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Thread: NEC 25 Foot Tap Rule Applied to Fusible Switch

  1. #1

    Default NEC 25 Foot Tap Rule Applied to Fusible Switch

    The NEC Rule:

    "25 foot tap rule;

    COPIED SECTION NEC 2002;

    240.21.B
    (2) Taps Not Over 7.5 m (25 ft) Long. Where the length of the tap conductors does not exceed 7.5 m (25 ft) and the tap conductors comply with all the following:
    (1) The ampacity of the tap conductors is not less than one-third of the rating of the overcurrent device protecting the feeder conductors.
    (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.
    (3) The tap conductors are suitably protected from physical damage or are enclosed in a raceway.

    Refers in (2) above to termination of tap conductors in the single circuit breaker or a set of fuses. It is not clear if the fusible switch can be interpreted a set of fuses since the tap conductors are actually connected to the fusible switch line terminals rather than directly to fuses. This appears to be potentially unsafe for a switch rated less than one third of the upstream overcurrent device, if a short circuit or overload happens between the swithch load side and fuses.

    Please, may a set of fuses be interpreted as a fusible switch in the above rule?

  2. #2

    Default

    First the word tap is the beginning of this certain feeder where it is getting its power source. The word termination is the panel at the end of line where this power is being delivered through this certain feeder to a load side panel or diconnect.

    This 25 foot tap is designed for example having a feed through distribution panel that is 200 amp rated and you want to use the feed through direct connect 200 amp protected lugs at the bottom of the buss bar in this power source panel to run power to a sub panel by way of this tapped feeder using this 25' tap rule. This feeder would then go to say a 100 amp sub panel with a main breaker rated 100 amp. This 100 amp main breaker in this sub panel would be the termination of one breaker. Then this 100 amp sub panel with this 100 amp main breaker could have 20 or more branch circuits in this sub panel fed by this 100 amp rated 25' tapped feeder from the 200 amp rated lugs inside the main service rated panel. The 100 amp main breaker in that load side sub panel dictates the feeder size because this 100 amp braeker connected at the tapped feeder's termination is protecting that feeder on the load side from overcurrent causing this feeder damage. A tap is where you have a larger power source tapped by a smaller feeder when this smaller feeder is protected on the load side aka termination of that feeder protecting that feeder from overloading. This is one of very few overcurrent or overload protection is allowed on the load side instead of the beginning of that feeder being the line side. Taps are exception from the normal.

    Good Luck

    Wg

  3. #3
    Join Date
    Dec 2003
    Location
    Kent, WA
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    8,431

    Default

    He asks a good question, and I don't know the answer. What if in the middle of that feeder tap there is a switch. Can that switch be rated at 100A if the tap is sourced from a 200A feed through and terminates in a 100A breaker at the other end? Or must be switch be rated at 200A because of the potential overload that could happen upstream of the 100A breaker?
    Mark
    Kent, WA

  4. #4
    Join Date
    Dec 2003
    Location
    Springfield
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    947

    Default

    Quote Originally Posted by suemarkp View Post
    He asks a good question, and I don't know the answer.
    Yep, he sure does.

    (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.
    A strict interpretation of the above would seem to rule out even a service rated fused disconnect switch, even if the fusing of such device limits the load to the ampacity of the tap conductors. Since a feeder connects directly to the switch portion of the fused disconnect switch, that in itself goes against section (2).

    The funny thing is that a service rated fused disconnect switch can be fed by a completely unfused feeder (right from the meter base).

    Section (2) seems not to reflect this fact.

    Homer
    Depending on your skills, doing your own electrical work may risk the health and safety of the community. Always find out how to do things safely before beginning.

  5. #5

    Default

    Homer, this tap rule is for feeders not requiring service rating equipment not being a service conductor but a tapped feeder after the main breaker in the service rated panel providing this tapped feeder its power source. When you install the fused disconnect or a distribution panel with a main breaker with the fuse or the main breaker sized not larger in ampacity than the ampacity rating of the feeder tap itself then you just ended the tapped conductor installation providing that single set of fuses or main breaker on the load side of that tapped feeder. Anything after that is fed from teh load side of that properly sized fused disconnect or main breaker that ended the tapped feeder conductor run. The tapped conductor would connect to the line side of that nonservice rated fused disconnect or nonservice rated distribution panel with a main breaker. That set of fuses or that main breaker is the amp limiting factor protecting that tapped feeder on the load side of that feeder as allowed for a tapped conductor.

    Hope this helps

    Wg

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