Please explain this 705.12 rule

[electrofelon]

1. (2)Taps. In systems where power source output connections are made at feeders, any taps shall be sized based on the sum of 125 percent of the power source(s) output circuit current and the rating of the overcurrent device protecting the feeder conductors as calculated in 240.21(B).

 

[wwhitney]

Say you have a 150A feeder from a meter/main disconnect running to a 150A main breaker panel with loads. You want to do a feeder interconnect for PV and intercept the feeder somewhere in between. Your PV inverter has a 40A inverter output current, so you to splice on some #8 / 75C conductors which run 20' to a 50A OCPD and then the inverter.

That's a problem, because while 50A complies with the 25' tap rule with respect to the 150A breaker at the service, it is smaller than 1/3 of 50+150. So you need to run conductors with an ampacity of at least 67A to comply with the rule in the OP.

Cheers, Wayne

 

[pv_n00b]

I have a different take on it than wwhitney. This is not about sizing the PV system interconnection conductor.
Say you have a feeder from a service entrance panel to a distribution panel and that feeder has a PV system interconnection tap already in it. You want to add another tap to that feeder to feed another distribution panel. To size the conductor for that tap you have to add the rating of the feeder OCPD in the service entrance panel to 125% of the PV output and use that to enter 240.21(B) to size the new tap conductor.

 

[electrofelon]

Say you have a 150A feeder from a meter/main disconnect running to a 150A main breaker panel with loads. You want to do a feeder interconnect for PV and intercept the feeder somewhere in between. Your PV inverter has a 40A inverter output current, so you to splice on some #8 / 75C conductors which run 20' to a 50A OCPD and then the inverter.

That's a problem, because while 50A complies with the 25' tap rule with respect to the 150A breaker at the service, it is smaller than 1/3 of 50+150. So you need to run conductors with an ampacity of at least 67A to comply with the rule in the OP.

Cheers, Wayne

Okay, I was racking my brain trying to figure out how the tap conductor would be overloaded. It's only overloaded in the sense of the 1/3 or 1/10 sizing considering all of the sources that can feed the tap?

 

[wwhitney]

I have a different take on it than wwhitney. This is not about sizing the PV system interconnection conductor.

It's not specifically about that feeder, but it does apply to that feeder. Was trying to come up with an example you might run into in the field.

Here's another example: You have a 200A feeder that supplies a 200A panel. That 200A feeder also has a 70A 20' long tap to a 70A OCPD. You'd like to add 32A PV inverter output to the 200A panel. If you do that, you'll have to upsize the feeder tap conductors to at least 80A, as 70A < 1/3 * (200 + 125% * 32).

Cheers, Wayne

 

[wwhitney]

Okay, I was racking my brain trying to figure out how the tap conductor would be overloaded. It's only overloaded in the sense of the 1/3 or 1/10 sizing considering all of the sources that can feed the tap?

Yes. And arguably this rule is unnecessary, in so far as the 1/3 and 1/10 limits of the tap rules have to do with the withstand rating of conductors during a fault, and in so far as they are based on the available fault current you might see from a service. Because even though the PV inverter is another potential source of energy into the fault, its fault current contribution is probably negligible compared to the utility service.

Cheers, Wayne

 

[Zee]

Caution: Many AHJ's dont allow interconnecting with a tap onto feeders, period.

 

[electrofelon]

I have a different take on it than wwhitney. This is not about sizing the PV system interconnection conductor.
Say you have a feeder from a service entrance panel to a distribution panel and that feeder has a PV system interconnection tap already in it. You want to add another tap to that feeder to feed another distribution panel. To size the conductor for that tap you have to add the rating of the feeder OCPD in the service entrance panel to 125% of the PV output and use that to enter 240.21(B) to size the new tap conductor.

Hmmm

 

[electrofelon]

Caution: Many AHJ's dont allow interconnecting with a tap onto feeders, period.

Any idea why not? Seems arbitrary.

 

[Zee]

Been a while since I ran the thought experiment on load side taps..... but essentially it is easier to overload the feeder conductors than when tying in line side. Something about the solar current being ADDITIVE downstream towards sub panel.
I think the AHJ has a point.

 

[tortuga]

arguably this rule is unnecessary, in so far as the 1/3 and 1/10 limits of the tap rules have to do with the withstand rating of conductors during a fault, and in so far as they are based on the available fault current you might see from a service. Because even though the PV inverter is another potential source of energy into the fault

I think it could be argued the 125% of the inverter output is unnecessary but my understanding is inverters can feed into a bolted fault much like other syncronized generation sources and of course motors.

 

[wwhitney]

Caution: Many AHJ's dont allow interconnecting with a tap onto feeders, period.

What new idiocy is this?

Been a while since I ran the thought experiment on load side taps..... but essentially it is easier to overload the feeder conductors than when tying in line side. Something about the solar current being ADDITIVE downstream towards sub panel.

2023 NEC 705.12(A)(2) takes this into account.

Cheers, Wayne

 

[jaggedben]

@ electrofelon Seems like you quoted the 2017 version. It might be interesting to look at the 2020:

1. Where power source output connections are made at feeders, all taps shall be sized based on the sum of 125 percent of all power source(s) output circuit current(s) and the rating of the overcurrent device protecting the feeder conductors for sizing tap conductors using the calculations in 240.21 (B).

and the 2023:

For taps sized in accordance with 240.21 (B) (2) or (B) (4), the ampacity of taps conductors shall not be less than one-third of the sum of the rating of the overcurrent device protecting the feeder plus the ratings of any power source overcurrent devices connected to the feeder.

FWIW I put in a PI to improve the grammar in 2020 and they rejected it although they came up with something similar in 2023. I think the 2023 wording best gets at the intent (leaving aside that they now simply don't address 10ft taps). The other versions don't do well at avoiding the potential confusion that the tap needs to be the sum of both sources.

 

[jaggedben]

I have a different take on it than wwhitney. This is not about sizing the PV system interconnection conductor.
Say you have a feeder from a service entrance panel to a distribution panel and that feeder has a PV system interconnection tap already in it. You want to add another tap to that feeder to feed another distribution panel. To size the conductor for that tap you have to add the rating of the feeder OCPD in the service entrance panel to 125% of the PV output and use that to enter 240.21(B) to size the new tap conductor.

As the addition of the word 'all' in 2020 addresses, it's both taps to sources and taps to loads. Any tap that can be fed from two (or more) overcurrent devices.

 

[wwhitney]

I think it could be argued the 125% of the inverter output is unnecessary but my understanding is inverters can feed into a bolted fault much like other syncronized generation sources and of course motors.

Sure, but my understanding is that a PV inverter will have a maximum fault current of at most 2x its rated output. While the fault current from the utility is anywhere from 10x to 1000x the feeder rating. Hence my statement that in terms of fault current, the PV inverter's contribution is likely negligible. So if the 1/3 and 1/10 rules are based on fault current considerations, ignoring the PV contribution is a much closer approximation to reality than combining the feeder OCPD rating and the PV rating on an equal footing.

Cheers, Wayne

 

[jaggedben]

What Wayne said the post #15. He beat me to it while I was typing.

Perhaps on very large PV systems it might not be as true. In a resi context it certainly is.

 

[wwhitney]

(leaving aside that they now simply don't address 10ft taps).

That makes me wonder if maybe the 1/3 factor in the 25' tap rule has some other basis, for which it would make sense to add the ratings, while the 1/10 factor in the 10' rule is in fact based on fault current considerations, and so the omissions recognizes the argument from post #15 with respect to the 10' tap rule. In which case the question would be what is the basis for the 1/3 factor in the 25' tap rule?

Cheers, Wayne

 

[ggunn]

Been a while since I ran the thought experiment on load side taps..... but essentially it is easier to overload the feeder conductors than when tying in line side. Something about the solar current being ADDITIVE downstream towards sub panel.
I think the AHJ has a point.

As long as the sub panel has OCP on the load side of the tap there is no problem.