Supply-Side Connection and Conductor Sizing
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Original post from USE_solar: I am designing a PV system that has a continuous output of 79 A. The ampacity of the overcurrent protection is 98.8 A. The system taps on the line/supply side of the 200 A main breaker of the 120/240 V service. If this was a connection on the load side of the main breaker, the conductors would be 3 AWG and rated at 100 A. I need clarification on the conductor size for a line-side tap. I read that they need to be sized as service-entrance conductors, but what would that be? Would the ground need to be the same size?
Marvin Hamon, PE: With a supply-side interconnection, several things dictate conductor sizing. Most of it comes out of NEC Article 230. I assume you are using a 100 A fused disconnect between the inverter and the tap point.
The conductors between the disconnect and the tap are unprotected service-entrance conductors. You size them so they will not be overloaded, since you can’t size them to be protected from a fault on the supply side. Start with 230.42(A) and (B). The conductors must be sized for the inverter output and for the disconnect size. So in this case, you size the conductors for the 100 A disconnect.
On the load side of the disconnect, you size the conductors so the 100 A fuses protect them.
Grounding is a little more difficult. Since your ac disconnect is a service entrance, it has the neutral-to-ground bond and has to be connected to the grounding-electrode system. You size the grounding-electrode conductor from Table 250.66 based on the size of the conductor from the tap to the disconnect. You size the equipment grounding conductor from the disconnect to the inverter using Table 250.122. If there is a ground conductor from the utility, you should run it to the disconnect.
USE_solar: Marvin, I understand the load side of the disconnect sizing, but after looking at 230.42(A) and (B), I’m confused about how to size the conductors. Do I size by 125% of the continuous load of the solar? Or since this is a 200 A 120/240 Vac residential service, do I feed the line side of the ac disconnect with 2/0 AWG as stated in 310.15(B)(6)?
Marvin Hamon: You have to comply with 230.42(B) only. You size the conductors to carry 100 A since that’s more than 125% of the inverter current of 79 A. NEC 310.15(B)(6) says you don’t have to count the grounding conductor when adjusting conductor size for conduit fill. I’m not sure how you are using it to get a 2/0 AWG conductor size.
156.25 [third poster]: Where does it say in the NEC that supply-side PV connections have to follow 230 rules? In 230.82(6), it says that PV can connect to the supply side, but I can’t see where it says PV is considered a service. I see why it should be, but help me connect the dots.
Marvin Hamon: It does not say explicitly to follow 230. Article 705 says you can connect on the supply side, but it does not say how. If we consider it a service entrance, all the requirements are called out in the Code.
Think of it this way: You need to connect a PV system on the supply side of the existing main-service disconnect and you need to connect at a dedicated OCPD. Article 690 covers everything from the inverter to the OCPD but not to the service entrance conductors. Since a connection between the service entrance conductors and the service disconnect is considered a service entrance, it is covered in 230. Article 230 gets you from the service entrance conductors to the main disconnect, which is typically a fused disconnect and therefore the inverter point of common coupling, and from there Article 690 gets you to the inverter.