Inverter Output Circuit Sizing (terminals)

[Andrew445]

I am having trouble figuring out how to deal with 75?C terminal temperature rating requirements and 90?C cable with respect to Inverter output circuit sizing.

The inverter FLA is 1176A at 270V. 1176 * 1.25 = 1470A. A 1600A breaker is used, and the conductor must be rated for 1600A. - 240.4(C)
The temperature range for this environment is 41?-45?C.

My proposed method is the following:

I can use (5) parallel sets of 400 kcmil, Cu cable. Per the 90?C column: 380 * 0.87 * 5 = 1653A > 1600A. Checks out.

Now when it comes to terminals, the argument is that they will not see the full 1600A. The terminals will only see 1176A. The 1.25 is only used for OCPD devices not fully-rated, and doesn't make sense to use here. Per the 75?C column: 335 * 0.82 * 5 = 1373A > 1176A. This also checks out.

Is this the correct line of thought?

 

[ggunn]

I am having trouble figuring out how to deal with 75?C terminal temperature rating requirements and 90?C cable with respect to Inverter output circuit sizing.

The inverter FLA is 1176A at 270V. 1176 * 1.25 = 1470A. A 1600A breaker is used, and the conductor must be rated for 1600A. - 240.4(C)
The temperature range for this environment is 41?-45?C.

My proposed method is the following:

I can use (5) parallel sets of 400 kcmil, Cu cable. Per the 90?C column: 380 * 0.87 * 5 = 1653A > 1600A. Checks out.

Now when it comes to terminals, the argument is that they will not see the full 1600A. The terminals will only see 1176A. The 1.25 is only used for OCPD devices not fully-rated, and doesn't make sense to use here. Per the 75?C column: 335 * 0.82 * 5 = 1373A > 1176A. This also checks out.

Is this the correct line of thought?

Where did the 0.87 come from? I see 0.87 in the 41-45 degree C row of the 60 degree conductor column.

 

[jaggedben]

I should start with a disclaimer: This level of amperage is way above my qualifications. But applying what I know for smaller systems (and knowing 240.4(C) )...

Now when it comes to terminals, the argument is that they will not see the full 1600A. The terminals will only see 1176A.

Nothing will see more than 1176A, but I think that according to 690.8 you still have to rate the conductor 125% according to 110.14(C). So I don't follow this argument.

The 1.25 is only used for OCPD devices not fully-rated, and doesn't make sense to use here.

So your saying the OCPD meets the Exception to 690.8(B)(1)? If you had a 1200A OCPD that met the exception, you could use that. But I think that is all irrelevant to the conductor rating.

Per the 75?C column: 335 * 0.82 * 5 = 1373A > 1176A. This also checks out.

Well, I think this calculation has to meet 1600A. But I don't think you don't have to derate temperature for the terminals. So it's actually just 335*5= 1675 > 1600. So you're fine.

That's how I learned to do it from all the good folks on this forum.

Where did the 0.87 come from? I see 0.87 in the 41-45 degree C row of the 60 degree conductor column.

You should be looking at the table that's based on 30C, not the one based on 40C.