parallel feeders?

[Mike01]

Existing emergency switchboard with two available 1600AF/1600AT breakers new 3200A switchboard located adjacent to it. In the current project they will be installing the new switchboard, with 3200A MCB the generator to feed this switchboard are to be included in a future project, for the time until the new generators are purchased would it be compliant to provide two 1600A feeders one from each of the breakers to the 3200AF breaker to supply emergency power to the board for the time until the new generators are purchased and installed (as part of the next phase / addition). Article 240.8 talks about circuit breakers and fuses in parallel but it is really the conductors that are being paralleled. Any thoughts?

 

[LarryFine]

Any thoughts?

Conductor size?

 

[Mike01]

size..

size..

4-sets of 600kcmil per phase (ea. brkr.) would be protecting the feeder in question. also the associated feeder would only be apprx. 25-50' in total length.

 

[charlie b]

Article 240.8 talks about circuit breakers and fuses in parallel but it is really the conductors that are being paralleled.

No, it is the breakers that would be in parallel. They would be connected together at their line side, since they connect to the same bus. They would be connected together at their load side, since they connect to the same (downstream) bus. The fact that the two buses are some distance apart, so that the conductors that connect the two breakers in parallel are long conductors, does not alter the fact that the two breakers are in parallel.

This would not be allowed, and you correctly spotted 240.8 as the reason.

 

[Mike01]

Loop Feed

Loop Feed

But is this not essentially a loop feed? We do this with primary services all of the time with double-ended subs that sometimes operate in the closed transition mode? on large sites?

 

[charlie b]

A loop system does not place two breakers in parallel, because between the two breakers there are paths leading to loads.

Two breakers would be in parallel if and only if two things are true. One is that there is essentially zero impedance between the line sides of the two breakers, and essentially zero impedance between the load sides of the two breakers. The other is that that current has no path in which to flow, once it leaves the load side of one breaker, other than the path that leads to the point at which the load side of other breaker is connected. From that point, current heads off to loads, and the path eventually leads back to the line side of the two breakers. But if current can take a side path towards loads, before it reaches the point of connection to the other breaker, then the two breakers are not in parallel.