Is the conductor sizing of a collection panel based on the inverter outputs or OCPD?

[ryangittens]

Quick question. If you have a multiple inverter system being collected at a subpanel do you size the conductor based on the breaker size or inverter output currents?

For example, a 4 inverter system with max output current of 42A each would need a 50A breaker each at the collection panel with a 200A main breaker. However, the max combined current would be 4*42=168A. Which is value is the conductor size after the collection panel based on? And what is the code reference that defines this?

Thanks for the help!

 

[ggunn]

Quick question. If you have a multiple inverter system being collected at a subpanel do you size the conductor based on the breaker size or inverter output currents?

For example, a 4 inverter system with max output current of 42A each would need a 50A breaker each at the collection panel with a 200A main breaker. However, the max combined current would be 4*42=168A. Which is value is the conductor size after the collection panel based on? And what is the code reference that defines this?

Thanks for the help!

View attachment 17045

Any conductor is sized for the current it will be expected to carry. In this case it is 125% of the total inverter current. I do not know if the code explicitly calls for this. BTW, you do not need a main breaker on your solar distribution panel.

The conductor you pick must be adequately protected by the OCPD at the interconnection end.

 

[ryangittens]

Thanks a lot!

Thanks a lot!

Thanks for the quick reply! That's what I thought, otherwise the conductors would be unnecessarily large.

 

[jaggedben]

I believe the code reference is 705.60.

If the 42A doesn't already include a 125% factor then unfortunately you'll need 60A breakers and a 225A breaker for the combined output.

 

[ryangittens]

Good Catch!

Good Catch!

I believe the code reference is 705.60.

If the 42A doesn't already include a 125% factor then unfortunately you'll need 60A breakers and a 225A breaker for the combined output.

You are absolutely right. Thanks for catching that!

 

[pv_n00b]

Quick question. If you have a multiple inverter system being collected at a subpanel do you size the conductor based on the breaker size or inverter output currents?

For example, a 4 inverter system with max output current of 42A each would need a 50A breaker each at the collection panel with a 200A main breaker. However, the max combined current would be 4*42=168A. Which is value is the conductor size after the collection panel based on? And what is the code reference that defines this?

Thanks for the help!

It has to be sized for both, but not how you stated it. It has to be sized to carry the PV current and it has to be protected by the OCPD that you do not show that is feeding the AC combiner. Say for instance that is a 250A CB because that happens to be there already. You would have to size the conductor so that it was protected by that size CB from the utility fault current.

 

[ryangittens]

I got you

I got you

`

It has to be sized for both, but not how you stated it. It has to be sized to carry the PV current and it has to be protected by the OCPD that you do not show that is feeding the AC combiner. Say for instance that is a 250A CB because that happens to be there already. You would have to size the conductor so that it was protected by that size CB from the utility fault current.

I see what you're saying. Most times that OCPD is chosen, but if it already exists and is larger than the next std. up from the conductor ampacity, it must still follow 240.4(B) & (C), which states that the OCPD should be the next std up for < 800A.

So for example, if I have 168A max current and I select 3/0AWG (ampacity of 225) but I have a 300A OCPD I would have to upsize the conductors to 250AWG (ampacity of 290) which makes the OCPD the next std up?

 

[ggunn]

`

I see what you're saying. Most times that OCPD is chosen, but if it already exists and is larger than the next std. up from the conductor ampacity, it must still follow 240.4(B) & (C), which states that the OCPD should be the next std up for < 800A.

So for example, if I have 168A max current and I select 3/0AWG (ampacity of 225) but I have a 300A OCPD I would have to upsize the conductors to 250AWG (ampacity of 290) which makes the OCPD the next std up?

You have to derate the 250kcmil conductors for conditions of use (temp and conduit fill); that derated ampacity must be greater than 250A for the conductors to be protected by 300A OCPD..