service and back to back panel

Status
Not open for further replies.
R

Rearic

Member
Location
Medina, NY, usa
Guys, I'm in need of some re-education.
I've got a service ( 200 amp) with a 200 amp meter/main combo on the outside. #2/0 cu. service conductors.
Let's say I feed an A/c outside with a 50 a and then come off the bottom of the panel go through the wall to the inside of the garage and feed a 200 amp panel.
1) does the feeder through the wall have to be #4/0 because it is no longer covered by the .83 reduction?
2) does the inside panel need a main?

any thoughts ?
 
Last edited:
My opinion, the service entrance conductors would need to be 3/0 CU (Table 310.16, 75C column allows 200 amp)because of what you described, but mostly, there needs to be a location that all disconnects can be thrown, either inside or outside and grouped together to comply with 230.72. I would not approve one outside and one inside. Note: I am currently still utilizing the 2008 Edition for my jurisdiction.
 
102 Inspector said:
My opinion, the service entrance conductors would need to be 3/0 CU (Table 310.16, 75C column allows 200 amp)because of what you described, but mostly, there needs to be a location that all disconnects can be thrown, either inside or outside and grouped together to comply with 230.72. I would not approve one outside and one inside. Note: I am currently still utilizing the 2008 Edition for my jurisdiction.
Click to expand...
Good point. I just assumed that the wires to the inside panel were feeders that were protected by a breaker in the meter main.
But the OPs description could also apply to taking a parallel feed off the load side of the meter main and running it to an MCB panel inside. In that case the wires would be service conductors rather than a feeder and it would violate the grouping rule for service disconnects.
 
102 Inspector said:
My opinion, the service entrance conductors would need to be 3/0 CU (Table 310.16, 75C column allows 200 amp)because of what you described, but mostly, there needs to be a location that all disconnects can be thrown, either inside or outside and grouped together to comply with 230.72. I would not approve one outside and one inside. Note: I am currently still utilizing the 2008 Edition for my jurisdiction.
Click to expand...

A panel with a main breaker inside in the OPs scenario would not be a service disconnect and is allowed. Even in the 2008.
 
102 Inspector said:
My opinion, the service entrance conductors would need to be 3/0 CU (Table 310.16, 75C column allows 200 amp)because of what you described, but mostly, there needs to be a location that all disconnects can be thrown, either inside or outside and grouped together to comply with 230.72. I would not approve one outside and one inside. Note: I am currently still utilizing the 2008 Edition for my jurisdiction.
Click to expand...
He said a 200A meter/main combo outside... and I am taking that to mean the 50A A/C is a branch circuit supplied by the meter/main panel, while the inside run is a feeder supplied by feed-thru lugs at bottom of meter/main panel bus (i.e has 200A OCP).

The 2/0 service entrance conductors are on the supply side of the meter/main, and I believe qualify as compliant under 310.15(B)(6) per 2008 NEC.
 
jumper said:
They are. The feed through lugs are at the bottom of the bus and protected by the main breaker.
Click to expand...

I fell victim to the age old confusion between coming off the bottom of the panel (the assembly of innards only) and coming out the bottom of the enclosure, originating anywhere inside.
 
I also agree that Table 310.15(B)(6) would allow 2/0 CU for the feeder. Not sure if the OP is under 2008 or newer edition. The Meter/Main combo would be accepted if the combo pack disconnects the 50A and 200 Amp panel are at the same location. was not sure if OP had considered location, that is why I tossed that in there.
 
What I mean is the service disconnect is the 200 amp breaker in the meter/main.
coming off the feed through lugs at the bottom I go through the wall to an MLO panel.
considering that the conductors through the wall are now feeders , must the wire size be raised from the #2/0 to #3/0 ?
does this make sense?
 
Last edited:
102 Inspector said:
I also agree that Table 310.15(B)(6) would allow 2/0 CU for the feeder. Not sure if the OP is under 2008 or newer edition. The Meter/Main combo would be accepted if the combo pack disconnects the 50A and 200 Amp panel are at the same location. was not sure if OP had considered location, that is why I tossed that in there.
Click to expand...


I would also agree that the feeder need not be larger than the Service conductors also. Even if Back to back. Regardless if they carry the entire load or not. They sure carry less than the entire load. IMHO
 
Rearic said:
What I mean is the service disconnect is the 200 amp breaker in the meter/main.
coming off the feed through lugs at the bottom I go through the wall to an MLO panel.
considering that the conductors through the wall are now feeders , must the wire size be raised from the #2/0 to #3/0 ?
does this make sense?
Click to expand...

Yes the wire must be changed to 3/0 and yes it makes no sense. Proposals for sanity in changing this keep getting shot down.
 
I guess I was hoping there was a little exception somewhere that finally cleared up this age old conundrum. The service wires can use the service reduction factor. But, after the main disconnect, the wire must then be sized by 310-15. even when common sense tells us that the panel on the inside will have less load after the A/C power load is subtracted from the outside meter main combo. Its just hard to look the contractor in the eye and explain this .
 
jumper said:
Wait a minute ....maybe not

(3) In no case shall a feeder for an individual dwelling unit
be required to have an ampacity greater than that speci-
fied in 310.15(B)(7)(1) or (2)
Click to expand...

Yes that is correct, 310.15(B)(7)(3) applies here. That said, notice that it is ampacity not physical size so in some cases you might end up with a physically bigger conductor than the service conductors if you had to derate for some reason. So assuming that no derating is required you could use 2/0 CU in this case.
 
texie said:
Yes that is correct, 310.15(B)(7)(3) applies here. That said, notice that it is ampacity not physical size so in some cases you might end up with a physically bigger conductor than the service conductors if you had to derate for some reason. So assuming that no derating is required you could use 2/0 CU in this case.
Click to expand...
But we are not feeding an individual dwelling anymore, one circuit got left off at the supply end of the feeder.


Quite frankly I doubt it would hurt to run same size feeder as the service conductor, but that is not what is written IMO.


OP did say 4/0. Maybe he is using aluminum? If so 4/0 is allowed to be protected by 200 amp as long as load calculation doesn't exceed 180.
 
kwired said:
But we are not feeding an individual dwelling anymore, one circuit got left off at the supply end of the feeder.
Click to expand...
I've been tossing this around for a bit now, and I believe it'll come down to the AHJ's interpretation... but technically, the feed-thru conductors are just extensions of the main busbar. As such, the one circuit (50A A/C) is just a circuit that is pulled off before the feeder enters the structure. The feeder itself, be it busbar or wire, is still the main power feeder for the dwelling unit.
 
Smart $ said:
I've been tossing this around for a bit now, and I believe it'll come down to the AHJ's interpretation... but technically, the feed-thru conductors are just extensions of the main busbar. As such, the one circuit (50A A/C) is just a circuit that is pulled off before the feeder enters the structure. The feeder itself, be it busbar or wire, is still the main power feeder for the dwelling unit.
Click to expand...
We all know the feeder will draw less then the service conductor, NEC is not written to acknowledge that fact though.
 
kwired said:
We all know the feeder will draw less then the service conductor, NEC is not written to acknowledge that fact though.
Click to expand...
If you understand the point I was attempting to make, you would see it draws the same. It just draws less after the A/C circuit. The feeder starts at the load side of the 200A main breaker in the meter/main panel. It may be busbar at that point, but it is still a feeder.
 
Status
Not open for further replies.
Top