I need to run a feeder from a metered pedestal with a 200-amp main breaker (it used to serve a manufactured home on the property many years ago) to the 200-amp service panel inside a house. The pedestal also has a breaker on a line that sends power directly to a well pump. I'm hoping I can use the 83% rule to size the feeder with 4/0 aluminum conductors. Is the feeder considered to provide 100% of the dwelling unit load despite the well pump running on a separate line?
Can you use the 83% rule for a feeder if a well pump runs on a separate line?
- Thread starter jamiec
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- Location
- Chapel Hill, NC
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- Retired Electrical Contractor
That's a good one but imo the nec will allow it simply because the feeder will never have to be larger than the service conductors. Thus if you have 4/0 al as the service conductors then the feeder can be 4/0, or so I am told--haha
Either way 4/0 is still good for 180 amps so if the load on the dwelling is 180 amps or less than 4/0 is still compliant on a 200 amp breaker even if the inspector complains about using the 83% rule.
Either way 4/0 is still good for 180 amps so if the load on the dwelling is 180 amps or less than 4/0 is still compliant on a 200 amp breaker even if the inspector complains about using the 83% rule.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Do you teach electrical?
- Location
- Illinois
- Occupation
- retired electrician
310.15(B)(7)(3) in the 2017 or 310.12(C) in the 2020 code.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Yes I am aware... I said that because I always believed you had to use a larger conductor based on ampacity. I still can't see how, let's say, 4/0 which may have to be de-rated for some reason or other is allowed to be used at 83% when the rating of the conductor would be less than the service conductors. IMO, in that case the conductors would need to be larger than 4/0
In the op case I can see the 4/0 al being allowed but only at 180 amps the way the nec is written.
I know everyone here thinks differently so I agreed in my post. I really don't buy it
Thank you, Dennis and Don. The service conductors are 4/0 copper. No, I don't teach electrical. It sounds as if there isn't any clarity about whether a well pump would count as part of the dwelling's load. I see that a somewhat similar question was discussed here: https://forums.mikeholt.com/threads/310-15-b-7-3.2560589/
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
IMO, the well is part of the load for the dwelling and therefore would not get the 83% however what Don was reference and what I was stating that there is no need to run feeder conductors larger than the service conductors. Of course, 4/0 is plenty large so I am assuming you have a long run.
Thank you for clarifying, Dennis. The conductors going to the metered pedestal are 4/0 copper, much bigger than I need for the 35 feet or so of feeder from the pedestal to the service panel in the house. 4/0 aluminum should be adequate for the feeder, but I was hoping the simplicity of the 83% rule could apply.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
It is far from clear IMO. Suppose you get a meter main with a 200 amp breaker feed thru lugs and additional space for an a/c unit or whatever.
The meter main has buss bars not conductors so are those busbars valued at 83% of the 200 amps? I don't think so so I would assume they are rated at 200 amps.
Now I want a 200 amp feeder from the feed thru lugs to a home and as in the op's scenario there is a well pump coming off the meter main. What size conductor must I use to feed the house? I don't think 4/0 al would do it.
The meter main has buss bars not conductors so are those busbars valued at 83% of the 200 amps? I don't think so so I would assume they are rated at 200 amps.
Now I want a 200 amp feeder from the feed thru lugs to a home and as in the op's scenario there is a well pump coming off the meter main. What size conductor must I use to feed the house? I don't think 4/0 al would do it.
electrofelon
Senior Member
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- Cherry Valley NY, Seattle, WA
Dennis, but even if the gods come down and tell us no 83% rule for the second part, can't I evoke my constitutionally guaranteed next size up rule and use 4/0?
- Location
- Chapel Hill, NC
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- Retired Electrical Contractor
Yes that is what I said earlier but suppose you needed the full 200 amps or your calculated load was more than 180 amps.
wwhitney
Senior Member
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- Berkeley, CA
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- Retired
First, I would say busbars are conductors, just not wire-type conductors.
But if the service is for a residence (and we say the well pump is associated with the dwelling unit, which I would), and the service has a 200A main breaker, then the service entrance conductors could have had only 83% * 200A = 166A ampacity, regardless of what was actually provided, or whether or not there are any service entrance conductors. So any downstream feeder need only have 166A ampacity.
Note that 310.15(B)(7)(3) refers to the ampacity "specified in 310.15(B)(7)(1) or (2)." Not the actually ampacity of anything installed.
Cheers, Wayne
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
OK, that clarifies that the busbars aren't subject to Article 310. They are still conductors, as they conduct.
Cheers, Wayne
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
But do they? Or is that just what the mainstream media has been telling us???
Carultch
Senior Member
- Location
- Massachusetts
But do they? Or is that just what the mainstream media has been telling us???
If it's a dumb idea for me to touch it when it is electrically energized, it is a conductor.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I believe I get it. My thoughts were going into the idea that a conductor has a different ampacity as a feeder but with the next size up rule it shoots my theory down.
There was an issue years ago when SE cable was rated 60C then I believe it would cause some issues. For instance 2/0 copper ser at 60C would only be good for 150 amps and here you could use 2/0 for the service conductors but not for the feeder, IMO. Still using the scenario above where the feeder cannot use the 83% rule.
Of course, now that se cable is 75C that is no longer an issue.
There was an issue years ago when SE cable was rated 60C then I believe it would cause some issues. For instance 2/0 copper ser at 60C would only be good for 150 amps and here you could use 2/0 for the service conductors but not for the feeder, IMO. Still using the scenario above where the feeder cannot use the 83% rule.
Of course, now that se cable is 75C that is no longer an issue.
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