Underhand Pitch....MC Cable Size

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blingbling4r

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Location
Seattle, WA
I'm trying to size my MC feeders (coming from the metered portion of a main switch gear) going to various utility panels at this housing complex. Its basically like a big apartment.

Also from the main switch gear there is a tap to the unmetered portion of the switch gear which goes to various meter cabinets on 4 floors of this housing unit. From the meter cabinets they branch off and pick up 100A panels in every residential unit of the building. The loads for all the panels have been figured. All I have to go by is the panel size.

So for the 3phase utility panels if its a 225A panel do I size it for 225A when fed by 4 wire 3 phase MC? Any derating factors for that?

For the residential portion where tenants will be living in those units do I size it for 100A 2 pole? Any derating for that?


This is a slam dunk easy question but I'm spacing right now as to how it goes....any help would be greatly appreciated.
 

augie47

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Location
Tennessee
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State Electrical Inspector (Retired) DINOSAUR
I may not understand the question, but if the loads for the panels have already been figured, you would size your conductor based on those loads and protect it accordingly (unless you desired extra capacity).
If you have a 225 amp panel you could feed it with any size feeder up to 225 amps. If the load was only 150 amps, you could size the feeder and protect it for the 150 amp load.
Calculate the load, select the conductor, size the overcurrent.
Normally for a 4 wire MC on residential there would not need to be any derating.
 

augie47

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Location
Tennessee
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State Electrical Inspector (Retired) DINOSAUR
how large of an mc feeder would I use for a 600/3 OCPD?

It would depend on your load.
If you needed the full 600 amp capacity, it would be a 1250 kcmil (actually 590 amps), but that I wold image is a little hard to come by much less handle (if its manufactured). More likely you would parallel 350 kcmil cables.
 

augie47

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Tennessee
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State Electrical Inspector (Retired) DINOSAUR
(Southwire does make a 500 kcmil AL MC Riser cable for such an application 310 x 2 = 620 amps. be about 2# per foot lighter than 350Cu)
 
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blingbling4r

Member
Location
Seattle, WA
btw...

btw...

I just got off the phone with our rep from AFC...they make it up to 750kcm. Ok well How would I parallel the MC i figured it would be 2 x 300 kcm from a 600 Amp OCPD. But its 3 phase...so would I have to run 3 of the 2x300kcm runs (1 per phase)?


I'm thinking about just saying screw it and piping the larger stuff. If its a 600 A OCPD im figuring 1000kcm wire with a 1 ground? Gah....i hate deadlines!
 
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Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
I'm thinking about just saying screw it and piping the larger stuff. If its a 600 A OCPD im figuring 1000kcm wire with a 1 ground? Gah....i hate deadlines!

You must not be the one installing it, I don't know anybody who likes handling anything larger than 500. If something needs more ampacity than a 500(typically), our shop does it with parallel runs.
 

augie47

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Location
Tennessee
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State Electrical Inspector (Retired) DINOSAUR
I just got off the phone with our rep from AFC...they make it up to 750kcm. Ok well How would I parallel the MC i figured it would be 2 x 300 kcm from a 600 Amp OCPD. But its 3 phase...so would I have to run 3 of the 2x300kcm runs (1 per phase)?


!

all 3 phase, the neutral and grounding means must be in each cable (300.3)(B) This is where you have to be careful paralleling "standard" MC
(where the sheath is not a ground) because for 600 amp you need a #1 ground (assuming cu). "off the shelf" MC may not have a large enough equipment ground.
 

blingbling4r

Member
Location
Seattle, WA
250.122 (f) 2008

250.122 (f) 2008

Says you can run parallel as permitted in 310.4. each parallel EGC must be sized based on the overcurrent device protecting the circuit. to T250.122.

So for a 400A phase OCPD would it be safe to run 1 set of 500 kcmil MC?
 

augie47

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Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired) DINOSAUR
If your load is 380 amps or less, a 500 kcmil Cu MC cable would normally be Code compliant as a 400 amp feeder to a subpanel.
 
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