Am I interpretting properly that all the conductors on the PV system from the array to the inverter and the inverter to a circuit breaker backfed on a panel bussbar do not need any additional overcurrent protection if the conductors are rated 156% of the short circuit currents available?
Overcurrent Protection
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Are you referring to 690.9(A) Exception?
Are you backfeeding a service supplied panel? If so, Exception part (a) is out, so under part (b) you'd need to determine the scc of the service-supplied power at the panel and compare to the ampacity of your conductors. It's not very likely to qualify, IMO, but it is possible since your question was not limited to specific conditions.
Yes I am using a scenerio in my question of backfeeding a residential utility fed service panel with a plug in circuit breaker on the bussbar. And if the conductors from the inverter to the panel are #10 cu THHN to a 30 amp circuit breaker on the bussbar, wouldnt the short circuit current from the panel be limited to the 30 amps of the circuit breaker?
No.
Short-circuit current is the amount of instantaneous fault current available at the point of consideration. The 30A rating of the breaker is the overcurrent rating, and that number is based on its time response curve. In the initial instances of a short circuit, or overcurrent for that matter, the level of current passing through the breaker can be substantially greater than the breaker's overcurrent rating, which is why breakers also have to be rated to interupt without failure the short-circuit current available at their supply connection. For example, a typical breaker may have 10,000A RMS Symmetrical Interupt Rating, which means it will not fail to interupt a short-circuit that draws 10,000A in the milliseconds or so before the breaker reacts and trips open.
AdrianWint
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This would certainly be true for the (DC) conductors between the PV panels & the inverter, since, by the natural of the PV panels, they cannot provide a short cicruit current of more than about 120% of their rated current - so any OCPD would be unlikey to operate.
However, this would not hold true for the conductors between the inverter & the breaker in the supply panel. Although the current output of the inverter is limited, a fault would be 'backfed' by the utility supply which would be capable of supplying a high fault current.
By Article 690.9 (A) Exception (b), in most cases the conductors from the array to the inverter are large enough (156% of the short current value) and do not need any overcurrent protection. So is it reasonable that the only overcurrent protection for the system is that 30 amp circuit breaker backfeeding the panel? Dont believe many inverters have built in overcurrent protection. Just seems like I am overlooking something. And thanks for you input, gets my brain working.
Well here's food for thought...
690.17 states disconnecting means have to comply with all the listed requirements thereafter. Item 4 of 4 states the "Switch or Circuit Breaker" must have an interrupting rating sufficient for the voltage and current available. Do non-fused disconnects have an interrupting rating?
Yes. After all, regardless of how inverters operate, we are talking short-circuit current. It is obvious they are connected in some manner or normal current could not pass through the inverter.
Now whether the industry considers this possibility for SCC...???
jaggedben
Senior Member
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- Solar and Energy Storage Installer
A couple things...
In the original question, it is not mentioned how many strings of panels are on the array. The practical import of the exceptions in 690.9(A) is that if there are only one or two strings in the array, OCP is not required. If three or more strings are combined in parallel, the exceptions are generally not met and OCP is required on the ungrounded conductor of each string. OCP is usually done with a fused combiner. The purpose is, say, to prevent the current from three strings from flowing through the conductors for a fourth string. (That could happen in case of a ground fault in said fourth string.)
Other than this, with the information mentioned, the answer to the original question is yes, you are interpreting the code properly.
Second, regarding backfeed current from the inverter...
Nowadays most inverters have (a) a transformer* and (b) relays that cut the circuit if the inverter is not operating. So inverter backfeed to the array is not a real possibility, and with proper OCP on the AC side shouldn't exceed the rating of the conductors even it if did happen. If you are using a UL listed solar inverter, that should qualify for the exception (a) to 690.9 (A).
*See the thread on ungrounded PV arrays. Transformerless models are coming onto the market. They still have the relays though.
In the original question, it is not mentioned how many strings of panels are on the array. The practical import of the exceptions in 690.9(A) is that if there are only one or two strings in the array, OCP is not required. If three or more strings are combined in parallel, the exceptions are generally not met and OCP is required on the ungrounded conductor of each string. OCP is usually done with a fused combiner. The purpose is, say, to prevent the current from three strings from flowing through the conductors for a fourth string. (That could happen in case of a ground fault in said fourth string.)
Other than this, with the information mentioned, the answer to the original question is yes, you are interpreting the code properly.
Second, regarding backfeed current from the inverter...
Nowadays most inverters have (a) a transformer* and (b) relays that cut the circuit if the inverter is not operating. So inverter backfeed to the array is not a real possibility, and with proper OCP on the AC side shouldn't exceed the rating of the conductors even it if did happen. If you are using a UL listed solar inverter, that should qualify for the exception (a) to 690.9 (A).
*See the thread on ungrounded PV arrays. Transformerless models are coming onto the market. They still have the relays though.
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