question of sizing LV switchgear

[daveR]

Can anyone help - I am upgrading a building supply to 1000KVA at 400V and installing new switchgear and a standby generator. The client has a spare generator rated at 1250KVA. The electrical contractor says the size of the main switchgear should be based on the rating of the generator not the incoming power supply which means we need 2000A rated incoming MCCB's (generator amps = 1800A max).

I don't agree with this as I think the generator can't supply more than the actual building load - which means I can use MCCB's rated at 1600A. Is this ok?

 

[cuky2000]

You are OK. A 1600 A frame circuit breaker set to trip at the appropriate level is a technical sound solution. The 1250 kVA generator will be limited by the circuit breaker to operated below maximum loading capability, approx. 1000 kVA)

A good engineering practice is to double check the interrupting capacity of the breaker for the worst-case scenario (utility vs. genset) and also check the generator capacity to move all connected loads

 

[dpc]

It's OK as long as you are sure you will never need the additional capacity that a larger breaker and switchboard would provide. You just won't be able to operate the generator at its full output.

 

[rbulsara]

1000kVA at 400V is 1443A. At 125% its 1800A. Unless you are using 100% rated breaker in appropriate enclosure you should use 2000AF breaker set at 1800A trip to utilize full 1000kVA.

Also if you apply utiltiy company logic, they normally undersize the supply transformers and if need be they do not mind running the transformer overloaded by 10-15% for long periods of time (more true for oil insulated units).

I personally do like to see the main service breaker or switchboard loaded anywhere near 80%, its too close for comfort and short sighted. You do not want to be making changes to the main service because a slight addition to a faciltiy. I would vote for 2000A breaker with adjustable trip unit.

Incremental cost may not be too significant. Do not lose sight of good engineering/ buisness sense in a zeal to make a technical point to a contractor. We engineers are too often guilty of that.

No offence intended.

 

[Mahase30]

Utility company do cater for additional load. I agree with rbulsara that you should rate the switchgear according to the generator and ensure the right trip level in protection... all in an effort to avoid additional spending and inconvenience if the overall load do increase in the future.

 

[jbartos]

Suggestion: The original posting may need further clarifications. Specifically, is the generator in the building the standby generator or emergency generator? (60sec to start or 10second to start?). Will there be an automatic transfer switch and what will be its rating? What is the current switchgear/switchboard short circuit current rating? What is the generator Xd", or short circuit current rating?