the nameplate of max 40A, that part is compliant.

ETA: I think the nameplate is also wrong as the MCA of 36.9 and MOCP of 40A is the same. Usually you have a larger MOCP than the MCA.