Connecting Fuel Cells to Distribution System

[charlie b]

Calculated load is 2500 amps at 480 volts. To leave room for future growth, I pick a 2500 KVA transformer and a 4000 amp main switchboard. Newly manufactured fuel cells will be tested at this facility. The intent is to use the building’s distribution system, instead of a separate load bank, as the load for the fuel cells. At maximum capacity, the fuel cells can supply up to 1500 amps. If I connect this energy source to the main switchboard (i.e., in parallel with the utility), do I have to upsize it to 5000 amps?

One viewpoint is that the main switchboard woud have two energy sources, one rated at 3000 amps and the other at 1500 amps, and therefore the bus must be rated for at least that much current. The other viewpoint is that any current supplied by the fuel cells is simply current that is not supplied by the transformer, and the total current supplied will never be higher than the calculated demand load, and therefore the 4000 amp board is adequate.

 

[don_resqcapt19]

It appears that if the connection is on the load side of the service disconnect that the sum of the main breaker and the fuel cell system breaker cannot exceed 120% of the bus rating. 705.12(D)(2). If the fuel cell breaker is a "service disconnect", then it can have any rating that is equal to or less than the rating of the service.

 

[gar]

121026-1043 EDT

The switchboard should never see current greater than the load current. The distribution of that load current in the busbars will depend upon where the two power sources feed into the switchboard. Depending upon this distribution there might be less heat generated in the switchboard than with just a single feed point.

The inverter from the fuel cell essentially will be a unity power factor source. Its current and voltage are both essentially in-phase with the utility voltage.

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