Line/Load Side Taps - Ground Wire Size

[ChargedUp]

Correct ground wire size for line side tap scenario -
From line side of existing service entrance MCB to PV disconnect - should the new ground wire routed with the new phase conductors be based on table 250.102 and treated as a supply side bonding jumper or should it be based on table 250.122 and treated as an EGC since load flow is only FROM PV system TO existing service entrance equipment. Assuming table 250.102 should be used but just want to confirm.

Correct ground wire size for load side tap scenario -
From load side of existing service entrance MCB to PV disconnect, should the new ground wire routed with the new phase conductors be based on table 250.122 and size according to existing service entrance MCB size, or should it be sized according to PV disconnect size since load flow is only FROM PV system TO existing service entrance equipment. Assume it should be based on existing service entrance MCB size, but just want to confirm.

Could the use of cable limiters for line or load side taps have an impact on ground size?

Thanks all.

 

[ggunn]

Correct ground wire size for line side tap scenario -
From line side of existing service entrance MCB to PV disconnect - should the new ground wire routed with the new phase conductors be based on table 250.102 and treated as a supply side bonding jumper or should it be based on table 250.122 and treated as an EGC since load flow is only FROM PV system TO existing service entrance equipment. Assuming table 250.102 should be used but just want to confirm.

Correct ground wire size for load side tap scenario -
From load side of existing service entrance MCB to PV disconnect, should the new ground wire routed with the new phase conductors be based on table 250.122 and size according to existing service entrance MCB size, or should it be sized according to PV disconnect size since load flow is only FROM PV system TO existing service entrance equipment. Assume it should be based on existing service entrance MCB size, but just want to confirm.

Could the use of cable limiters for line or load side taps have an impact on ground size?

Thanks all.

The direction of current flow in non faulted operation is not relevant, IMO. OCPD is there to protect conductors from fault current from the utility, not from the inverter.

Cable limiters have no effect on grounding conductor size.

 

[jaggedben]

It's 250.102.

This has been very vague and unaddressed in the code for a long time, until the 2020 NEC. See 250.25. While I think it's still less than clear, it seems to be saying that you treat the PV disconnect like an additional service disconnect. At any rate, 250.24(C) makes it pretty clear that you need a ground fault path sized to 250.102.

 

[ChargedUp]

So 250.102 for line side tap. Thanks for your feedback, that is my thinking as well. Looks like the blue commentary in NEC 2020 250.25(B) Also addresses PV systems very clearly.

What about 250.122 for a load side tap, which I would consider an EGC, not a supply side bonding jumper (even though that goes a little against the thought above as treating this as a service disconnect - I'm assuming that was meant for line side only application)...

Do you all agree load side tap ground for PV should be sized to 250.122, not 250.102? If so, I assume it should be sized according to existing MCB/buss rating its connected to, or to the PV disconnect, whichever is larger of the two. Thoughts?

 

[ggunn]

So 250.102 for line side tap. Thanks for your feedback, that is my thinking as well. Looks like the blue commentary in NEC 2020 250.25(B) Also addresses PV systems very clearly.

What about 250.122 for a load side tap, which I would consider an EGC, not a supply side bonding jumper (even though that goes a little against the thought above as treating this as a service disconnect - I'm assuming that was meant for line side only application)...

Do you all agree load side tap ground for PV should be sized to 250.122, not 250.102? If so, I assume it should be sized according to existing MCB/buss rating its connected to, or to the PV disconnect, whichever is larger of the two. Thoughts?

250.122, sized to the OCPD protecting the conductors from fault current coming from the utility.

 

[pv_n00b]

Correct ground wire size for line side tap scenario -
From line side of existing service entrance MCB to PV disconnect - should the new ground wire routed with the new phase conductors be based on table 250.102 and treated as a supply side bonding jumper or should it be based on table 250.122 and treated as an EGC since load flow is only FROM PV system TO existing service entrance equipment. Assuming table 250.102 should be used but just want to confirm.

First, identify the major source of fault current, protect from that.
For a supply side interconnection, the disconnect switch has to be fused. Since the disconnect switch is fused the grounding conductor from it to the inverters is an EGC and is sized based on the rating of the fuses.
A good rule of thumb is that if there is no OCPD protecting the conductors from the source of fault current then the ground is a supply side bonding jumper since it is sized based on the size of the phase conductors and not an OPCD rating. If the conductors are protected by an OCPD then the ground conductor is an EGC and is sized based on the OCPD rating.

Correct ground wire size for load side tap scenario -
From load side of existing service entrance MCB to PV disconnect, should the new ground wire routed with the new phase conductors be based on table 250.122 and size according to existing service entrance MCB size, or should it be sized according to PV disconnect size since load flow is only FROM PV system TO existing service entrance equipment. Assume it should be based on existing service entrance MCB size, but just want to confirm.

Could the use of cable limiters for line or load side taps have an impact on ground size?

This is a tap so you size the EGC based on the MCB but it does not have to be larger than the tap conductors. So if your tap conductors are for a 200A PV interconnection into a 2,000A SWBD bus you don't have to size the EGC for 2,000A. Tap conductors are pretty much expected to burn up in a fault. That's why they are as short as possible and protected.
Current limiters don't do anything for you in the NEC. They are not considered protection in the same way an LV OCPD is.