4/0 Al SER to subpanel

[SolarAlternativesDesign]

Does the 83% rule for dwellings (310.15(B)(7)) apply to a subpanel being fed off the service panel? In this case we have a 200A MSP and a branch to an Enphase Enpower smart switch panel and a branch from the smart switch panel to a backed up loads panel. The contractor proposes to protect both panels with 200A breakers. He proposes to use 4/0 Al SER conductors having 180A rating.
For the MSP - smart switch circuit it seems to me the maximum current would be the same as the 83% of the service conductors, 166A, so 4/0 Al SER would work. But these conductors are branch conductors, not service conductors, so would the 83% rule apply?
For the smart switch - BU panel there would be additional power sources from the PV and battery, so I think we need to use a smaller breaker.

 

[don_resqcapt19]

As long as the load is 180 amps or less, 4/0 aluminum with an ampacity of 180 amps can be protected by a 200 amp OCPD per 240.4(B), assuming all terminations are listed for 75°C.

 

[jaggedben]

Does the 83% rule for dwellings (310.15(B)(7)) apply to a subpanel being fed off the service panel?

Yes, if the subpanel feeder isn't rated lower than the service conductors are required to be.

In this case we have a 200A MSP and a branch to an Enphase Enpower smart switch panel and a branch feeder from the smart switch panel to a backed up loads panel. The contractor proposes to protect both panels with 200A breakers. He proposes to use 4/0 Al SER conductors having 180A rating.
For the MSP - smart switch circuit it seems to me the maximum current would be the same as the 83% of the service conductors, 166A, so 4/0 Al SER would work. But these conductors are branch feeder conductors, not service conductors, so would the 83% rule apply?
For the smart switch - BU panel there would be additional power sources from the PV and battery, so I think we need to use a smaller breaker.

Feeders are covered by the same rule. And you are talking about feeder conductors, not branch circuit conductors. (Fixed your quote for you.) Therefore part (3) of 310.15(B)(7) applies, and it says "In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.15(B)(7)(1) or (2).

So either 310.15(B)(7)(1) or (2) applies and 4/0 AL SER is fine.

 

[SolarAlternativesDesign]

Thanks for your correction.
One other clarification: since the backed up loads panel is being fed not only by the service but by the solar and the battery, would we then size the conductors for 83% X 200A + 1.25 X (solar continuous current + battery continuous current)? It seems obvious that the backed up panel will not serve more load than the MSP, but do we need to protect the conductors in case of fault current? I was thinking of protecting that circuit with a 175A breaker.

 

[don_resqcapt19]

Does the 83% rule for dwellings (310.15(B)(7)) apply to a subpanel being fed off the service panel?

If there is more than one subpanel, 310.15(B)(7) does not apply to any of them no matter what. If there is a single subpanel, but the main panel has branch breakers in addition to the subpanel feeder breaker that section also does not apply.

In this case we have a 200A MSP and a branch to an Enphase Enpower smart switch panel and a branch from the smart switch panel to a backed up loads panel. The contractor proposes to protect both panels with 200A breakers. He proposes to use 4/0 Al SER conductors having 180A rating.

You don't need the dwelling unit service conductor rule for this. 4/0 aluminum is 180 amp wire in the 75°C column. Assuming a load equal to or less than 180 amps, 240.4(B) permits the use of a 200 amp OCPD.

For the MSP - smart switch circuit it seems to me the maximum current would be the same as the 83% of the service conductors, 166A, so 4/0 Al SER would work. But these conductors are branch conductors, not service conductors, so would the 83% rule apply?

The 83% rule almost never applies to any feeder or service conductor that does not carry 100% of the dwelling unit load. It does not apply for your application.

 

[jaggedben]

Ah yes, this makes a difference. 705 requires that the downstream portion of the feeder be either sized for both sources or that it be protected by a breaker. In my opinion you could protect it with a 200A breaker and still invoke 310.15(B)(7). (A 175A breaker is certainly ok.) If you choose to upsize the conductors that's a grayer area.

 

[jaggedben]

If there is more than one subpanel, 310.15(B)(7) does not apply to any of them no matter what. ..If there is a single subpanel, but the main panel has branch breakers in addition to the subpanel feeder breaker that section also does not apply..

I strongly disagree. There is nothing that prohibits me from installing two 100A subpanels on a 100A service. In this case all conductors could be 4awg cu. This seems to be the entire point of the 3rd subsection of 310.15(B)(7) which I quoted above.

The 83% rule almost never applies to any feeder or service conductor that does not carry 100% of the dwelling unit load. It does not apply for your application.

It's not clear that his feeder doesn't carry the entire load, and in any case his feeder conductors are never required to be larger than the service conductors.

 

[augie47]

I strongly disagree. There is nothing that prohibits me from installing two 100A subpanels on a 100A service. In this case all conductors could be 4awg cu. This seems to be the entire point of the 3rd subsection of 310.15(B)(7) which I quoted above.

Unless there is a 100 amp device ahead of those two panels, how do you justify a #4 when it is not carrying the full load associated with the dwelling ?

 

[kwired]

I strongly disagree. There is nothing that prohibits me from installing two 100A subpanels on a 100A service. In this case all conductors could be 4awg cu. This seems to be the entire point of the 3rd subsection of 310.15(B)(7) which I quoted above.




It's not clear that his feeder doesn't carry the entire load, and in any case his feeder conductors are never required to be larger than the service conductors.

Unless I read it wrong conductors in question are potentially carrying load from two different sources though. One of the sources would likely be continuous current at fairly fixed level during the production hours of the day. But if the actual load is still under 180 then the next size up rule still allows 200 amp protection, but I don't believe 310.15(B) (7) would apply so if the actual load were 181-200 amps you would need larger conductor.

 

[wwhitney]

Unless there is a 100 amp device ahead of those two panels, how do you justify a #4 when it is not carrying the full load associated with the dwelling ?

2017 NEC 310.15(B)(7)(3) says: "(3) In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.15(B)(7)(1) or (2)"

Because it makes no sense to require a higher ampacity for a subset of the load.

Cheers, Wayne

 

[jaggedben]

Unless there is a 100 amp device ahead of those two panels, how do you justify a #4 when it is not carrying the full load associated with the dwelling ?

A main breaker ahead of them is what I meant to describe. However, if the calculated load on the service were 100A or less then the service conductors and the subpanel conductors could all be 4awg regardless. That's per the code section Wayne and I have now cited three times.

 

[jaggedben]

Unless I read it wrong conductors in question are potentially carrying load from two different sources though. One of the sources would likely be continuous current at fairly fixed level during the production hours of the day. But if the actual load is still under 180 then the next size up rule still allows 200 amp protection, but I don't believe 310.15(B) (7) would apply so if the actual load were 181-200 amps you would need larger conductor.

The alternate intermittent source is not really relevant as the utility alone will likely supply the maximum load at some point or another.

 

[winnie]

2017 NEC 310.15(B)(7)(3) says: "(3) In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.15(B)(7)(1) or (2)"

Because it makes no sense to require a higher ampacity for a subset of the load.

Cheers, Wayne

That is quite correct, but must be understood to be a specific situation.

The 83% rule applies only to conductors that carry the full load of a dwelling. But after you've sized the conductors that carry the full load of the dwelling, nothing needs to be larger than these.

So if you have a 200A main disconnect, and conductors sized for it, other feeders 'downstream' of the main disconnect need not be larger than the main service conductors. A 200A main followed by a 200A feeder you indirectly size the feeder using the 83% rule.

But say you have a 100A feeder downstream of this 200A main, then you can't use the 83% rule for the 100A feeder.

-Jon

 

[SolarAlternativesDesign]

As long as the load is 180 amps or less, 4/0 aluminum with an ampacity of 180 amps can be protected by a 200 amp OCPD per 240.4(B), assuming all terminations are listed for 75°C.

Thank you for bringing that up. I've been confused about when we can use the next higher OCPD. It seems like usually people size conductors connected to service panels for the breaker actually used, rather than next size up. I think it's because 240.4(B)(1) says the next size up cannot be used if the conductors being protected are part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads. So I thought I couldn't use it for conductors to a backed up loads panel. I do use it for the PV circuits up to the inverter though. Any clarification?

 

[jaggedben]

Thank you for bringing that up. I've been confused about when we can use the next higher OCPD. It seems like usually people size conductors connected to service panels for the breaker actually used, rather than next size up. I think it's because 240.4(B)(1) says the next size up cannot be used if the conductors being protected are part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads. So I thought I couldn't use it for conductors to a backed up loads panel. I do use it for the PV circuits up to the inverter though. Any clarification?

The thing is, it's moot. 310.15(B)(7) allows your 4/0 AL conductors even if the load calc is 190A. I'd also wager that no one has done a load calc on this residence, and that the load calc is less than 180A. But either way you're fine.

And yes, you can use the next size up rule for inverter output conductors.

 

[jaggedben]

That is quite correct, but must be understood to be a specific situation.

The 83% rule applies only to conductors that carry the full load of a dwelling. But after you've sized the conductors that carry the full load of the dwelling, nothing needs to be larger than these.

So if you have a 200A main disconnect, and conductors sized for it, other feeders 'downstream' of the main disconnect need not be larger than the main service conductors. A 200A main followed by a 200A feeder you indirectly size the feeder using the 83% rule.

But say you have a 100A feeder downstream of this 200A main, then you can't use the 83% rule for the 100A feeder.

-Jon

Agreed. It is pretty clear that it applies to our OP though, because he has a 200A main service panel and a 200A feeder breaker.

 

[jaggedben]

Thank you for bringing that up. I've been confused about when we can use the next higher OCPD. It seems like usually people size conductors connected to service panels for the breaker actually used, rather than next size up. I think it's because 240.4(B)(1) says the next size up cannot be used if the conductors being protected are part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads. So I thought I couldn't use it for conductors to a backed up loads panel. I do use it for the PV circuits up to the inverter though. Any clarification?

Meant to add...
I think residential electricians often size wires to breakers because they can't be bothered with doing accurate load calcs. Customer wants a 200A upgrade means they get a 200A breaker, they don't know what the wire is supposed to be. Commercial projects are more likely to involve real load calcs.

 

[SolarAlternativesDesign]

I should have attached the SLD to eliminate confusion. For the MSP - smart switch circuit I think you have all confirmed we can use the 240.4(B) next size up rule or the 83% rule and the 180A 4/0 Al is fine.

However in the smart switch - backed up panel circuit, the max current could be 166A from MSP + 16.9A from solar + 10.6A from battery = 193.5A which is greater than the 180A ampacity of the 4/0 Al. We could solve this by limiting the current to the backed up loads panel with a 175A breaker.

 

[jaggedben]

....We could solve this by limiting the current to the backed up loads panel with a 175A breaker.

Or a 200A breaker, which will save you some money.

This is why the Enpower switch has a provision to put a breaker there for the backup loads. You'll need it most of the time.

 

[SolarAlternativesDesign]

I don't understand how the 200A breaker would solve the problem of the 193.5A operational current being greater than the 180A ampacity of the 4/0 Al cable.