3 phase breaker question with "2" phase loads

[SparksBFlying]

I came across a 3 phase 208 breaker feeding 6) 208V single phase loads. Each load is tapped to 2 phases of the 3 phase breaker. The 6 loads are generally distributed evenly across the phases. There is a 8/3 MC pulled to a Jbox. From there, 12-2 MC is distributed to each HVAC units

1) Can a 3 phase breaker be tapped like this (rather than using multiple double pole breakers and another wire out to loads) Theoretically the branches will not see the entire load of the circuit, only their individual draws. Isn’t there a tap rule for this situation? I haven’t found anything yet

2) Can the 12/2 taps be in a circuit fused at 40 amps, without additional step down fusing to 20 amp?

3) Since there is a fractional HP fan motor, do I also need to simultaneously disconnect both phases on overload, or will the manufacturers safeties handle that?

Thanks

 

[Little Bill]

I came across a 3 phase 208 breaker feeding 6) 208V single phase loads. Each load is tapped to 2 phases of the 3 phase breaker. The 6 loads are generally distributed evenly across the phases. There is a 8/3 MC pulled to a Jbox. From there, 12-2 MC is distributed to each HVAC units

1) Can a 3 phase breaker be tapped like this (rather than using multiple double pole breakers and another wire out to loads) Theoretically the branches will not see the entire load of the circuit, only their individual draws. Isn’t there a tap rule for this situation? I haven’t found anything yet

2) Can the 12/2 taps be in a circuit fused at 40 amps, without additional step down fusing to 20 amp?

3) Since there is a fractional HP fan motor, do I also need to simultaneously disconnect both phases on overload, or will the manufacturers safeties handle that?

Thanks

Would you please update your profile and list your occupation.

 

[SparksBFlying]

Done

 

[Little Bill]

Done

Thank you!

 

[steve66]

So its a 40 amp breaker?

I think we need to know what the HVAC labels say. Minimum circuit ampacity, and maximum overcurrent protection?

Edit; Thinking about it some more, it might be a violation no matter what the labels say. I'll let others chime in.

 

[ptonsparky]

So its a 40 amp breaker?

I think we need to know what the HVAC labels say. Minimum circuit ampacity, and maximum overcurrent protection?

Edit; Thinking about it some more, it might be a violation no matter what the labels say. I'll let others chime in.

I tried to think of a way it might be acceptable but gave up. Then again, I already didn't like it.

 

[wwhitney]

If the HVAC unit label says MOCP 40A (or higher), MCA 20A (or less), what would the violation be? Although maybe that's an unlikely combination.

Cheers, Wayne

 

[ptonsparky]

If the HVAC unit label says MOCP 40A (or higher), MCA 20A (or less), what would the violation be? Although maybe that's an unlikely combination.

Cheers, Wayne

Where are our HVAC guys?
I thought it kind of a stretch myself.

 

[steve66]

If the HVAC unit label says MOCP 40A (or higher), MCA 20A (or less), what would the violation be? Although maybe that's an unlikely combination.

Cheers, Wayne

Could this be an application where #12 is allowed to be protected at 25 amps? I don't remember the "small conductor" rules of the top of my head, but 40 MOCP & 25 MCA might not be too much of a stretch for the equipment label.

But now we have multiple 25 Amp loads on one pole of a 40A breaker (3 of them to be exact).

So I agree - no way this could be code compliant. Since the minimum circuit ampacity puts a minimum size on the circuit breaker too, now the breaker is too small to supply the 3 loads.

 

[wwhitney]

Could this be an application where #12 is allowed to be protected at 25 amps? I don't remember the "small conductor" rules of the top of my head, but 40 MOCP & 25 MCA might not be too much of a stretch for the equipment label.

Good point, I forgot about that. HVAC equipment is an exception to 240.4(D), so the 25A ampacity of the 75C #12 Cu conductor is not restricted.

But now we have multiple 25 Amp loads on one pole of a 40A breaker (3 of them to be exact).

4, actually, and I did my math wrong.

I'm less familiar with Article 440, so let's just look at it as an Article 430 application (motors only). So treat the eqiupment like a big motor, with FLA X. Then at FLA, the current on phase A from one unit A-B and one unit A-C is sqrt(3) * X. And we have two sets of 3, so the current on phase A would be 2 * sqrt(3) X. Then we need to add another 25% of the largest FLA, which brings us to (2*sqrt(3) + 0.25) * X as the minimum feeder ampacity.

#8 Cu MC cable has a 75C ampacity of 50A. So if it's not undersized, we have 50 >= (2*sqrt(3) + 0.25) * X, which means X <= 13.5A. The unit would be labeled with an MCA of 1.25 * X = 16.8A at most. Then the OCPD would be limited to 2.5*X = 33.7A, which can be rounded up to 35A per 430.52(C)(1) Exception 1.

So the upshot, I think, is that if the #8 Cu is big enough for the feeder ampacity, then the 40A is too big for SCGF protection, if the equipment were motors instead of HVAC, and if 430.52(C)(1) Exception 2 doesn't apply. Of course maybe the Article 440 rules are a bit different and it just squeaks by somehow.

Cheers, Wayne