tap conductor sizing and fusing

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khoops's picture
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Joined: 2014-06-22

I have a question in regards to tap conductors and protection. I understand the tap conductors as per the NEC, however, I'm having trouble with understanding the sizing allowed. I have a 400A distibution panel with no more fuse or breaker space available. I would like to tap its bus to feed another panel. What I need to know is what the maximum amperage is that I can tap to? For example, can I tap the bus bar and feed a 125A disconnect?
Any help understanding this section of the Code would be greatly appreciated.

jmorrissette's picture
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Joined: 2014-04-29

How far away is the proposed panel? Need to use either the 10' or 25' tap rule. As i understand the code, the distance is as the crow flies, not actual footage. [ up, down etc]

khoops's picture
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Joined: 2014-06-22

Hey, thanks for the response. I do understand the tap rule and it would only be 10 ft. My question is how large of a load can I pull from this tap?

skd76's picture
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Joined: 2013-09-11

RE: jmorrissette
Careful when sharing "as I understand" opinions. We are all experts just looking for someone to 'help'. Joking aside, the Code is what it says, not what we think it meant or what is should say. It says what it says. Section 240.21 has long used the specific language "...length of the conductors...". So unless your crow flies the exact path of those cables it is the conductor length - not straight-line distances.

skd76's picture
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Joined: 2013-09-11

RE: khoops
Your answer is all in the load calculations. The feeder to your 400A panel will allow a maximum load. The overcurrent device protecting the 400A panel will allow a particular maximum load. The tap conductors and their required overcurrent device (required within 10 ft or 25 ft for a common tap scenario) will allow a maxim load that will also be part of the 400A panel load calculation. EC&M Magazine has published some great articles by Mike Holt on the topic. Take a look at the magazine archive.

W A Werning's picture
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Joined: 2014-02-12

khoops
You can't "tap" the bus bars. You need to remove one of the single- or three-phase breakers or if it is a fuse OCPD and use that space for a feeder overcurrent device and re-feed that one that you removed from a feeder tapped panel. And you may be able to supply other loads from this feeder tapped panel because you must terminate the feeder tap conductors in a disconnect or panelboard with space for additional loads.

A 100 ft. tap conductor is at least one third of the rating of the OCPD protecting the feeder conductors, for high-bay Mfg. buildings.
A feeder tap of up to 25 ft.is at least one-third of that of the feeder tap conductor.
A feeder tap of not over 10 f. shall have an ampacity of not less than one-tenth of the rating of the feeder tap OCPD.

What is your length you're looking for? You can go up to 100 ft. if it is a high-bay manufacturing building.

JOHN123153's picture
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Joined: 2013-04-02

Tapping the bus of a distribution panel will likely void its UL listing. Why not replace the panel with a new panel with sub panel feed thru lugs? Then, you mount a new sub panel. Using the same wire size as the incomer, you are making a fully rated tap.

W A Werning's picture
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Joined: 2014-02-12

That could get very expensive and you must consider the down time. You can't work it hot, either. If you do check NFPA 70E for the incident energy per square centimeter and PPE per HRC level.

Advice: Don't do it! Go with the feeder tap rules, 240.21(B)(1) thru (B)(4)

FYI: these are not transformer tap rules.

W A Werning's picture
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Joined: 2014-02-12

The 10 ft. tap rule is found in 240.21(B)(1 (4), 2014 NEC. The ampacity of the tap conductors is not less than one-tenth of the rating of the OCPD that protects the feeder conductors OCPD.

So, what is the size of the OCPD protecting the feeder?

I believe you need to make some changes in your switchboard/panelboard to allow for a feeder OCPD and then run your feeder conductors to a J-box that is properly sized to make your tap and then set a MCB panel about eight foot from the J- box, the length of the tap conductors are based on conductor length and not conduit length.

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