Tap rule question

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Jeff Roeger

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Electrical Inspector
Hello,

We have a 62.5 Kva UPS with a 1200 amp DC breaker that feeds a bank of 1200 amp-hour batteries (120) volt DC via a 700 amp disconnect. There is a DC panel tapped off of the UPS side of the DC disconnect that has a 100 amp main. Can you please tell me what size wire I need from the DC panel to the disconnect? IF I use the tap rule at less than 25 feet it seems we need 1200/3 (400 amp wire) 250 kcmil. If we mount the panel within 10 feet of wire we can go down to 120 amp wire or #2 wire. Am I correct in this? The engineer on site told us we had to use 350 kcmil cable and that size will not fit in the 100 amp breaker.

Thanks for your help,
 
My understanding of your setup:
The feeder is 1200A
The tapped circuit is 100A

Once you are on the load side of the OCPD at the end of the tapped circuit, you no longer classify as a tap, and no longer have to think about the 1200A feeder affecting the sizing of the circuit conductors. It now classifies as a feeder conductor, and only needs to be sized for its own load and own OCPD.

If you follow the 10 ft tap rule, the tapped circuit has to have at least 120A of wire and termination ampacity. I.e. one tenth of 1200a. #2 Cu is NOT 120A in most situations, because most examples in practice require terminating with the 75C ampacities. You'd need at least #1 Cu to get the 120A worth of ampacity. In the event that the tap conductors do not leave the original enclosure, the "one tenth" part of the ten foot rule no longer needs to apply. One example where you would see this, is 15A voltage taps, terminated on inline fuses immediately within the enclosure of the main feeder wires. The #14Cu wire is good enough both before and after the 15A fuses.

If you follow the 25 ft tap rule, the tapped circuit has to have at least 400A (i.e. 1/3 of 1200A) of wire and termination ampacity. E.g. 600 kcmil Cu. This is usually more common if your tapped circuit is already a significant fraction of the feeder rating (quarter, third, half, etc), and bigger equipment means you aren't as flexible with where you place the OCPD at the end of the tap conductors. A 100A disconnect takes a lot less space than a 400A unit, so a 400A unit is more likely to require you to locate it a little farther from the source.

Regardless of what tap rule you follow, 1200A means you'll have at least 3 sets in parallel, and possibly 4. It is important to preserve the symmetry of how you tap, so that current divides uniformly among the sets. Just tapping 1 conductor breaks the symmetry, and the feeder conductors are no longer "terminated in the same manner". Either tap where the conductors already are terminated on a common block, or use multiport tap connectors to connect to all sets of the feeder.
 
Last edited:
Carultch said:
My understanding of your setup:
The feeder is 1200A
The tapped circuit is 100A

Once you are on the load side of the OCPD at the end of the tapped circuit, you no longer classify as a tap, and no longer have to think about the 1200A feeder affecting the sizing of the circuit conductors. It now classifies as a feeder conductor, and only needs to be sized for its own load and own OCPD.

If you follow the 10 ft tap rule, the tapped circuit has to have at least 120A of wire and termination ampacity. I.e. one tenth of 1200a. #2 Cu is NOT 120A in most situations, because most examples in practice require terminating with the 75C ampacities. You'd need at least #1 Cu to get the 120A worth of ampacity. In the event that the tap conductors do not leave the original enclosure, the "one tenth" part of the ten foot rule no longer needs to apply. One example where you would see this, is 15A voltage taps, terminated on inline fuses immediately within the enclosure of the main feeder wires. The #14Cu wire is good enough both before and after the 15A fuses.

If you follow the 25 ft tap rule, the tapped circuit has to have at least 400A (i.e. 1/3 of 1200A) of wire and termination ampacity. E.g. 600 kcmil Cu. This is usually more common if your tapped circuit is already a significant fraction of the feeder rating (quarter, third, half, etc), and bigger equipment means you aren't as flexible with where you place the OCPD at the end of the tap conductors. A 100A disconnect takes a lot less space than a 400A unit, so a 400A unit is more likely to require you to locate it a little farther from the source.

Regardless of what tap rule you follow, 1200A means you'll have at least 3 sets in parallel, and possibly 4. It is important to preserve the symmetry of how you tap, so that current divides uniformly among the sets. Just tapping 1 conductor breaks the symmetry, and the feeder conductors are no longer "terminated in the same manner". Either tap where the conductors already are terminated on a common block, or use multiport tap connectors to connect to all sets of the feeder.
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tap rule question
The above link shows a photo of what I need help with. I have the ability to move the DC panel closer to the 700 amp breaker not towards the 1200 amp UPS battery breaker. Thanks
 
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