Magic wire or blatant violation?

[s-park-er]

So I have been helping a friend of mine with a commercial historic building, rewired by a local contractor with a very good reputation. While troubleshooting some dead receptacles, I found this: Similar on the opposing side but both are 50A. Those are 10AWG solid in MC cable.
They are supplying four HVAC units (large ducted mini-split type, so both OSU and air handler).
I told the owner this is a problem that he should hold the contractor to. Owner told me that contractor says this is correct.
Where - outside of Never Never Land - would this be appropriate?

 

[ActionDave]

It's totally appropriate, necessary, and safe. It's done all the time for air conditioning equipment and other loads that involve motors. 240.4(G) is the code reference.

 

[zbang]

First thing I twigged on is the color- is there a specific code provision that makes the white wire acceptable as an ungrorunded/phase conductor? I'd expect at least a little black or red tape on them. (Heck, even some black magic marker....)

 

[s-park-er]

It's totally appropriate, necessary, and safe. It's done all the time for air conditioning equipment and other loads that involve motors. 240.4(G) is the code reference.

So I see that 240.4(G) does in fact mention HVAC equipment, but I don't see where I can get a table or other reference to the gauge allowed for amperages. What table would it be in 310?
I just can't wrap my head around a 60A breaker protecting a 10 gauge wire in an historic building.

 

[Little Bill]

So I see that 240.4(G) does in fact mention HVAC equipment, but I don't see where I can get a table or other reference to the gauge allowed for amperages. What table would it be in 310?
I just can't wrap my head around a 60A breaker protecting a 10 gauge wire in an historic building.

The motors on the HVAC equipment have overload protection that takes care of overloads/overcurrent. The breakers are for short circuit protection and can be oversized as opposed to normal breaker-to-wire protection. The motors need a larger breaker to allow the motors to start and not trip on in-rush overload. The nameplates on the equipment will have the info as how to size the breaker or fuse and the wire/conductors. The plate will have "MCA" and max breaker or fuse. The "MCA" stands for minimum circuit ampacity which is for the conductors/wires, max breaker/fuse is self explanatory.

 

[s-park-er]

The motors on the HVAC equipment have overload protection that takes care of overloads/overcurrent. The breakers are for short circuit protection and can be oversized as opposed to normal breaker-to-wire protection. The motors need a larger breaker to allow the motors to start and not trip on in-rush overload. The nameplates on the equipment will have the info as how to size the breaker or fuse and the wire/conductors. The plate will have "MCA" and max breaker or fuse. The "MCA" stands for minimum circuit ampacity which is for the conductors/wires, max breaker/fuse is self explanatory.

I understand the need for inrush on most motor/compressor circuits, and regarding the internal protections afforded. In this case, inrush is moot because these are all inverter systems with low-start current. unit protections are digitally monitored by the control board.
In the real-world these units will seldom, if ever draw anywhere near the full current rating of the breaker. I get all that.

The area I am confounded by is what if a short develops at the OSU conduit entrance, inverter circuit power supply board or disconnect? What protects the wires then?

 

[James L]

In this case, inrush is moot because these are all inverter systems with low-start current. unit protections are digitally monitored by the control board.
In the real-world.....

Have you verified your hunch by referencing the nameplate data?

Pardon my candid response, but it seems you need to climb down off your high horse (and hobby horse).

You erroneously appointed yourself as corrector-in-chief, took a fictitious problem to the property owner, didn't like the (most likely) correct answer given to you from who you admit to be a very reputable contractor, asked a forum full of experts, and have challenged two of those answers - on the sole basis that you can't wrap your head around it

 

[Dsg319]

I understand the need for inrush on most motor/compressor circuits, and regarding the internal protections afforded. In this case, inrush is moot because these are all inverter systems with low-start current. unit protections are digitally monitored by the control board.
In the real-world these units will seldom, if ever draw anywhere near the full current rating of the breaker. I get all that.

The area I am confounded by is what if a short develops at the OSU conduit entrance, inverter circuit power supply board or disconnect? What protects the wires then?

The breaker protects the wire. The manufacture is not going to have a Conductor to small for the allowable max circuit breaker.

Just trust the name plate, they have designed and tested it I am sure.

 

[infinity]

Until you provide the nameplate data there is not much to say other than we're all speculating as to whether or not this is code compliant. Here's my speculation, #10 AWG on a 60 amp OCPD is not going to be within the sizes on the nameplate.

 

[augie47]

I agree that, without nameplate data, there is no substantiation of your claim (other than conductor identification). As infinity suggests, the particular breakers might be somewaht oversized but the practice of matching breakers and wire sizes to HVAC nameplate per Article 440 and 240..4(G) is standard practice. If there is indeed a "short", a 50 or 60 amp breaker will adequately protect a #10.

 

[Gene B]

First thing I twigged on is the color- is there a specific code provision that makes the white wire acceptable as an ungrorunded/phase conductor? I'd expect at least a little black or red tape on them. (Heck, even some black magic marker....)

Those are the colors you get with 2 conductor cable, no choice. Yes, it's supposed to be marked in some way.

 

[roger]

Here's an example of using small conductors for motor loads

Using # 12 conductors

Table 430.148 gives a 3HP motor an full load current of 17 amps
430.22(A) requires conductors be sized at 125% of full load current
17 x 125% = 21.25 amps
Table 310.16, #12 THHN 75 deg C column = 25 amps
Table 430.52 allows up to 250% for the inverse time breaker
17 x 250% = 42.5 amps
240.6 states we can go UP to the next STANDARD size breaker
you can use a 45 amp breaker and # 12 conductors

 

[s-park-er]

Here's an example of using small conductors for motor loads

Using # 12 conductors

Table 430.148 gives a 3HP motor an full load current of 17 amps
430.22(A) requires conductors be sized at 125% of full load current
17 x 125% = 21.25 amps
Table 310.16, #12 THHN 75 deg C column = 25 amps
Table 430.52 allows up to 250% for the inverse time breaker
17 x 250% = 42.5 amps
240.6 states we can go UP to the next STANDARD size breaker
you can use a 45 amp breaker and # 12 conductors

Thank you so much for your kind, informative and professional reply @roger! This is the kind of help I am used to here.

So in my application, I have a Minimum Circuit Amps rating of 35A (according to the manufacturers installation data)
35*1.25=43.75
310.16 for THHN shows #10 @ 40A, therefore #8 would be required.
Manufacturer's data also indicates Max OCP is 50A
Unless I am missing something, my instincts were correct.

 

[david luchini]

So in my application, I have a Minimum Circuit Amps rating of 35A (according to the manufacturers installation data)
35*1.25=43.75
310.16 for THHN shows #10 @ 40A, therefore #8 would be required.
Manufacturer's data also indicates Max OCP is 50A
Unless I am missing something, my instincts were correct.

Not 35*1.25....

MCA is just 35. #10 has a 75deg ampacity of 35, so #10 would be sufficient (assuming no adjustment or correction factors needed.)

 

[retirede]

Thank you so much for your kind, informative and professional reply @roger! This is the kind of help I am used to here.

So in my application, I have a Minimum Circuit Amps rating of 35A (according to the manufacturers installation data)
35*1.25=43.75
310.16 for THHN shows #10 @ 40A, therefore #8 would be required.
Manufacturer's data also indicates Max OCP is 50A
Unless I am missing something, my instincts were correct.

There is no need to multiply the MCA by 1.25.
#10 wire is fine, but the 60A breaker is wrong.

 

[infinity]

MC cable for this application can be sized according to the 75° C ampacity which is 35 amps so the wire size with a 50 amp OCPD is correct. A 60 amp OCPD is not correct.

 

[s-park-er]

You all are AWESOME, Thanks so much.

One last question - more opinion:
There is only one 4T unit; the other three are 3T. MCA specs for those are 30A, and MOCP remains at 50.

Considering the historic nature of this building, would any of you choose smaller breakers for the three 3T units, just for added level of safety?
In my years with State parks, most of the work we did and the contract specifying we called out more than minimum for safety in electrical and plumbing (especially waste plumbing).

Thanks again

 

[petersonra]

It's totally appropriate, necessary, and safe. It's done all the time for air conditioning equipment and other loads that involve motors. 240.4(G) is the code reference.

I think maybe he was worried about the wire color coding. 240.4(g) does not allow white wire to be used as an ungrounded conductor.

I think there is some provision to use the white conductor of a cable assembly as a hot but it is supposed to be marked.

 

[infinity]

I think there is some provision to use the white conductor of a cable assembly as a hot but it is supposed to be marked.

That's correct. It must be within a cable assembly.

 

[don_resqcapt19]

I understand the need for inrush on most motor/compressor circuits, and regarding the internal protections afforded. In this case, inrush is moot because these are all inverter systems with low-start current. unit protections are digitally monitored by the control board.
In the real-world these units will seldom, if ever draw anywhere near the full current rating of the breaker. I get all that.

The area I am confounded by is what if a short develops at the OSU conduit entrance, inverter circuit power supply board or disconnect? What protects the wires then?

The breaker on these types of circuits provides short-circuit and ground-fault protection for the conductors, and the overload device that is part of the equipment provides the overload protection. Short-circuits and ground-faults are high current events and operate the breaker in its magnetic range. For a high current event, there will be little difference in the time to trip between a 30 amp and a 60 amp, or even a 100 amp breaker.