Parallel Circuits allowed?

E

elec_eng

Senior Member
During the site observation in one of my customers facilities, I found this very interesting set up. They have this piece of equipment that requires 150A, 208V, 3ph input. Instead of providing (1)150A 3ph circuit to this equipment, they provided (3) 50A 3ph circuits, paralleled. All three circuits are coming from the same panel and they have provide some sort of handle ties for three CBs. They have enough lugs to accept 3 circuits at the equipment.

I think they couldn't install 150A 3Ph CB in the panel and they have (3) 50A spare cbs so they installed it this way. I never seen this kind of set up and not quite sure if this is allowed by the code but couldn't pinpoint the code violation. Shouldn't there be a common trip in additional to handle tie?
 
infinity said:
Is this piece of equipment listed and designed to have three separate 50 amp circuits?
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It's the custom equipment and not sure if this was specifically listed and designed for that. I doubt it, though. Does it make any difference?
 
NEC Inspector said:
A lot of electric tankless water heaters are wired this way. The way I understand it, you aren't actually paralleling conductors because they each serve a separate heating element in the unit.

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Thanks for the info. Looks like this is a little bit different situation, though.
 
See NEC 240.8.

My understanding of the water heater installation is that the separate circuits feed separate loads internal to a single device. The installation described by the OP has 3 separate supply breakers feeding conductors which then all land on the same lugs of the equipment, so the circuit breakers are actually in parallel.

I rather doubt that these 3 breakers are were a single listed factory assembled unit, so you would have a violation of 240.8

I would argue that these conductors are in parallel, and likely smaller than 1/0 AWG, so you would have a violation of 310.10(H) (One might argue that the conductors originate from different breakers and are not completely in parallel, but IMHO a fault in one of the conductors would be fed by all three breakers, so they are in parallel.)

But IMHO if you had a listed factory assembly of 3 50A breakers, feeding a bunch of 1/0 AWG conductors to the load, then this installation would be kosher. (And much more expensive than going the normal route of a 150A breaker and correctly sized conductors)
 
elec_eng said:
Thanks for the info. Looks like this is a little bit different situation, though.
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Restating infinity's question. Does each set of three lugs feed a different load? Another nuance is, where did you get the information that the equipment requires 150A 3Ø? If from a label plate, then it would absolutely be a violation. If you just added up the breaker ratings, then assuming the loads are sized right, this isn't a violation of code. If you confirm the first, but still feel the need to do further investigation then you would perform load calculations on each circuit. As an
 
elec_eng said:
During the site observation in one of my customers facilities, I found this very interesting set up. They have this piece of equipment that requires 150A, 208V, 3ph input. Instead of providing (1)150A 3ph circuit to this equipment, they provided (3) 50A 3ph circuits, paralleled. All three circuits are coming from the same panel and they have provide some sort of handle ties for three CBs. They have enough lugs to accept 3 circuits at the equipment.

I think they couldn't install 150A 3Ph CB in the panel and they have (3) 50A spare cbs so they installed it this way. I never seen this kind of set up and not quite sure if this is allowed by the code but couldn't pinpoint the code violation. Shouldn't there be a common trip in additional to handle tie?
Click to expand...
3-50's don't equal 150to start with. Handle ties will not create a simultaneous trip condition.
 
winnie said:
See NEC 240.8.

My understanding of the water heater installation is that the separate circuits feed separate loads internal to a single device. The installation described by the OP has 3 separate supply breakers feeding conductors which then all land on the same lugs of the equipment, so the circuit breakers are actually in parallel.

I rather doubt that these 3 breakers are were a single listed factory assembled unit, so you would have a violation of 240.8

I would argue that these conductors are in parallel, and likely smaller than 1/0 AWG, so you would have a violation of 310.10(H) (One might argue that the conductors originate from different breakers and are not completely in parallel, but IMHO a fault in one of the conductors would be fed by all three breakers, so they are in parallel.)

But IMHO if you had a listed factory assembly of 3 50A breakers, feeding a bunch of 1/0 AWG conductors to the load, then this installation would be kosher. (And much more expensive than going the normal route of a 150A breaker and correctly sized conductors)
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Thanks Winne. I think 240.8 is the winner since I doubt those breakers are a single listed factory unit.
 
Fred B said:
3-50's don't equal 150to start with. Handle ties will not create a simultaneous trip condition.
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Is common trip required for this application?

It's a custom equipment for this facility and the feeder rating is 150A but I believe the load will be a lot less, roughly ~110A. It has contain an inverter unit inside this equipment for their processing equipment downstream.
 
Eddie702 said:
Unless the equipment is listed for separate breakers it is a violation. The name plate and manual should tell the story
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It's a custom unit and it doesn't provide any listing for the separate breakers. Which section is prohibiting this then?
 
I would state it differently. Putting fuses or breakers in parallel requires detailed engineering of the system to ensure proper current distribution and OCPD operation.

High current fuses are often build as smaller fuse links in parallel in a single large package.

Some residential main breakers are 4 pole devices 100A with pairs of poles in parallel to create a 2 pole higher 200A device.

In both cases these are engineered by the OCPD manufacturer, not field assembled by electricians.

If an OCPD manufacturer builds a device with, or describes in a datasheet how to set up parallel operation, then I'd be comfortable with it. It would be bad engineering practice to try to invent this without the resources to test it out.
 
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