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Joined: Jul 2004
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OK lets go a little farther. What if you had a dual lug breaker and maybe even another tap to another transformer? Is that then the feeder? Now if you just step back a little and see what we are accomplishing you see the stub of 3/0 and the Polaris is unnecessary. The "feeder" is just the lug on the breaker.
At the end of the day the important thing is that the conductor is adequately protected from overload and we have done that with the load side fuses. Short circuit protection is adequately provided from the line side O/C device and we have the conductor in a raceway to deal with the momentary heating until the line side breaker opens.
Greg Fretwell
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But the definition of Feeder is: All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device. It specifically states conductors and not lugs. I think they need to change the wording because you can tap the secondary on a transformer and it's obvious there that you need to use lugs.
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Joined: Jul 2004
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It does not say wire, it says conductor. What is a conductor? If a piece of conduit is a conductor, why can't a lug be a conductor? It is really undefined.
An inspector who would insist on a stub of 3/0 and a Polaris connector, simply to settle a semantics problem is why we lose respect in the community.
Greg Fretwell
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Greg
I agree 100% but in the NEC definitions they lean towards a conductor being a wire. A conductor is a conductor of electricity in the dictionary. Excellent point!!
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Joined: Mar 2005
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The "good reason" you can't use undersized cables is because the OCP won't protect it in case of a fault. If there was a fault, the cable could burn up before the OCP trips. Allowances for bus taps are highly restrictive because of this.
However, if full ampacity (#3/0) cable is used, it's not a tap- in fact, the #3/0 can be considered a bus just as well as if it was a solid piece of bus-bar inside an enclosure. You can safely legally feed a bus-tap from the end of this cable, so even if you consider the polaris to be a bus-tap, it's OK because it's only a few inches long and meets all the other requirements of 240.21(B).
If you're going to feed it from 200A OCP, I'd recommend you use #3/0 and polaris lugs. It's safe, simple, and would be able to withstand fault conditions long enough for the breaker to open and protect it.
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Joined: Jul 2004
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The "good reason" you can't use undersized cables is because the OCP won't protect it in case of a fault. If there was a fault, the cable could burn up before the OCP trips. I suppose you know that puts you at odds with 240.21(B). As long as the tap is protected at the load end from overload they believe the larger line side breaker will protect you from overcurrent (short circuit) faults in the tap. That is why there are different rules for 10 foot and 25 foot taps.
Greg Fretwell
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If you can tap a feeder you ought to be able to feed a tap from a breaker if its lugs are listed for the smaller wire. This seems common sense to me. I cannot find the wording to allow exception or provision in the code for a breaker to feed a smaller wire than the breaker rating unless perhaps if you consider the breaker lug a conductor. If so then the code also says the tap shall feed no other conductor so where does that leave us? Anyone have experience submitting revisions to the NEC committee? Am I missing something in the interpretation? Steve, how is feeding the tap with a breaker more dangerous than feeding it from a feeder protected by the same size breaker? Your input is appreciated!
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Absolutely you can connnect a smaller wire to the load lugs of a breaker. Example 1: Start with a 100A protective device feeding #3CU @75°, go 300' and terminate them in a 200A breaker, leave the 200A with #3 conductors and go 20' to a 100A breaker.
Example 2: Start with a 200A protective device feeding 3/0, again go 300' and terminate in a 200A device, leave the 200A with #3 conductors and go 20' to a 100A breaker.
In my examples, the oversized breaker is acting as nothing more than a very expensive method to 'butt splice' the conductors together.
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Can you please cite the exact verbiage in the NEC that allows you to do that while simultaneously complying with 240.21?
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Joined: Jul 2001
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240.21(B) says conductors may be tapped to a "feeder". The article 100 defines a "feeder" as all circuit conductors between the source and the final branch overcurrent device.
No where in the definition of feeder does it ever specifically mention either main or feeder circuit overcurrent devices, but we know these devices must exist (see 215.3). So breakers are either considered 'conductors' or they are ignored when defining feeder circuits.
As far as UL and breaker manufacturers go, they clearly allow 'taps' directly from circuit breakers as there are Listed power distribution lugs for many industrial breakers (typically up to 6 #14-4AWG openings).
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Tom
Shinnston, WV USA
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