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....

 

#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.

 

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.

 

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.

 

Metallic conduit can be used as a grounding means. Do you have metal conduit?

 

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?