Fused Disconnects, Line Side Tap, and 230.42 (B)

[Matu]

Going to keep is straight forward and simple.

I have a SolarEdge 6000H-US inverter (single phase, 120/240V) feeding a single family home, requiring an OCPD of 35A. Since we are doing a line side tap, we are using a fused disconnect to feed the main service panel, 60A Disco with a 35A fuse. The MSP has a rating of 150A.

Our one line calls for a 60A disconnect (understandable), but is calling for #6AWG conductors and a #6 AWG ground? Before hearing about 230.42 (B) and 230.79 A through D, we would have just used #10 AWG conductors with a #8 ground (minimum would be a #10), all THHN.

My question is:

Which is correct? Do these codes (230.42 B and 230.79 A-D) refer to the service disconnecting means as the fuse/breaker in these systems or the rating of the disconnect itself? Bonus: Would my ground be sized to 250.6/250.102(c)(1) based on the load of my inverter, or the load of the service feeding into the house, which is 150Amps max, or simply 250.122?

Thank you

 

[jaggedben]

Good questions. Some parts of your questions have concrete answers, so I'll try to start there, although there's a big 'it depends' part involved, see last part of my answer.

First, 230 specifies the rating of the line side conductors and the disconnect, but the fuses can be smaller. But if 230 matters is a question...
Second, you didn't ask about this, but you cannot use 10awg with a 35A OCPD. Because of 240.4(D). So you'll have to use at least 8awg regardless.
Third, you can't really use 250.122 if you don't have an OCPD upstream. So if you need a supply-side bonding jumper between your tap point and your disconnect, then 250.102 is the correct section.
Fourth, the main service load and breaker is pretty much irrelevant to anything your asking about.

Regarding those ifs ...
The general question of whether/how to apply 230 to supply side connections is something which the code offers little concrete guidance on. A few years ago now I was participating in vehement arguments in this forum on such subjects, but eventually had to admit the code is not explicit about it.

On the one hand, it makes sense from a physics and safety perspective to view your disconnect as another service disconnect, given that fault current and other issues are basically the same. So you'll find some AHJs treating supply side connections that way, and if they do, then those rules on conductor size would apply.

On the other hand, the code has tended to talk about supply side connections as if they may be some other kind of animal, such as a feeder. And attempts to change the code to say otherwise have been rejected.

So...
Find out if you're AHJ is going to treat it as a service disconnect, and if so then apply 230 accordingly. In that case you can use a grounded conductor to bond the disconnect and follow 250.24(C) and such and maybe not need a supply-side bonding jumper. If you're AHJ doesn't treat it like a service disconnect and you don't want to either, maybe you can run 8awg conductor and an SSBJ sized to 250.102 instead.

 

[PWDickerson]

It's a small point, but that inverter model does not require a 35A OCPD. Per the manufacturer, 35A is the minimum, and 50A is the maximum.

 

[Matu]

Regarding those ifs ...
The general question of whether/how to apply 230 to supply side connections is something which the code offers little concrete guidance on. A few years ago now I was participating in vehement arguments in this forum on such subjects, but eventually had to admit the code is not explicit about it.

On the one hand, it makes sense from a physics and safety perspective to view your disconnect as another service disconnect, given that fault current and other issues are basically the same. So you'll find some AHJs treating supply side connections that way, and if they do, then those rules on conductor size would apply.

On the other hand, the code has tended to talk about supply side connections as if they may be some other kind of animal, such as a feeder. And attempts to change the code to say otherwise have been rejected.

So...
Find out if you're AHJ is going to treat it as a service disconnect, and if so then apply 230 accordingly. In that case you can use a grounded conductor to bond the disconnect and follow 250.24(C) and such and maybe not need a supply-side bonding jumper. If you're AHJ doesn't treat it like a service disconnect and you don't want to either, maybe you can run 8awg conductor and an SSBJ sized to 250.102 instead.

Thank you for your response, Ben.
Could you argue that 230.79 (B) could apply to the inverter installation, meaning that we are technically installing a branch circuit, so that we would not have to worry about it falling under 230.79 (D), other installations? Meaning that we would need a 60A disconnect, but since 230.42(B) states that the ampacity cannot be less than what's specified in 230.79 A-D, we would be fine since what is specified is a no more than two-circuit installation of 2-wire branch circuits?

 

[jaggedben]

Thank you for your response, Ben.
Could you argue that 230.79 (B) could apply to the inverter installation, meaning that we are technically installing a branch circuit, so that we would not have to worry about it falling under 230.79 (D), other installations? Meaning that we would need a 60A disconnect, but since 230.42(B) states that the ampacity cannot be less than what's specified in 230.79 A-D, we would be fine since what is specified is a no more than two-circuit installation of 2-wire branch circuits?

I think you could argue that, but is it really worth saving $2 on wire?