NEC 705.12(B)(1)(b) Load Side Connection at Feeder

wwhitney

Senior Member
Location
Berkeley, CA
Hello,

Is NEC 705.12(B)(1)(b) commonly interpreted to require the feeder on the load side of the power source connection point to terminate on a single overcurrent device? Because if not, it seems the topology below would be allowed, but that feeder segment (B) could be overloaded (when the energy storage is charging).

Cheers, Wayne

Code:
200A Utility OCPD
|
| 200A Feeder Segment A
|
|---- 75A OCPD -- PV Inverter
|
| 200A Feeder Segment B
|
|---- 125A OCPD -- Energy Storage
|
|  200A Feeder Segment C
|
200A Main Breaker Load Panel
 

jaggedben

Senior Member
As far as the 'commonly interpreted' part, I don't know if the question has come up often enough.

I'd agree you have a problem with segment B. Either put another OCPD on that segment, or (oddly enough) switch the PV inverter and energy storage positions.

You're always coming up with the most boundary-pushing thought experiments. ;)
 

wwhitney

Senior Member
Location
Berkeley, CA
In this case, the example is based on thinking about the new Tesla Backup Gateway 2 for their Powerwall. It has an extra set of lugs on the line side of the transfer switch (although only 100A rated) as well as the same double lugs on the load side as the previous version. To my understanding it doesn't create the problem illustrated in the OP, but the topology is pretty close, so I started thinking about similar topologies.

My other interest is in reconciling the rules in 705.12(B) on busbars versus feeders, as a busbar is just a feeder with a bus topology.

Cheers, Wayne
 

jaggedben

Senior Member
In this case, the example is based on thinking about the new Tesla Backup Gateway 2 for their Powerwall. It has an extra set of lugs on the line side of the transfer switch (although only 100A rated) as well as the same double lugs on the load side as the previous version. To my understanding it doesn't create the problem illustrated in the OP, but the topology is pretty close, so I started thinking about similar topologies.
It certainly seems like they are trying to confuse people. Why did they do that?

My other interest is in reconciling the rules in 705.12(B) on busbars versus feeders, as a busbar is just a feeder with a bus topology.
To my understanding busbars are treated differently because that topology calls for different rules. The thermal issues are just different when you have so many connections crammed right next to each other. At least, the CMP has not assumed they are just the same, and I see why not.
 

Carultch

Senior Member
Location
Massachusetts
My other interest is in reconciling the rules in 705.12(B) on busbars versus feeders, as a busbar is just a feeder with a bus topology.
The essential difference is that most busbars are expected to be a part of a load center/panelboard/switchboard/etc, and have multiple loads branched off of them. By contrast, a feeder is expected to start without any tapped circuits that would complicate the current flow. The title of the busbar section uses the word "busbars" instead of "panelboards", to generalize it for applying to switchboards and other similar 1200A+ equipment. The rule didn't consider the whether it would also have to apply to extensive length busbars, being used like feeders usually would be used. Such as what you might see in high rise construction, to deliver power to the top floors.

With feeders, the rule that one should apply, even if not explicitly written in the NEC, is to make sure that Kirchhoff's current law doesn't ever give the possibility of impose any current that exceeds the feeder ampacity, without tripping an OCPD. The general concept that a circuit needs an ampacity for the load, is what tells us we would need to do this. Thus avoiding situations where normal operating current would locally exceed the ampacity of any section of conductor.
 

wwhitney

Senior Member
Location
Berkeley, CA
It certainly seems like they are trying to confuse people. Why did they do that?
To make it easy to add a panel (limited to 100A supply) for non-backed up loads. Previously the easiest way do that was to install a pair of Polaris-type connectors in the Backup Gateway 1 on the line side.

The Backup Gateway 2 also accepts an optional six space 200A panelboard, which you can feed from either the extra line side set of lugs, or from one of the load side set of lugs. In the former case they tell you to backfeed a breaker on the internal panelboard to comply with 705.12(B)(1)(b).

Cheers, Wayne
 

wwhitney

Senior Member
Location
Berkeley, CA
By contrast, a feeder is expected to start without any tapped circuits that would complicate the current flow.
But that expectation is no longer reasonable given the increasing prevalence of power sources other than the utility. And the overly strict rules on busbars drive the use of more complicated wire-type feeder topologies.

For example, 705.12(B)(3) does not allow a panelboard with a 200A breaker with utility feed, a 200A breaker with PV/ESS feed, a 200A breaker for a load panel, and no other breakers. But replace that busbar with a wire-type feeder, and 705.12(B)(1) allow the same topology.

With feeders, the rule that one should apply, even if not explicitly written in the NEC, is to make sure that Kirchhoff's current law doesn't ever give the possibility of impose any current that exceeds the feeder ampacity, without tripping an OCPD.
I agree. Can we put that in code language and rewrite 705.12(B)? : - )

Cheers, Wayne
 
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