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#61041 - 01/15/06 07:05 PM Feeder Tap
master66 Offline

Registered: 04/18/02
Posts: 260
Loc: Masontown, PA, USA
I'm trying to apply one of the tap rules and I just want to make sure I'm doing it correctly.

Here is the situation. Working on the 8th floor of an 8 story office building. There is a 1600 amp 120/208v buss duct running vertically up a chase that is tapped and metered to feed each tennant space as needed.

The drawings show a 400A disconnect being installed to tap the buss duct. The disconnect will be fused at 350 amps.

The problem is, there is no room to install another disconnect and meet all working space clearances.

So, I was thinking that I could install a tap box on the buss and run a feeder to the office space behind the buss where a new closet will be built to house the new electrical equipment.

I will first have to go through a meter then to a fused disconnect. All wiring will be installed in EMT or greenfield.

I know that I won't be able to do this within 10 feet so I am trying to apply the 25' tap rule.

If I am reading 240.21(2)(3) correctly. I can do this if I tap the buss with parallel 300MCM to the disconnect.

Am I correct or can I parallel smaller conductors in this situation.

This is the first time that I ever encountered this situation.


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#61042 - 01/15/06 08:22 PM Re: Feeder Tap
Bob Offline

Registered: 02/05/02
Posts: 182
Loc: Mobile, AL, USA
240.21B2 requires that the conductor capacity be equal to at least 1/3 of the OC rating. 1600 x 1/3 = 528 amps. 2 #300 at
285 amps = 570 amps. It appears that you have met the requirements of B2 1,2 and3.

#61043 - 01/16/06 02:55 AM Re: Feeder Tap
iwire Offline

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
I will first have to go through a meter then to a fused disconnect.

That will be a problem.

240.21(B)(2)(2) The tap conductors terminate in a single circuit breaker or a single set of fuses that will limit the load to the ampacity of the tap conductors. This device shall be permitted to supply any number of additional overcurrent devices on its load side.
Bob Badger
Construction & Maintenance Electrician


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