100A subpanel for detached garage

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  #1  
Old 03-15-18, 11:26 AM
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100A subpanel for detached garage

I need to run a 100A feeder to a new construction detached garage.

The main house has 200A main panel, and a 100A subpanel. Both panels are full.

code is NEC 2008 but requires conduit.

Please sanity check my plan....

1) Install 3rd panel in main house to supply the garage.
- feed 3rd panel via 100A breaker in main panel.
- move 2 or 3 circuits to the new 3rd panel to make room for the 100A breaker
- extend those circuits with wire nuts and wire extensions to reach the new 3rd panel
- Install new 100A breaker at the bottom of the main panel, on the phase opposite the 1st 100A subpanel breaker (did that make sense?)


2) Run 2-2-2-4 MHF (mobile home feeder) to a subpanel in the new garage.
- Run the MHF in 2" conduit (yikes) to the house exterior under the back deck
- switch to 2" PVC on the outside
- run above ground under deck a length of 50ft
- then burry 18" and run underground 30ft to the garage service entry.
- bring the 2" PVC up and punch it through the building.
- enter the Garage subpanel through the bottom.

a few questions:
- extensions/wire nuts are allowed inside panel boxes?
- can the 100A breaker be at the bottom? I've only seen them at the top.
- what wire to supply the 3rd subpanel from the main panel? the same 2-2-24 MHF? the existing subpanel has a 3 wire setup, but that may no longer be code?
- does under a deck count as "under a building" per NEC code? NEC 2008 allows exterior above ground runs in conduit under a building.

here's pics of the existing main panel & sub panel....

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Name:  sub panel.jpg
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Last edited by PJmax; 03-15-18 at 12:45 PM. Reason: removed duplicate pics
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  #2  
Old 03-15-18, 12:13 PM
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#2 MHF is only rated at 90A when used as a branch feeder. For a 100A sub feeder you need to use #1 Al or #3 Cu. You also need to verify that your panel allows branch circuit breakers as large as 100A.
 
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Old 03-15-18, 12:34 PM
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hey are you the same pattenp from garage journal?

so the mhf I was looking at is rated USE2/RHH/RHW-2. if i'm reading 310.15 correctly it states #2 AL is good for 100A serive or feeder? 310.16 describes insulated conductors in conduit and also states 100A for USE2/RHH/RHW-2

what am I missing?
 
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Old 03-15-18, 02:25 PM
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Originally Posted by sky jumper View Post
hey are you the same pattenp from garage journal?

so the mhf I was looking at is rated USE2/RHH/RHW-2. if i'm reading 310.15 correctly it states #2 AL is good for 100A serive or feeder? 310.16 describes insulated conductors in conduit and also states 100A for USE2/RHH/RHW-2

what am I missing?
Yep... that's me. Anyway the allowable amp rating when used as a main service supplying the full service to a dwelling vs. being used as a branch feeder to a subpanel is different. When used as a branch feeder you use table 310.15(b)(16) at the 75deg-C column. The 90deg-C column is only used for derating purposes. There are no breakers with terminal ratings over 75deg-C.
 
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Old 03-15-18, 06:34 PM
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ok I think I get it.

So the existing sub panel is fed by (3) #1 Cu wires. but no 4th wire. Is that because the conduit handles the ground? I'm assuming I'll have to run a ground wire to the detached garage, though? where would I connect it in the house? tie it to the panel? if you notice in the pics there's no ground terminals in either of my panels.
 
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Old 03-15-18, 06:38 PM
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Metallic conduit can be used as a grounding means. Do you have metal conduit?
 
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Old 03-15-18, 06:57 PM
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yes metal conduit. so does that mean if I run metal conduit to the detached garage I don' t need a 4th ground wire in the conection?
 
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