Is this tap legal?

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bkludecke

Senior Member
Location
Big Bear Lake, CA
Occupation
Retired Electrical Contractor
Here's the situation. I've got a 200amp 3pole 480volt 3phase CB feeding a 112.5kva xfrmr. Out of the xfrmr is a set of 500kcmil thhn cu conductors landing in a 350amp 3pole CB on the 208volt panel board. What I want to do is tap the line side of the 350amp CB (very convenient) with a set of 4/0 landing them in a 225amp main CB new panel board using the 25' tap rule.

What I'm not sure about is, is this tapping a tap? Or is it NEC compliant?
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Not permitted. See 240.21.

"no conductor supplied under the providions of 240.21(A) through (G) shall supply another conductor under those provisions, except through an overcurrent protective device....."

Basically, conductors on a transformer secondary are tap conductors as permitted by 2401.21(B). And the rule above is the "You can't tap a tap" rule.

Steve
 

bkludecke

Senior Member
Location
Big Bear Lake, CA
Occupation
Retired Electrical Contractor
Yeah that's kinda what I was thinkin' too. Thanks for clearing that up. I'll just add lugs to the panel bussing which would be tapping the load side of the CB.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
transformer feeding two OCP devices

transformer feeding two OCP devices

If your transformer has proper overcurent protection on the primary (125%), there would be no reason you could not supply your second breaker from the LV side transfromer terminals.
 

bkludecke

Senior Member
Location
Big Bear Lake, CA
Occupation
Retired Electrical Contractor
augie47 said:
If your transformer has proper overcurent protection on the primary (125%), there would be no reason you could not supply your second breaker from the LV side transfromer terminals.

So with the proper primary protection, the secondary wires coming off of the transformer are not considered tap conductors?

I appologize for seeming ignorant but this is an area where I'm not as familiar as I would like to be. Thanks.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
I think they are considered "tap" conductors. (Although I think someone once argued with my use of the word "tap", saying taps were items listed under 240.21(A), not 240.21(B). )

But if you come directly off the transformer lugs with another set of wires, you are not tapping the set of wires that are already there. I think that might be OK.

But you also have to look at the rules for transformer protection in article 450. Look at table 450.3(A). If secondary protection is required, then note #2 would apply.

Steve
 

ItsHot

Senior Member
scary!

scary!

bkludecke said:
Actually, that's what got me thinking of all this.:-?
Yeah, and now you got me thinking, and thats scary! Where is the "brains" of this operation when you need them? Now I am actually drawing this out? Good post! I will be checking back on this one for suggestions/comments...sorry I "am tapped out"!
 

bkludecke

Senior Member
Location
Big Bear Lake, CA
Occupation
Retired Electrical Contractor
Well I guess that these are not tap conductors now that I've studied this a bit more. So I now declare that it is not an NEC violation so long as the primary side is protected at no more than 125%. I Think.
 

bkludecke

Senior Member
Location
Big Bear Lake, CA
Occupation
Retired Electrical Contractor
I take it all back. I just saw Mike Holt's webpage on transformer secondary conductors which says they are all essentially tap conductors. I'm dying out here.:confused: :-?
 
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