120% Conductor Rule?

[Dennis Alwon]

The 4/0 aluminum is SER so depending on the code cycle that code be rated 60C which means 150amps. If that is the case then it is not compliant since it is on the load side of the 200 amp breaker

 

[shortcircuit2]

I don't think that the feeder to the second panel qualifies under 310.15(B)(7) and even if it did, I don't think that section changes the ampacity of the conductor. The ampacity of the 4/0 AL is 180 so the maximum solar input breaker would be 15 amps for that 200 amp service.

I too agree that 310.15(B)(7) would not change the ampacity. It just allows the reduced value for dwellings.

Could you please show the formula used to come up with the 15amp solar input?

 

[don_resqcapt19]

I too agree that 310.15(B)(7) would not change the ampacity. It just allows the reduced value for dwellings.

Could you please show the formula used to come up with the 15amp solar input?

120% of the 180 amp rated ampacity of the 4/0 AL is 216 so that is the total of the two breakers that can supply power to the 4/0. The 200 main and a 15 amp solar breaker (or a custom 16 amp breaker).

If you would change the main to 175 amp, then you could still install a 40 amp solar breaker without exceeding 120% of the conductor ampacity.

 

[jaggedben]

Having thought about this further, it makes a difference if the second panel is an MLO panel or a main breaker panel. If it is an MLO panel and the only overcurrent protection is the main breaker in the first panel, then installing solar in the first panel violates 705.12(D)(7) for the second panel.

It seems to me the easiest way to address the situation, if possible, is to re-land the feeders to the second panel on a smaller breaker in the first panel.

 

[Dennis Alwon]

It seems to me the easiest way to address the situation, if possible, is to re-land the feeders to the second panel on a smaller breaker in the first panel.

I would guess that that is not possible as the bus for those panels typically cannot have more than 125amp breakers on them.

 

[Sierrasparky]

Having thought about this further, it makes a difference if the second panel is an MLO panel or a main breaker panel. If it is an MLO panel and the only overcurrent protection is the main breaker in the first panel, then installing solar in the first panel violates 705.12(D)(7) for the second panel.

It seems to me the easiest way to address the situation, if possible, is to re-land the feeders to the second panel on a smaller breaker in the first panel.

Well I guess it is time to chime in here. If lthe feeder 4/0 SER cable carries the entire load then it is rated at 200 AMPS.
If the breaker feeding this is reduced to 175 then the EGC would need to be increased.

IF however the wire is rated at only 150 ( 60 deg. table) because it does not carry the entire load then there is no issue of using a smaller breaker on this feeder other than buss issues.

I thought I would chime in on this thread and the sillyness of the broad scope of 250.122 and how it affects/effects this thread/topic

 

[Smart $]

The 4/0 aluminum is SER so depending on the code cycle that code be rated 60C which means 150amps. If that is the case then it is not compliant since it is on the load side of the 200 amp breaker

Well I guess it is time to chime in here. If lthe feeder 4/0 SER cable carries the entire load then it is rated at 200 AMPS.
...

IF however the wire is rated at only 150 ( 60 deg. table) because it does not carry the entire load then there is no issue of using a smaller breaker on this feeder other than buss issues.

OP'er stated installation under 2011 NEC. SER cable used as feeder is only subject to 60?C temperature rating where installed in thermal insulation: 338.10(B)(4)(a).

If the breaker feeding this is reduced to 175 then the EGC would need to be increased.
...
I thought I would chime in on this thread and the sillyness of the broad scope of 250.122 and how it affects/effects this thread/topic

4/0 Aluminum SER cable has an EGC smaller than #4???

 

[Sierrasparky]

OP'er stated installation under 2011 NEC. SER cable used as feeder is only subject to 60?C temperature rating where installed in thermal insulation: 338.10(B)(4)(a).



4/0 Aluminum SER cable has an EGC smaller than #4???

4/0 has a #4 EGC, but if the wire was used as 200 amp and rated for such and then reduced rating to 175 then as per 250.122 the EGC would need to be #3 ..... Pretty foolish don't ya think!
250.122 does not give the option as to why the wire was upsized. So if you run the calculation based on the percentage upsize then you will see that #4 is too small. You must use #3.

In other words if derating the feeder makes you comply with the Solar PV regulations you now are Violating the Grounding Regulations. Silly Yes :?

 

[Dennis Alwon]

4/0 has a #4 EGC, but if the wire was used as 200 amp and rated for such and then reduced rating to 175 then as per 250.122 the EGC would need to be #3 ..... Pretty foolish don't ya think!
250.122 does not give the option as to why the wire was upsized. So if you run the calculation based on the percentage upsize then you will see that #4 is too small. You must use #3.

In other words if derating the feeder makes you comply with the Solar PV regulations you now are Violating the Grounding Regulations. Silly Yes :?

Yes it is silly to some degree and the reality is that most inspectors would not catch it or perhaps care but.....

if the 4/0 ser is rated 75C then it is rated for 180 amps and thus could be used on a 175 amp breaker. How is that upsizing it.

What I am getting at is that a 4/0 aluminum with the same insulation may be rated 150, 180 or 200 amps.

This is where it gets even crazier because the same wire could be rated 60C and only be rated 150 amps.

 

[shortcircuit2]

120% of the 180 amp rated ampacity of the 4/0 AL is 216 so that is the total of the two breakers that can supply power to the 4/0. The 200 main and a 15 amp solar breaker (or a custom 16 amp breaker).

If you would change the main to 175 amp, then you could still install a 40 amp solar breaker without exceeding 120% of the conductor ampacity.

Thanks Don...you come up with some very good angles to look at things...

 

[shortcircuit2]

One thing I was trying to point out here is a situation where a conductor is affected by the 120% rule of 705.12(D) (2) of the 2011 NEC

Other printed articles and forum posts I've read have discussed that the CONDUCTOR part of the 120% rule is of little concern and only would apply to taps to a feeder where a source is at each end. I have pointed out in my example that there are existing installations in the real world that the 120% rule would apply to a conductor.

 

[shortcircuit2]

Having thought about this further, it makes a difference if the second panel is an MLO panel or a main breaker panel. If it is an MLO panel and the only overcurrent protection is the main breaker in the first panel, then installing solar in the first panel violates 705.12(D)(7) for the second panel.

It seems to me the easiest way to address the situation, if possible, is to re-land the feeders to the second panel on a smaller breaker in the first panel.

I don't see how landing the PV in the 1st panel violates the wording of 705.12(D) (7)?

705.12(D) (7) just says to use the breaker of the upstream panel in the 120% calculation.

705.12(D) says the inverter output can be connected to the load side of any distribution equipment on the premises.

 

[Sierrasparky]

I need to correct something.
I just looked at the Southwire website and the 4/0 AL SER has a 2/0 ground not a #4 as I thought.

Dennis , Section 250.122 as elaborated by others and possibly you states any or all upsizing of the ccc. We are only playing symantics by using Dwelling feeder table when it best suits one. or use the ampacity table for conductors when it suits then. In the case here the ampacity is 200amps for a dwelling feeder. use table 310.15 (B) (7). Now if you use a Lower breaker one could say you have an upsized feeder in this particular installation. If you now use the wire at 175 A the difference in circular mil is roughly 1.26%. So if you calculate the current EGC of 2/0 by 1.26 you arrive at a 3/0 wire for the EGC. This is silly.

I have no understanding where you get that 4/0 SER is rated for 175 in a dwelling that is not the Main power feeder. This is a modern home and the service generally originates on an outside wall and not is installed in a wall or ceilinig in insulation I would be very surprised. ( I suppose it is stapled to the outside and run directly to the basement somehow and not contacting insulation)
So the rating would be from the 60 deg table and be only 150.
In that case the EGC is too small as per some opinions in other threads on this forum.
i guess we just pick an choose what works for ourselves at the time. What a wonderful code.

 

[Dennis Alwon]

So there are many installations existing in the real world that I have described that were code compliant at the time. Also there are parts of the country that are still under the 2005 NEC or earlier editions.

IMO, this was not compliant in the 2005 but was misread or misinterpreted. The 2008 just clarified the intent of the 2005

 

[Dennis Alwon]

Dennis , Section 250.122 as elaborated by others and possibly you states any or all upsizing of the ccc. We are only playing symantics by using Dwelling feeder table when it best suits one. or use the ampacity table for conductors when it suits then. In the case here the ampacity is 200amps for a dwelling feeder. use table 310.15 (B) (7). Now if you use a Lower breaker one could say you have an upsized feeder in this particular installation. If you now use the wire at 175 A the difference in circular mil is roughly 1.26%. So if you calculate the current EGC of 2/0 by 1.26 you arrive at a 3/0 wire for the EGC. This is silly.

I have no understanding where you get that 4/0 SER is rated for 175 in a dwelling that is not the Main power feeder. This is a modern home and the service generally originates on an outside wall and not is installed in a wall or ceilinig in insulation I would be very surprised. ( I suppose it is stapled to the outside and run directly to the basement somehow and not contacting insulation)
So the rating would be from the 60 deg table and be only 150.
In that case the EGC is too small as per some opinions in other threads on this forum.
i guess we just pick an choose what works for ourselves at the time. What a wonderful code.[/QUOTE]

I think this discussion belongs in a different thread or its own thread but not here. I will be happy to discuss it there.

 

[Sierrasparky]

No problem to move to another post.
i was just trying to demostrate the sillyness and the overly broad interpetation of 250.122 and how it can inadvertainly make a perfectly legitimate installation unlawful.

 

[shortcircuit2]

IMO, this was not compliant in the 2005 but was misread or misinterpreted. The 2008 just clarified the intent of the 2005

Would it be compliant if the computed load were 180amps and then next size up breaker rule for the 200amp protection?

 

[Dennis Alwon]

Would it be compliant if the computed load were 180amps and then next size up breaker rule for the 200amp protection?

That is if the second panel had the total load of the service and it was not ser. I am quite certain that the 4/0 SER is only rated 150 amps and was compliant at the 75C rating years ago. In that case then the calculated is 180 or less then it would be compliant. It depends on when the install was made.

Quite frankly I don't think this is a major issue in a residence.

 

[jaggedben]

I don't see how landing the PV in the 1st panel violates the wording of 705.12(D) (7)?

705.12(D) (7) just says to use the breaker of the upstream panel in the 120% calculation.

705.12(D) says the inverter output can be connected to the load side of any distribution equipment on the premises.

Perhaps it doesn't violate the wording exactly. But feeding an MLO panel from one end with supplies that total more than its rating seems to go against the spirit of it.

 

[Smart $]

I need to correct something.
I just looked at the Southwire website and the 4/0 AL SER has a 2/0 ground not a #4 as I thought.

Dennis , Section 250.122 as elaborated by others and possibly you states any or all upsizing of the ccc. We are only playing symantics by using Dwelling feeder table when it best suits one. or use the ampacity table for conductors when it suits then. In the case here the ampacity is 200amps for a dwelling feeder. use table 310.15 (B) (7). Now if you use a Lower breaker one could say you have an upsized feeder in this particular installation. If you now use the wire at 175 A the difference in circular mil is roughly 1.26%. So if you calculate the current EGC of 2/0 by 1.26 you arrive at a 3/0 wire for the EGC. This is silly.

...

Silly should be a clue that you're not quite thinking right :slaphead:

Not saying it don't happen... but I am saying rarely.

Base upsizing on the minimum EGC size. 2/0 AL EGC for a 200A OCPD is substantially oversized. The 4/0's would have to be six sizes oversized for the OCP before upsizing the EGC comes into play (in the neighborhood of a 40A OCPD).