Derating MSP w/ feed through conductors

ohmti787

Member
Location
Orlando, FL
Occupation
Electrical Engineer
If I derate the MCB at a MSP w/ feed through lugs , should I still consider the potential of overloading the feed through conductors? On a regular panel, even if you derate the MCB, you would still need to install your PV breaker at the opposite end of the busbar from the MCB. But having feed through conductors kinda presents a whole different issue since you're technically not at the opposite end. Can I get some insight on this manner?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
If I derate the MCB at a MSP w/ feed through lugs , should I still consider the potential of overloading the feed through conductors? On a regular panel, even if you derate the MCB, you would still need to install your PV breaker at the opposite end of the busbar from the MCB. But having feed through conductors kinda presents a whole different issue since you're technically not at the opposite end. Can I get some insight on this manner?
As long as there is an appropriate main breaker on the sub at the other end of the feeder I think you are OK with the existing feeder.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
The feed through conductors must be rated for the combined current of the utility plus 125% of PV sources that can add together on them, or else protected by their own overcurrent device.

For example, if you had a 200A/200A main panel with feed through lugs, and you derated the main breaker to 175A, you could probably only put 20A of inverter output (25A after multiplying by 125%) without protecting the feed through conductors with a 200A or less device. That is unless the feed through conductors and their subpanel are miraculously rated for more than 200A.

This is addressed in 705.12, I believe it's 705.12(2) in the 2020 numbering.
 

ohmti787

Member
Location
Orlando, FL
Occupation
Electrical Engineer
As long as there is an appropriate main breaker on the sub at the other end of the feeder I think you are OK with the existing feeder.
That's the way I see it. Problem is this particular inspector won't let me tap the feed through conductors and put a breaker downstream, but he'll let me derate the mcb at the main without the additional breaker at the sub. 🤷‍♂️
 

PWDickerson

Senior Member
Location
Clinton, WA
Occupation
Solar Contractor
Looks like Florida is on the 2017 NEC. 2017 NEC 705.12(B)(2)(1)(b) states "Feeders: Where the power source output connection is made to a feeder at a location other than the opposite end of the feeder from the primary source overcurrent device, that portion of the feeder on the load side of the power source output connection shall be protected by one of the following: An overcurrent device on the load side of the power source connection shall be rated not greater than the ampacity of the feeder". You should be able to just read him this section of the code.
 

ceb58

Senior Member
Location
Raeford, NC
We have ran into a problem now with feed through lugs in a meter combo. We install a 40 amp back fed in the combo for the inverter and the inspector is now failing it because the feeder now doesn't carry the full load as described in 310.3 7 (2) we now have to carry the conductors from the lugs to a disconnect and fuse the feeders down to 175amps.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
because the feeder now doesn't carry the full load as described in 310.3 7 (2)
I don't quite follow how that's true, but regardless doesn't the next subsection exempt you from having to downsize the OCPD? I.e. on a 200A residential 3-wire service, 2/0 Cu with 75C terminations is sufficient for any 200A feeder.

Cheers, Wayne
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
We have ran into a problem now with feed through lugs in a meter combo. We install a 40 amp back fed in the combo for the inverter and the inspector is now failing it because the feeder now doesn't carry the full load as described in 310.3 7 (2) we now have to carry the conductors from the lugs to a disconnect and fuse the feeders down to 175amps.
I'm not following you completely, but if you have installed a 40A backfed PV breaker in a 200A feedthrough meter main and the subpanel connected to the feedthrough is 200A MLO, then the inspector is correct. The current available to the subpanel is 200A plus 125% of the inverter current and it all comes in on the same end of the bus, so it is a 705.12(B) violation.
 

ohmti787

Member
Location
Orlando, FL
Occupation
Electrical Engineer
Looks like Florida is on the 2017 NEC. 2017 NEC 705.12(B)(2)(1)(b) states "Feeders: Where the power source output connection is made to a feeder at a location other than the opposite end of the feeder from the primary source overcurrent device, that portion of the feeder on the load side of the power source output connection shall be protected by one of the following: An overcurrent device on the load side of the power source connection shall be rated not greater than the ampacity of the feeder". You should be able to just read him this section of the code.
Oh I have, trust me. But he keeps coming back at me saying that adding the breaker downstream will protect the subpanel, but it wont protect the meter combo. Sometimes you just can't win with these guys.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Oh I have, trust me. But he keeps coming back at me saying that adding the breaker downstream will protect the subpanel, but it wont protect the meter combo. Sometimes you just can't win with these guys.
I feel your pain. We had a situation a few years ago where the inspector's claim would only be valid if additive currents could flow both directions in a conductor simultaneously; the inspector did not understand basic electricity and he had no technical oversight in his chain of command.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Oh I have, trust me. But he keeps coming back at me saying that adding the breaker downstream will protect the subpanel, but it wont protect the meter combo. Sometimes you just can't win with these guys.
What is your PV output and your main panel busbar rating?
 

ceb58

Senior Member
Location
Raeford, NC
I don't quite follow how that's true, but regardless doesn't the next subsection exempt you from having to downsize the OCPD? I.e. on a 200A residential 3-wire service, 2/0 Cu with 75C terminations is sufficient for any 200A feeder.

Cheers, Wayne
Its the same as if you installed a 30 amp breaker for a A/C unit in the meter combo. That feeder is now not carrying the full load. So now the 4/0 alum feeder is only good for 180 amps.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Its the same as if you installed a 30 amp breaker for a A/C unit in the meter combo. That feeder is now not carrying the full load. So now the 4/0 alum feeder is only good for 180 amps.
Sure, except that the next section (2017) 310.15(B)(7)(3) says that as 4/0 Al is large enough for a 200A dwelling unit service carrying the full load, it's large enough for any 200A feeder in the dwelling unit, even if it's not carrying the full load.

From the subsequent discussion, it sounds like the reason for downsizing the breaker was not 310.15(B)(7) related.

Cheers, Wayne
 
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