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#176077 - 03/19/08 04:16 PM Sub Panel Feeder Size
Micmann Offline
Member

Registered: 04/22/05
Posts: 20
Looking for insight on a few questions:

Are the feeders for a subpanel sized at the table 316.10 size because the wires are considered branch circuits coming from the main panel?

Example:
200A main panel single phase
100A subpanel
75 degree rated lugs
SER from main panel to sub panel
Given the loads for the sub panel are an unknown mix of continuous and non continuous I would suppose sizing at 100% continuous.
Am I correct the cable would need to be #1 copper or 2/0 aluminum because 125% of 100A puts me there or am I oversizing?

Second:
200A breaker in 1600A main switchgear three phase 120/208
200A subpanel three phase
Conduit run to subpanel
Again unsure of continuous vs non-continuous
I assume needing to size at 100% continuous
Would I need 250kcmil copper, or could I go smaller because I thought there was a three phase multiplier to reduce the needed amperage size. Or again am I missing something and oversizing?

Thanks for the help.

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#176081 - 03/19/08 06:47 PM Re: Sub Panel Feeder Size [Re: Micmann]
sparkyinak Offline
Member

Registered: 07/08/07
Posts: 1286
Loc: Alaska
The breaker is sized to protect the wire is a a good way to look at it. How did you come about for #1 wire for an 100 amp panel? on a 100 amp panel a #3 usually is required. #1 is for 125 amp panel I do believe without looking at 316.
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#176083 - 03/19/08 07:39 PM Re: Sub Panel Feeder Size [Re: sparkyinak]
Micmann Offline
Member

Registered: 04/22/05
Posts: 20
Thanks for the reply Sparky.

A #3 is good for 100 amps at 75 degrees, but don't i have to size the wire with continuous load figured in? 100a x 1.25= 125a. Needing a #1 good for 130a. Or am I going about this wrong?

Thanks

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#176085 - 03/19/08 09:14 PM Re: Sub Panel Feeder Size [Re: Micmann]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9012
Loc: Estero,Fl,usa
Mic Short answer ... no. This is a feeder. 310.16 is fine at 100% and you could even use the round up rule (240.4(B)). #3 is 100a so it is a perfect fit on a 100a feeder (no rounding). As a design issue you might not want to "round up" but generally you have pretty good diversity in residential feeders.
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#176087 - 03/20/08 03:38 AM Re: Sub Panel Feeder Size [Re: gfretwell]
SteveFehr Offline
Member

Registered: 03/19/05
Posts: 1192
Loc: Chesapeake, VA
Micmann, if it's feeding a panel, it's a feeder circuit, and not a branch circuit. Not much difference, but enough little things to make it a real pain in the *** sometimes.

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#176198 - 03/24/08 08:15 PM Re: Sub Panel Feeder Size [Re: SteveFehr]
Micmann Offline
Member

Registered: 04/22/05
Posts: 20
Thanks for the clarification. I’ve been sizing them as feeders, but something in the back of my mind kept saying just check to make sure I was doing it right.

Mic.

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#176368 - 03/30/08 09:30 AM Re: Sub Panel Feeder Size [Re: Micmann]
JValdes Offline
Member

Registered: 06/03/07
Posts: 308
Loc: South Carolina
3 #3s and 1 #8. Four wire feeder. Seperate grounds and neutrals. Bond ground bus to electrodes and metal piping if available. If this is unattached you need a main breaker in the sub. If attached you can use a lug panel.

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