HVAC Max OCP

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augie47

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Location
Tennessee
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State Electrical Inspector (Retired)
A fellow inspector just asked me a question for which I had no answer.
Our SOP has been to use the manufacturer nameplate and confirm that a package heat-pump unit has the correct wiring and overcurrent protection based on nameplate data.
I've done this for years without questioning until he presented this interesting viewpoint.
A typical unit in our area may list a compressor with no heat installed as
requiring min ckt amps of 12 and a MaxOCP of 40. In that case that is our requirement.
An identical unit may have 9.6 kw of heat added and a nameplate that allows for "single point" connection. The MCA increases to 57 and the OCP to 60.
Again we accept it.
I understand that the compressor has OL protection, but why is it in 1 case the compressor must have a MOCP of 40, yet the same compressor, with the heat strips installed can have a MCOP of 60?
(There is no supplemental protection shown on the wiring for the units)
 

augie47

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Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Would it be because there are separate OCPDs in the unit itself, one for the motor and one for the heat strips?
When I open the units, I do not find seperate OCP and I don't see a requirement on the diagram.
I have not seen an install manual.
 

480sparky

Senior Member
Location
Iowegia
When I open the units, I do not find seperate OCP and I don't see a requirement on the diagram.
I have not seen an install manual.

The ones I've seen installed, there are two (sometimes three, if there's a 2-stage heat strip system) breakers installed at the factory. So I just basically wire them up as one branch circuit, and the breakers in the unit protect the various components.

There's no need to open the unit, the breakers are installed and visible through a factory-made opening. I'll see if I can find a pix of one.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I've seen those, especially, as you say, with more than 1 stage of heat.
These units definitely don't have provisions for internal breakers.
Fuse kits possibly, but I've seen no requirement for such, hence the question.
 

480sparky

Senior Member
Location
Iowegia
I just ran through my archive disk quickly and scanned the jobs where a geothermal system was installed, and couldn't find a pix of what I know I've seen.

I guess it boils down to something that is outside our scope of work. Being a complete unit, it falls under NEMA and listing rules, not the NEC. As long as the nameplate says so-and-so, that's what we install. But it's an intriquing question.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
A typical unit in our area may list a compressor with no heat installed as requiring min ckt amps of 12 and a MaxOCP of 40. In that case that is our requirement.
That's quite the range: #14 wire and a 40a breaker.

I understand that the compressor has OL protection, but why is it in 1 case the compressor must have a MOCP of 40, yet the same compressor, with the heat strips installed can have a MCOP of 60?
Maybe the heat and compressor cannot run simultaneously.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I think the OP said it was a heat pump.
He also mentioned adding 9.6kw of heat strips, so I took this to be a unit package that does heating and cooling, and contains the blower and ducting, and uses no separate air handler.
 

augie47

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Location
Tennessee
Occupation
State Electrical Inspector (Retired)
That's quite the range: #14 wire and a 40a breaker.

Maybe the heat and compressor cannot run simultaneously.

The numbers may not be exact. They were close enough to use for my illustration of the "problem". I've found with HVAC 14 on a 30 is very common.


I think the OP said it was a heat pump.

These are heat pumps. Heat strips can be on with the compressor but obviously not necessarily so. That's why it's questionable to me. If we are protecting the compressor under two circuit or cooling mode a 40 amp breaker is necessary, but with single connection we are allowed a 60.
 

elohr46

Senior Member
Location
square one
He also mentioned adding 9.6kw of heat strips, so I took this to be a unit package that does heating and cooling and contains the blower and ducting, and uses no separate air handler.

yes, but that's what a heat pump does, with a compressor. It's still a heat pump with or without the additional back up heat.
 
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LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
yes, but that's what a heat pump does, with a compressor. It's still a heat pump with or without the additional back up heat.
True, but the compressor is outside, the air handler and strips are inside, and they're usually supplied by two separate circuits.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Gus, this is the same problem with a gas pack. You wire it for the ability to run the a/c but when the heat is on then you can have a 50 amp breaker protecting the blower and gas controls on the unit.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Gus, this is the same problem with a gas pack. You wire it for the ability to run the a/c but when the heat is on then you can have a 50 amp breaker protecting the blower and gas controls on the unit.
To me, that's the manufacturer's responsibility; yours is to follow the labeling.

Our OCP (usually) protects our conductors, not necessarily their assemblies.
 

benaround

Senior Member
Location
Arizona
If the question is just for the compressor, 440.51 and 440.52 provide answers. For the

circuit in general 440.35.

In your example of the 57a MCA and 60a MOCP, that would be a normal circuit. Compare

that to, for example, a range circuit of 40a or 50a that has a 120v clock. That may not be

such a good comparison but you know what I'm getting at.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
...
I understand that the compressor has OL protection, but why is it in 1 case the compressor must have a MOCP of 40, yet the same compressor, with the heat strips installed can have a MCOP of 60? ...
The OCPD is only providing short circuit and ground fault protection. There will be very little difference in the trip points of the 40 and 60 amp breakers under short circuit or ground fault conditions. The compressor will have the same protection with either breaker, and the larger breaker will permit the heat to run with out tripping the breaker.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
True, and I understand that. It just sorta seems stupid that I say, sorry this compressor won't meet Code on that 50 amp breaker (the unit w/o heat) then turn around and say, o.k. this passes on the 60 amp (same compressor)
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
It just sorta seems stupid that I say, sorry this compressor won't meet Code on that 50 amp breaker (the unit w/o heat) then turn around and say, o.k. this passes on the 60 amp (same compressor)
That's like the #6 EGC required for a #6 circuit on a 30a breaker that can be #10 when on a 60a breaker.
 
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