Residential Feeder Calc

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honestjoe

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Hey all, thank you in advance for your consideration and feedback on this. I’m pretty sure I’ve covered all the necessary codes, but just in case I wanted to post this here in case anyone sees something off.

Residential project; sizing main feeders and pathway.

I have 2x 125A main panels that are installed approx 60’ from the meters and disconnects. For pathway I have an existing 1.5” pvc conduit that is empty. According to my research, I would be able to pull 6x #2 and 1x #6 within this pathway to service both main panels. My theory is based on the following:

-Main panel feeder can be sized to 83% of the total load; for 125A, 83% is 103.75A

-Derating for 6 current carrying conductors reduces the ampacity of #2 wire by 80%, 130A (90 degree wire) is good for 104A after derating

-Conduit fill calcs out to 39.86%

while I know some of these are right on the line, I don’t have the option to run a second pipe, but this should all work, correct?

thank you!
 
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honestjoe

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Also noting, I’m driving 2x new ground rods at the main panel locations and providing the min ground within the pathway to code, however I’m also not sure if I need to calc the ground for the total amps or for the highest rated conductor in the pathway. I think it’s the second
 
Dsg319

Dsg319

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West Virginia
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Wv Master “lectrician”
honestjoe said:
Hey all, thank you in advance for your consideration and feedback on this. I’m pretty sure I’ve covered all the necessary codes, but just in case I wanted to post this here in case anyone sees something off.

Residential project; sizing main feeders and pathway.

I have 2x 125A main panels that are installed approx 60’ from the meters and disconnects. For pathway I have an existing 1.5” pvc conduit that is empty. According to my research, I would be able to pull 6x #2 and 1x #6 within this pathway to service both main panels. My theory is based on the following:

-Main panel feeder can be sized to 83% of the total load; for 125A, 83% is 103.75A

-Derating for 6 current carrying conductors reduces the ampacity of #2 wire by 80%, 130A (90 degree wire) is good for 104A after derating

-Conduit fill calcs out to 39.86%

while I know some of these are right on the line, I don’t have the option to run a second pipe, but this should all work, correct?

thank you!
Click to expand...
Not quite sure if the 83% rule will be able to be used in your situation. As you have 2 panels. This works for service or feeder suppling the entire dwelling…. We’ll see what others have to say.

Assuming this is a single dwelling, now say you were feeding 2 separate apartments then yes you can use the 83% rule on each one.
 
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Dsg319

Dsg319

Senior Member
Location
West Virginia
Occupation
Wv Master “lectrician”
honestjoe said:
Also noting, I’m driving 2x new ground rods at the main panel locations and providing the min ground within the pathway to code, however I’m also not sure if I need to calc the ground for the total amps or for the highest rated conductor in the pathway. I think it’s the second
Click to expand...
I am unclear on what exactly you are asking. The EGC is not considered a current carrying conductor.
 
Dennis Alwon

Dennis Alwon

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ptonsparky said:
These are two dwelling units and you want to run both feeders in one conduit?
The EG is sized per the OC protection. Table 250.122
The minimum Grounded conductor is Table 250.102(C)(1)
Click to expand...
Why are you saying there are 2 dwellings? Sounds like one meter with 2 subs.
 
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honestjoe

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There are two dwellings, with two meters, and each panel supplies the entire load per dwelling. Why would the 83% rule not apply if each circuit supplies the full load of each dwelling? I sized the GRC off of the table, which is where I got #6 from, sized off of 200A, but I was asking if I need to size off of 125A or 250A. I’m assuming 125A. And yes, I understand the GRC doesn’t count towards the fill so I’m actually at 37.41%
Thank you for the responses!! Keep ‘em coming!
 
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honestjoe

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Each feeder supplies a panel of 125A. Did I size this correctly in this case? Or do I need to add up the total ampacity of both panels and size the GRC to that?
 
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honestjoe

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the primary GRC is going to be 2 local ground rods at the main panels. The #6 is just to satisfy the ‘GRC within a pathway.’
 
kwired

kwired

Electron manager
Location
NE Nebraska
sounds like you are ok on fill with what you have planned, but is possible you could reduce the neutral a size or two if it were putting you over a little bit. Good chance it doesn't carry same current as the ungrounded conductors in a typical dwelling.
 
kwired

kwired

Electron manager
Location
NE Nebraska
honestjoe said:
the primary GRC is going to be 2 local ground rods at the main panels. The #6 is just to satisfy the ‘GRC within a pathway.’
Click to expand...
GEC = Grounding electrode conductor

EGC = Equipment grounding conductor

Not aware of anyone using GRC or what that would be acronym for.
 
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