2 Phases From a 3 Phase Disconnect

[fifty60]

Is it code compliant to use a UL98 rated 3 phase fused disconnect if the customer is only using 2 of the 3 phases? I have a 460V 3 phase supply, but am only using 2 phases. My powerpack is remote, and will be powered by all 3 phases. The local machine however will only need a heater and it's control transformer powered. I can achieve this with just 2 phases of the customer's supply.

The customer has 3 phase power, if it were not for the remote power pack I would have to supply them with a 3 phase fused disconnect. I do not see a problem supplying a 3 phase disconnect and only fuse 2 poles and leave the 3rd unused pole without a fuse. Would there be any code violation with this?

 

[petersonra]

Is it code compliant to use a UL98 rated 3 phase fused disconnect if the customer is only using 2 of the 3 phases? I have a 460V 3 phase supply, but am only using 2 phases. My powerpack is remote, and will be powered by all 3 phases. The local machine however will only need a heater and it's control transformer powered. I can achieve this with just 2 phases of the customer's supply.

The customer has 3 phase power, if it were not for the remote power pack I would have to supply them with a 3 phase fused disconnect. I do not see a problem supplying a 3 phase disconnect and only fuse 2 poles and leave the 3rd unused pole without a fuse. Would there be any code violation with this?

There is no requirement that you even bring the unused conductor in.

 

[fifty60]

I was only planning on fusing 2 of the 3 poles and just leaving unused pole open. The customer would only have to bring in 2 phases, and hopefully see the lack of fuse on the 3rd phase and not hook anything to it.

 

[templdl]

There is no requirement that you even bring the unused conductor in.

There is no 2 Pass either in this instance as you may be taking power grow 2 of 3 phases but that power is single phase.

 

[GoldDigger]

I was only planning on fusing 2 of the 3 poles and just leaving unused pole open. The customer would only have to bring in 2 phases, and hopefully see the lack of fuse on the 3rd phase and not hook anything to it.

It tends to get confusing when you do not specify whether the three phase circuit is delta or wye.

If we label the three line conductors A, B and C, then a single phase wye load will be connected between one of the A, B, C wires and neutral. This allows you to refer to that single phase load being on phase A, phase B or phase C.

A single phase load connected to a delta supply will necessarily be connected line-to-line and that "single phase" will be A-B, B-C or C-A. You can then talk about needing to balance the load on the phases as meaning balancing the current on A, B and C conductors or balancing the current on A-B, B-C and C-A.
Fortunately the math tells us that doing one is exactly equivalent to doing the other.
But the existence of both ways of describing it allows us to confusingly refer to bringing in two phases (conductors) to supply a single phase load. <sigh>

 

[petersonra]

I might be inclined to run the third wire since the extra cost would likely not amount to all that much in the grand scheme of things. One never knows what one might want to do down the road, and having 3 phase power there might be advantageous someday.