LTPU Breaker Settings for a 375 kW load Bank

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Nardie

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North Augusta, SC USA
I need some advice on this, I have a new load bank rated at 451A. From the breaker to the load bank the vendor installed parallel #3/0 AWG conductors. Based on NEC table 310.16 at 90 degree C the cables are rated for 225A, in parallel this would be 450A. This is my first problem since 451A from the load bank exceeds the rating for the #3/0 AWG conductors. If I follow NEC article 110.14(C) the ampacity of the conductors should be sized for 75 degree C. Per this article the ampacity of the #3/0 AWG conductors are only 200A, in parallel this would be 400A. So my design organization is recommended replacing the #3/0 AWG conductors with #4/0 AWG conductors. At 75 degree C the #4/0 AWG parallel conductors would be rated at 460A. So, the design organization is recommending the LTPU breaker setting to be at 455A. I believe this is set to low since the FLA for the load bank is 451A. I plan to test the load bank for at least 4 hours at full load and I'm worried about nuisance tripping of the load bank breaker during this test, since the LTPU setting is so close. Is there something in the NEC that would allow me to set the LTPU breaker setting at the 90 degree C rating of 260A for #4/0 AWG conductors, in parallel 520A, or am I misunderstanding article 110.14(C)? Any help would be appreciated, thanks in advanced.
 
Look at your load profile versus the breaker time current curve. Chose a long time setting that is below the conductor 75C rating shown in the appropriate NEC table. You may want to consider how the ambient temperature affects the breaker trip setting if it is a thermal magnetic design. If you are using electronic trip breakers you could look into a 100% rated device if your enclosure/panel will accept it.
 
If you can run this load bank at 100% for more than 3 hours then wouldn't it be a continuous load requiring the conductors to be sized at 125% of the 451 amps? IMO the OCPD and the conductors should be rated for 600 amps.
 
infinity said:
If you can run this load bank at 100% for more than 3 hours then wouldn't it be a continuous load requiring the conductors to be sized at 125% of the 451 amps? IMO the OCPD and the conductors should be rated for 600 amps.
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I think yes, with 1.25*451 = 564-amp cable ampacity on a 600-amp breaker.
 
Is the load bank reactive (rare/expensive but recommended)? What is the load bank rating for 451A: 375 kVA/kW pf=1 or 300kW pf=0.8 assuming 480VAC 3 phase?
 
tjkusa said:
Is the load bank reactive (rare/expensive but recommended)? What is the load bank rating for 451A: 375 kVA/kW pf=1 or 300kW pf=0.8 assuming 480VAC 3 phase?
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Every load bank I've ever worked on was nothing more than a big bunch of resistors making it purely resistive with a pf of 1.0.
 
infinity said:
Every load bank I've ever worked on was nothing more than a big bunch of resistors making it purely resistive with a pf of 1.0.
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Noted, as stated, ‘rare but recommended’. There is a reason all gensets are rated with a 0.8 power factor. Good reason as well to excercise at said rating.
 
I would say about half of the load banks I've designed over the years have inductive or capacitive components. But we only sell specialty load banks. I think all the load banks that we use to test marine diesel gensets have inductive and resistive components.
 
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