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Maximum Number of Strings on a Combiner Calculation

Posted on 14 April, 2016 at 22:33

Q:

I am having trouble grasping an outside sourced practice question:
A 225 kW(DC) commercial system with a single 225 kW(AC) central inverter is being installed. The module specifications are:
Maximum power (Pmax): 200 W
Voltage at maximum power (Vmp): 28.9 V
Current at maximum power (Imp): 6.93 A
Open-circuit voltage (Voc): 36.2 V
Short-circuit current (Isc): 7.68 A
Maximum system voltage: 600 V
Maximum series fuse rating: 15 A

The grounded PV array is broken into subarrays, and each subarray is combined in a separate fused string combiner box that feeds a separate pair of PV output circuit conductors to the DC input bus of the inverter. Each ungrounded PV output circuit conductor connects through one pole of a three-pole DC safety switch rated at 100 A per pole to a 100 A fuse on the inverter's DC input bus. The string combiner boxes may hold up to 14 fuses.
What is the MAXIMUM number of module strings that may be connected to each fused string combiner???
I think the answer would be 8... but there is a lot going on in this question so I am not sure. Please help!

A:

This question is reality and too much information, so it is an exercise at sifting out the erroneous information, such as the rating of the inverter, voltages, etc.

What it comes down to is what would be the maximum number of strings that you can put on a 100A fuse.

The fuse is sized from 156% of Isc

Isc = 7.68A

7.68A x 1.56 = 12A

100A fuse / 12A = 8.33 strings

The answer is 8, so you have it right!

You are ready to pass the NABCEP PVIP Exam Saturday. I will bet pizza on it.

Thanks,
Sean White

Categories: NABCEP PV Installation Professional Exam Prep Course

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