DOUBLE LUGS - ATS

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dannyrzk

Member
Location
tulsa, OK
Hello I have the following situation:
an existing 1200 amp main switchboard (MSB), fed with 3 sets of 500 kcmil, I'm also adding a new load is a UPS (300amp load) fed by 1 set of 500 kcmil through a 400 amp disconnect switch.

I'm proposing a new ATS, which have inits load side 4 sets of lugs, here is my question: Can use three (3) lugs to feed the MSB and 1 set of lug to feed my 400 amp disconnect? (all together is 4 set of lugs)

Thanks
 

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ron

Senior Member
Seems that you are effectively tapping the 1200A feeder with a 400A feeder. Seems okay as long as you are compliant with 240.21(B).

BTW, I think you show the ATS incorrectly, witching the load terminals and the normal input
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I see no problem with the lug 'plan'. I do wonder how they got by with (3) 500s on a 1200 circuit.
 

dannyrzk

Member
Location
tulsa, OK
"Seems that you are effectively tapping the 1200A feeder with a 400A feeder. Seems okay as long as you are compliant with 240.21(B)."

if I understand correctly 2401.21(B1) is telling me I need an extra disconnect switch for the 1200amp MCB?

thank you all!
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
As long as you comply with the tap rule for the conductors supplying your computer disconnect you should be o.k. in an overall sense.
The rating of the 500s being less than 1200 amp would be a problem here but if its existing perhaps folks there don't view it as a violation. If it is allowable the only place I see the 240.21 tap rules coming into play would be supplying your UPS disconnect.
 

ron

Senior Member
"Seems that you are effectively tapping the 1200A feeder with a 400A feeder. Seems okay as long as you are compliant with 240.21(B)."

if I understand correctly 2401.21(B1) is telling me I need an extra disconnect switch for the 1200amp MCB?

thank you all!

The 400A disconnect you already show needs to be within 10' of conductor length from the upstream ATS, if you plan to use (B)(1)
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
The 400A disconnect you already show needs to be within 10' of conductor length from the upstream ATS, if you plan to use (B)(1)

Good point, I'm so used to the 25ft rule I overlooked that,
 
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