Breaker Sizing and HVAC MCA

[kyleapec]

I apologize for stepping out so long been busy with the wife and kids for the holidays. I appreciate all of the input. All of the wire sizes for these VAV'S will be quite oversized (I learned something new on this project about having to assume 60 degrees when when the temp rating of the equipment is unknown) so were fine on the ampacitys (#8 for the example used) . I wrote them a change order because I assumed that the OCPD needed to be at least larger than MCA, they did not want to pay it and said to leave it. It definitely is not the way I would like to build a building but I wanted to get some input from you guys. Back to post #7 from INIFNITY.

"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?"

Do you have any code references towards that? I assumed that was the case, but i couldn't find anything in the code saying that. Looking at post #14 from Little Bill, I believe that is exactly what the engineer is saying, but I still think it's not right (maybe a 110.12 violation ) Thanks again for all the input guys.

 

[LarryFine]

To me,it's not a code issue, it's a starting-current issue.

 

[kwired]

I apologize for stepping out so long been busy with the wife and kids for the holidays. I appreciate all of the input. All of the wire sizes for these VAV'S will be quite oversized (I learned something new on this project about having to assume 60 degrees when when the temp rating of the equipment is unknown) so were fine on the ampacitys (#8 for the example used) . I wrote them a change order because I assumed that the OCPD needed to be at least larger than MCA, they did not want to pay it and said to leave it. It definitely is not the way I would like to build a building but I wanted to get some input from you guys. Back to post #7 from INIFNITY.

"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?"

Do you have any code references towards that? I assumed that was the case, but i couldn't find anything in the code saying that. Looking at post #14 from Little Bill, I believe that is exactly what the engineer is saying, but I still think it's not right (maybe a 110.12 violation ) Thanks again for all the input guys.

Since this is an appliance with a marked MCA, it technically may not be a code violation. If it were only marked in volts-amps and you needed to use NEC to determine minimum conductor and minimum overcurrent protection they would both be the same minimum value and the overcurrent protection would have allowances to go higher if needed to start motors.

 

[bwat]

I must be missing a key point because I’m not seeing how it isn’t a code violation if the breaker isn’t at least at MCA.

MCA is Minimum Circuit Ampacity and the rating of a branch circuit is determined by the ocpd. So having a breaker less than MCA seems like a violation of manufacturers instructions and thus NEC. 110.3 or similar if I remember correctly.

This seems to go against what a lot of you are saying, so where am I wrong?

 

[david luchini]

I must be missing a key point because I’m not seeing how it isn’t a code violation if the breaker isn’t at least at MCA.

MCA is Minimum Circuit Ampacity and the rating of a branch circuit is determined by the ocpd. So having a breaker less than MCA seems like a violation of manufacturers instructions and thus NEC. 110.3 or similar if I remember correctly.

This seems to go against what a lot of you are saying, so where am I wrong?

The load on the branch circuit is the MCA, per 220.3.

220.18 says that the load shall not exceed the rating of the branch circuit.

 

[bwat]

The load on the branch circuit is the MCA, per 220.3.

220.18 says that the load shall not exceed the rating of the branch circuit.

Right, also a good point. So you’re in agreement with me that it is a violation, and is easily citable.

There were multiple posts here that seemed to suggest that it might not be, so I was wondering what I was missing.

 

[Jraef]

MCA or Minimum Circuit Ampacity is not actually defined in the NEC, but it is referenced in several places as being per 430.7(D), which reads:

(D) Multimotor and Combination-Load Equipment.
(1) Factory-Wired. Multimotor and combination-load
equipment shall be provided with a visible nameplate
marked with the manufacturer's name, the rating in volts,
frequency, number of phases, minimum supply circuit conductor
ampacity, and the maximum ampere rating of the
circuit short-circuit and ground-fault protective device.

So how can you have an MCA of 36.9A and a 35A breaker? Easy. You size the CONDUCTORS for 36.9A and if someone wants a 35A breaker on that, it is PERFECTLY legal, because it is not OVER the Maximum OCP value of 40A.

Might it nuisance trip? Yes. Is that your problem to solve up front? Not in my opinion if you have been DIRECTED by an engineer to install a 35A breaker. So if it DOES nuisance trip later, you get a change order. Ka-Ching!

 

[bwat]

MCA or Minimum Circuit Ampacity is not actually defined in the NEC, but it is referenced in several places as being per 430.7(D), which reads:



So how can you have an MCA of 36.9A and a 35A breaker? Easy. You size the CONDUCTORS for 36.9A and if someone wants a 35A breaker on that, it is PERFECTLY legal, because it is not OVER the Maximum OCP value of 40A.

Might it nuisance trip? Yes. Is that your problem to solve up front? Not in my opinion if you have been DIRECTED by an engineer to install a 35A breaker. So if it DOES nuisance trip later, you get a change order. Ka-Ching!

In my first post (#24) I bolded the word Circuit in the term MCA. I had a suspicion that this was where I was seeing this differently. Your point is that MCA isn’t defined in NEC but where it is referenced you’re saying that it’s done in a way that makes it about the conductors and only the conductors, yes?

But I’ve taken MCA to mean the minimum conductors and ocpd. It just seemed to make more sense since it’s the word circuit and not conductor, and since it’s not defined I have to take it at face value. And I’d like to point out that using MCA this way is still meeting your cited 430.7 but it’s also specifying the min ocpd with the same number.

If someone were to simply ask “Does a 35A breaker meet the minimum circuit ampacity of 36.9?” I would say no, regardless of wire size.

Using the interpretation that MCA is only about conductors then you could put a 5A breaker on this and it not be a violation. I do get that the code isn’t there to protect against a terrible design... but there are multiple places where it is a code requirement for the breaker to carry the load.

I’m willing to be wrong here.

 

[Little Bill]

I'm not an engineer but I'll make an attempt at explaining this. With motors and refrigeration equipment, OCP and conductor sizing are handled different than normal chapter 3 wiring methods. The OCPD in this circumstance is not for overcurrent, but only for short circuit since the motors have their own protection for overloads. The wires/conductors are sized to the MCA. The OCPD is represented by Max OCP, either fuse or breaker, unless fuse only is specified. So you often find something like, MCA: 19.8 Max OCPD: 40A, there you could have #12 conductors on a 40A breaker or fuse. The nameplate is based on the mfg already figuring in the the circuit size based on the rules for motors, such as 125% of FLA. The Max OCPD is just that, the max size allowed. You can certainly go smaller but you risk nuisance tripping.
Maybe this will help, or muddy it up more!

 

[bwat]

I'm not an engineer but I'll make an attempt at explaining this. With motors and refrigeration equipment, OCP and conductor sizing are handled different than normal chapter 3 wiring methods. The OCPD in this circumstance is not for overcurrent, but only for short circuit since the motors have their own protection for overloads. The wires/conductors are sized to the MCA. The OCPD is represented by Max OCP, either fuse or breaker, unless fuse only is specified. So you often find something like, MCA: 19.8 Max OCPD: 40A, there you could have #12 conductors on a 40A breaker or fuse. The nameplate is based on the mfg already figuring in the the circuit size based on the rules for motors, such as 125% of FLA. The Max OCPD is just that, the max size allowed. You can certainly go smaller but you risk nuisance tripping.
Maybe this will help, or muddy it up more!

I appreciate the explanation and agree with it if MCA is really only meant for min conductor sizing, but I think that's where I'm diverging. It has been my understanding (perhaps wrong) that MCA is a minimum requirement for the circuit, not just the conductors.

Similar to what David said in #25 that the MCA can be viewed as the load and it can't exceed the rating of the circuit. I viewed MCA as the manufacturer already did the 125% calcs for largest motor, number of motors, etc.. and stated that the equipment needs a circuit of at least the MCA.

And relating this to a motor circuit seems to help my case. Even if I weren't to bring up that article 210's requirement of the ocpd to be at least 125% of a continuous load, in 430.52(B) it states that the SC and GF device must be capable of carrying the starting current. Yes the overload provides the thermal overload protection, which allows the SC and GF device to be largely increased from what we'd typically see, but I don't think this means that there isn't a minimum for a motor circuit breaker serving as the SC and GF protection. Or am I also wrong here? Is it perfectly legal per NEC to put a 5A breaker on a 30HP motor? (3ph, 460v)


I do appreciate everyone entertaining my clarifications and questions here. This may be a little off in the weeds, but I'd like to get this straight for myself and the OP.

 

[Jraef]

As I said, the term "minimum circuit ampacity" is not officially defined in article 100 of the NEC, so how YOU choose to interpret it is your own business; everyone is entitled to their opinion. But if you want to know what MCA means for CODE COMPLIANCE, then you cannot use your personal interpretations, you use what the Code actually says.

Here are ALL references to the specific term "minimum circuit ampacity" in the NEC, starting first with Article 440 because your situation is in fact, Air Conditioning and Refrigeration Equipment (the title of Art. 440).

440.35 Multimotor and Combination-Load Equipment.
The ampacity of the conductors supplying multimotor and
combination-load equipment shall not be less than the minimum
circuit ampacity marked on the equipment in accordance
with 440.4(B).

430.25 Multimotor and Combination-Load Equipment.
The ampacity of the conductors supplying multimotor and
combination-load equipment shall not be less than the minimum
circuit ampacity marked on the equipment in accordance
with 430.7(D). Where the equipment is not factory wired
and the individual nameplates are visible in
accordance with 430.7(D)(2), the conductor ampacity shall
be determined in accordance with 430.24.

That's it.

Then because it is referenced in 440.35, I'll include some more:

440.4 Marking on Hermetic Refrigerant Motor Compressors
and Equipment. ...
(B) Multimotor and Combination-Load Equipment.
Multimotor and combination-load equipment shall be provided
with a visible nameplate marked with the maker's name,
the rating in volts, frequency and number of phases, mini-
mum supply circuit conductor ampacity, the maximum rating
of the branch-circuit short-circuit and ground-fault protective
device, and the short-circuit current rating of the
motor controllers or industrial control panel. The ampacity
shall be calculated by using Part IV and counting all the
motors and other loads that will be operated at the same
time. The branch-circuit short-circuit and ground-fault protective
device rating shall not exceed the value calculated
by using Part 111. Multimotor or combination-load equipment
for use on two or more circuits shall be marked with
the above information for each circuit. ...

(exceptions left off)

So ALL references to "minimum circuit ampacity" in the NEC refer ONLY to the conductor ratings, NOT the OCPD, because that is a SEPARATE requirement.

 

[bwat]

I do not mean to make it about my opinion. The meaning of MCA is what I was saying is important. If by MCA, the manufacturer is specifying a minimum ocpd in addition to minimum conductor, then it’s a violation. That’s not an opinion. Additionally, a manufacturer’s instruction term doesn’t have to be explicitly defined in NEC to make it a violation. For example, even if the 430 and 440 sections you cited didn’t exist, supplying conductors less than MCA is a violation of 110. So by the same logic just because there isn’t an NEC section that explicitly states the ocpd must be at least the MCA, it’s a violation of 110 if (and it’s the big IF) the MCA is meant to specify min ocpd as well.

All that said, I will concede defeat here and let this go. I came into this conversation thinking it was very straight forward as a violation. I still tend to see it as a violation of mfr instructions and therefor NEC, but I at least now see that there’s some gray area and it might not be a violation. It seems very possible that MCA is a mfr specification for conductor size only, and therefore would not be a violation.

Jraef, thank you for your detailed responses. I’ve read thousands of your posts over the years and have learned much from you; especially with motors and vfds.

 

[david luchini]

All that said, I will concede defeat here and let this go.

Why "concede defeat" when you are correct. The load on the branch circuit cannot be larger than the rating of the branch circuit.

 

[LarryFine]

Why "concede defeat" when you are correct. The load on the branch circuit cannot be larger than the rating of the branch circuit.

I guess the debate is whether the OCPD is included in "the rating of the branch circuit."

What's the rating of a circuit wired with #6 cu supplied by a 15a breaker?

 

[david luchini]

I guess the debate is whether the OCPD is included in "the rating of the branch circuit."

What's the rating of a circuit wired with #6 cu supplied by a 15a breaker?

15A

 

[bwat]

The question, for me at least, was whether the manufacturer is specifying a circuit rated at MCA or just the conductors. As David pointed out, and as I did earlier, the rating of a branch circuit is derived from the ocpd. That’s from somewhere early in 210 I believe, and there’s no debate on that. This is why I was originally so confident that it was a violation because of the use of the word circuit in MCA.



If it were my call, I would still say it’s a violation, but I can now see that there’s a decent argument to be made that it isn’t. So I’m not conceding to the point of saying it’s perfectly legal, I’m conceding to say that it’s at least debatable and not a clear cut violation (which is where I started).



David, how would you go about proving that it’s a violation because the ocpd is less than load? I’d like to hear how you can make a solid case for this. I tend to think that is what’s happening also, but there are some obstacles for me to state this. A couple would be:

• Showing that MCA is the load
• If MCA is the load, is it 125% of it?

 

[LarryFine]

Is there nothing in the NEC that says a circuit, including the OCPD, must be capable of supplying its intended load?

 

[david luchini]

David, how would you go about proving that it’s a violation because the ocpd is less than load? I’d like to hear how you can make a solid case for this. I tend to think that is what’s happening also, but there are some obstacles for me to state this. A couple would be:

• Showing that MCA is the load
• If MCA is the load, is it 125% of it?

220.3 says the load for an art 440 piece of equipment is per Art 440 part IV...Art 440 part IV tells you how to calculate the MCA....MCA=branch circuit load (not 125% of it).

And we've already seen that the load can't exceed the branch circuit rating (220.18)

 

[david luchini]

Is there nothing in the NEC that says a circuit, including the OCPD, must be capable of supplying its intended load?

See 220.18

 

[LarryFine]

Then the OP's engineer's instructions are non-NEC-compliant.