NEC 700.28 Selective Coordination on Small Central Emerg Lighting Inverters

[donohuco]

I just started a project where a small office building is utilizing a central emergency Inverter for egress illumination with the specs indicated below:

Inverter Spec: 120V 1PH 1.5 KVA MEYERS E-1150
Input: 120V, 1PH, 20A Branch Circuit
Output: 120V, 1PH, with (4) separate 20A branch Circuit Breakers.

I seem to see this setup / design a lot for commercial buildings where the inverter specified is between 1.5-5KVA. The input will be anywhere from a 20-50A s.p circuit breaker and the output will be 4-6 20A s.p circuit breakers.

I do not see how most smaller (1.5-5KVA) central emergency inverter designs like the ones above can meet selective coordination requirements. Can anyone let me know what i am mis-understanding on these.

Is selective coordination only required up to the input of the inverter?

 

[steve66]

As long as the output breakers trip faster than the input breaker, you would still have coordination.

But obviously you can't load up each output breaker to 1500 watts, when the whole UPS is only rated for 1500 watts. You would only have 375 watts per output.

I think they provide the multiple output breakers just so the output can be divided up, and if one ballast shorts out, it should only trip one breaker, and not take out the entire emergency lighting load.

 

[ron]

On most lighting inverters, there is no direct connection between the input power and the output. Only through solid state circuitry, which provides isolation.

An overload or short on one of the output breakers, would likely be delayed or isolated on the input side so the input breaker wouldn't trip as fast.

The 700 circuit, "begins" on the load side of the inverter. The input side of the inverter is just a regular branch circuit.