PV plant ? Fuses for 2 strings in parallel

[Paul-tecnico]

I?m an Italian electrical designer and i have a doubt about the fuses for the protection of 2 strings in parallel for a photovoltaic plant in USA.
The following table shows the system configuration:

• Modules power: 295Wp
• 19 modules for each string
• 2 strings in parallel (with ?Y? connector MC4)
• Short circuit current (Isc): 8.95A

For this scenario i chose a 30A fuse for the protection of each pair of strings connected in parallel (fuses installed in the combiner boxes. #1 for each input).

The size of these fuses is determined by the maximum circuit current output (Imax) = 1.25xIsctot
Imax=(Isc1+Isc2)x1.25=(8.95A+8.95A)x1.25=22.375A
In fuse=Imax x 1.25=27.97A
So I chose fuses with In=30A.

The fuse nameplate ampere ratings, for the PV modules is 15A.

Based on NEC 110.3(B) we are required to use the module as it was listed. This prohibits us from using a larger fuse (first problem).
Also, NEC 690.8(D) gives us a requirement for sizing module interconnection conductors of paralleled strings. Ampacity needs to be greater than [(n-1)*Isc*1.25]+Series Fuse Rating where n=number of parallel strings protected by the same fuse. Required ampacity would then be 41 A.

The PV module conductors are only #12AWG (4mm2) and can only carry 20A per NEC 240.4(D)(7) (second problem)

But, in case of fault, the fuse will break because of other strings, therefore i cannot understand the article NEC 690.8(D).

Can i solve this problem with MC4 fuse connector #1 for each string (15A fuse) ? as defined in NEC 690.9 (E)?

Thank you in advance.

Paolo

 

[GoldDigger]

The fuse nameplate ampere ratings, for the PV modules is 15A.

Based on NEC 110.3(B) we are required to use the module as it was listed. This prohibits us from using a larger fuse (first problem).
Also, NEC 690.8(D) gives us a requirement for sizing module interconnection conductors of paralleled strings. Ampacity needs to be greater than [(n-1)*Isc*1.25]+Series Fuse Rating where n=number of parallel strings protected by the same fuse. Required ampacity would then be 41 A.

The PV module conductors are only #12AWG (4mm2) and can only carry 20A per NEC 240.4(D)(7) (second problem)

But, in case of fault, the fuse will break because of other strings, therefore i cannot understand the article NEC 690.8(D).

Can i solve this problem with MC4 fuse connector #1 for each string (15A fuse) – as defined in NEC 690.9 (E)?

Thank you in advance.

Paolo

Hello Paolo,

The design justification for a fuse on each individual string does not come into play until you have three or more total strings. The situation which this will protect against is a fault in one string which allows current to flow in the reverse direction through the string, overheating it in places and causing damage or fire.

If there are only two strings in parallel, and no other strings in the system at all, the maximum fault current through on defective string will limited to the Isc of the other string.
The current that a string can withstand is presumed to be safely limited by the maximum series fuse size specified by the manufacturer.

If you have two strings in parallel with one fuse, the current through a failed string can be the sum of the current in the other string and the current allowed from all of the other strings based on the shared series fuse. Under almost all conditions, and especially if the shared fuse is set at the maximum allowed value, the resulting current will be more than the failed string has been tested against. So even a 15A fuse would not be allowed since it would not provide protection (and would likely blow during normal use.)

You may argue that the shared fuse will blow and leave only the current from the parallel string, but this is not a valid argument because a failure mode with a high resistance may not pull enough current to blow the fuse.

It is not only the requirement of the Code, it is also fully justified.
Protecting the connectors and the wiring is NOT the only purpose of the series fuse.

Your solution of a single 15A or smaller fuse in each string is exactly what the code calls for if there are more than two strings. For two parallel strings, a single 15A fuse would be allowed, but would probably trip from time to time.

 

[jaggedben]

Goldigger said more or less what I would have said. A 30A fuse will not protect the wiring from all the other strings in the system.

The "Y connectors" were discussed recently in this thread:
http://forums.mikeholt.com/showthread.php?t=151774
A lot of us do not understand why someone would use them.

Also...

The size of these fuses is determined by the maximum circuit current output (Imax) = 1.25xIsctot
Imax=(Isc1+Isc2)x1.25=(8.95A+8.95A)x1.25=22.375A
In fuse=Imax x 1.25=27.97A
So I chose fuses with In=30A.

It is actually (Isc1+Isc2)x1.56 when all is said and done. 690.8(A) calls for 125% of the rated Isc, and then 690.8(B) calls for 125% of that. That's for the fuse. In the case of the wiring, it can be even greater if the conditions of use call for it.

Finally, make sure you are allowed to use 1000V on this system. That's only allowed in special circumstances in the US.