Correct Sizing for Load side Switchgear wiring

[ABB]

I'm currently at odds with my in-house EE (he's stubborn but a very good guy) over the sizing of switchgear load side wiring.

His position is that load side wiring from a LVDO breaker (Low Voltage Draw Out) must be sized at 125% of the dialed in setting, since the breaker is rated for a continuous load at its setting. Compared to a molded case breaker whose continuous rating is typically 80% of its trip setting.

Unfortunately this has some huge implications for the project I'm on.

Haaalp!

 

[bob]

I'm currently at odds with my in-house EE (he's stubborn but a very good guy) over the sizing of switchgear load side wiring.

His position is that load side wiring from a LVDO breaker (Low Voltage Draw Out) must be sized at 125% of the dialed in setting, since the breaker is rated for a continuous load at its setting. Compared to a molded case breaker whose continuous rating is typically 80% of its trip setting.

Unfortunately this has some huge implications for the project I'm on.

Haaalp!

Check NEC 210.20(A) Exception.

 

[ABB]

I had checked that, but even adjusting the stated demand load per 210.20(A) does not exceed the breaker setting.

The known load is around 300 Amps on a downstream 800 Amp DPB w/MCB, and spares for future. The LVDO is 800 frame/800 trip.

 

[bob]

I had checked that, but even adjusting the stated demand load per 210.20(A) does not exceed the breaker setting.

I do not understand you point. If the breaker has a 100% rating, then you
are not required to add the 25%.

 

[ABB]

210.19(A)1 and 220.20(A) both point to sizing based on 100% of non-continuous load and 125% of continuous load.

While the breakdown in my case is not 100% clear, the TOTAL demand load is about 300 Amps, and the TOTAL connected load is under 400 Amps.

So even idf the 400 Amp number was used and rounded up to 500 Amps (400 X 125%), the number is still far below the 800 amp trip setting of the breaker.