OCPD for DC conductors

[Canton]

I'm having trouble understanding when it is required to have an OCPD on my PV output conductors. (NEC 2008 and NEC 2011)

I have two systems that were designed by someone else and i am the EC installing the PV system. They are both for residential applications at two different homes and less than 10kW. Both inverters are mounted outside next to the meter. The only conductors that will enter the house are the inverter output conductors to the back-fed breaker in the panel (Load-side connection)

The first is a transformer-less inverter and is an "Ungrounded Photovoltaic Power System". Following 690.35 am i correct that I will always have to have an OCPD in my DC positive and negative PV output conductors at DC disconnect before the inverter? Voltage is about 400VDC per string and has two PV output strings going to inverter, 8 Isc per string. Calculating with 690.8(A)1 and (B)1 with 156% x 8 = 12.48 amp conductor/OCPD.....?

The second design is exactly the same but with a "transformer" type inverter and will have the negative conductors grounded. The current design only shows the positive conductors being disconnecting in the DC disconnect and shows no OCPD anywhere in the PV DC side. I'm looking at 690.9(A) Exception, and i have no strings in parallel and no backfeeding possible. Is this compliant?

Thank you,

 

[jaggedben]

Typically overcurrent protection for PV source circuits is only required if you are using a combiner to combine three or more strings. (Since string conductors must be sized greater than the maximum possible current that can be delivered by the string of modules, only paralleling other strings can create an over-current condition.)

Your transformerless inverter is likely to have (at least) two electrically separate inputs; all of the ones I've seen do, but confirm it for yourself. In that case, since you have only two strings, you are not paralleling them and can invoke the code exception you mentioned.

Look carefully at the disconnect or disconnect section supplied with the other (transformer, grounded) inverter. SMA disconnects actually have fuses in them, and are actually a combiner, although this may not be readily apparent. Other inverter disconnects may or may not have the same features, read the manual carefully. In any case, with two strings, as long as your conductors are adequately sized, you can avoid an overcurrent protection requirement. You should most likely be using #10 wire for your DC conductors for this and other reasons (such as rooftop temperature adders). Even though you are paralleling two strings, if you have conductors rated for the combined short circuit current (25A) you qualify under the exception. With only two strings it is usually cheaper and easier to upsize conductors than to add separate equipment for overcurrent protection.

If you are installing separate DC disconnects then the ungrounded system should break both conductors and the grounded system should breaker only the ungrounded conductors.

 

[Canton]

Typically overcurrent protection for PV source circuits is only required if you are using a combiner to combine three or more strings. (Since string conductors must be sized greater than the maximum possible current that can be delivered by the string of modules, only paralleling other strings can create an over-current condition.)

Your transformerless inverter is likely to have (at least) two electrically separate inputs; all of the ones I've seen do, but confirm it for yourself. In that case, since you have only two strings, you are not paralleling them and can invoke the code exception you mentioned.

Look carefully at the disconnect or disconnect section supplied with the other (transformer, grounded) inverter. SMA disconnects actually have fuses in them, and are actually a combiner, although this may not be readily apparent. Other inverter disconnects may or may not have the same features, read the manual carefully. In any case, with two strings, as long as your conductors are adequately sized, you can avoid an overcurrent protection requirement. You should most likely be using #10 wire for your DC conductors for this and other reasons (such as rooftop temperature adders). Even though you are paralleling two strings, if you have conductors rated for the combined short circuit current (25A) you qualify under the exception. With only two strings it is usually cheaper and easier to upsize conductors than to add separate equipment for overcurrent protection.

If you are installing separate DC disconnects then the ungrounded system should break both conductors and the grounded system should breaker only the ungrounded conductors.

Thank you for the good information. I am interperting 690.35 as all ungrounded systems require OCP in each DC conductor? The install that i have the exception/exception would apply for OCPD for the ungrounded systems also?

Edit: I just re-read the article and the exception applies to Ungrounded systems also. Thank you again for the information.