I heard that the reason 240.4(B) (upsizing a CB size) does not apply to tap conductors because the tap conductors need to get overload protection from the CB ahead of the tap conductors. Then what about the feeder that is after the CB? The feeder conductor ampacity can be lower than the CB size due to 240.4(B). Don’t the feeder conductors need overload protection as well?
240.4(B) (upsizing a CB size)
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jumper
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A feeder and a tap are different, pick one.
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You're correct that the conductors after the tap may be a feeder and when they are they're protected just like any other feeder. Up to 800 amps if the conductor ampacity doesn't correspond to a standard OCPD size in 240.6 then you can go up to the next larger standard size but the calcuated load on those feeder conductors still cannot exceed the ampacity of the feeder conductors.
For example if you had 2 sets of 500 Kcmil THHN conductors (380*2=760 amps) feeding equipment with a load calculation value of 760 amps or less those conductors are considered adequately protected even when you use the next standard size OCPD of 800 amps.
240.21(B) requires all tap conductors have ampacity not less than the OCPD supplied by the tap conductor(s). Following that requirement and using your example, the tap conductors have to be two 600kcmil (2 x 420 = 840A for the 800A OCPD). If the OCPD adequately protects the two 500Kcmil feeder conductors against overload, it must also protects the tap conductors adequately against overload. So, why can't the tap conductors be two 500kcmils as well?
wwhitney
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If I understand correctly, the question is this: say you have the following one line diagram:
1200A OCPD -- 1200A feeder -- tap conductors -- 800A OCPD -- 760A conductors -- 760A load
The tap conductors need to have an ampacity of 800A. Why? What extra hazard exists for the tap conductors (that doesn't exist for the 760A conductors downstream of the 800A OCPD) which justifies the requirement for the greater ampacity?
Cheers, Wayne
1200A OCPD -- 1200A feeder -- tap conductors -- 800A OCPD -- 760A conductors -- 760A load
The tap conductors need to have an ampacity of 800A. Why? What extra hazard exists for the tap conductors (that doesn't exist for the 760A conductors downstream of the 800A OCPD) which justifies the requirement for the greater ampacity?
Cheers, Wayne
jumper
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If I am reading correctly the only time that is a problem is for taps less than 10’.
IIRC, taps under 10’ ,that do not leave the enclosure, have no minimum ampacity in regards to the feeder OCPD unlike the other taps which is 1/3 of the feeder OCPD.
So, it is kinda like a check on that allowance.
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For whatever reason the next up rule does not apply to tap conductors, same scenario with secondary conductors on a transformer.
How many taps do not leave the enclosure? Maybe for GFPE or something on a switchboard otherwise the 10' rule has a 1/10 ampacity minimum.
jumper
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Okay, I was confused. This is the OPs question?
Yes, I know. I was actually answering a different scenario.:slaphead:
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I believe so, the tap conductors for an 800 amp OCPD need to be a minimum of 800 amps but the load side of the OCPD can use the next size up rule and be less than 800 amps so the question is why?
jumper
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Well because the NEC says so!
Actually it is a matter as to the protection of the conductors as you know.
The tap conductors between the tap and the OCPD are only really protected by the upstream OCPD. As the old wording used to say you are (trying to) limiting the over load current (kinda) by the termination, say a 800A OCPD, but you do not have the same parameters of protection that the load side would.
Adamjamma
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Ok, student here asking the question, but, does not the next size up rule in part have to do with the fact the wires actually can take more load than they are using in breakers, but are protected by the breakers, due to the lower temp ratings of the breakers? Thus all the ocpd is to shut off before a fault gets to the fire stage...
or am I not understanding that section?
this is why we have all the sections over 60 degree, 75 degree and 90 degree wire ratings?
or am I not understanding that section?
this is why we have all the sections over 60 degree, 75 degree and 90 degree wire ratings?
jumper
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Adamjamma
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Answers what I meant.. there is safety factors already built into the load calculations and the wire ratings... but we still need to be sure of our load ratings to keep safe...
jaggedben
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Perhaps this speaks to your question...
Using the 'one size up' rule does not create a tap conductor. Those conductors are still considered to be protected at their ampacity by article 240. But 'two sizes up' creates a tap conductor.
Examples:
4awg conductors rated 85amps, on a 90A breaker, are not tap conductors because 85A is not a standard breaker size and thus 240.4(B) allows you to use a 90A breaker.
4awg conductors rated 85amps (say they're tapped off 2awg conductors on a 100A breaker). The 4awg are tap conductors because a 100A breaker doesn't protect them at their ampacity, not even per 240.24(B).
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The question is why does it matter? If you have an 85 amp tap conductor feeding a 90 amp OCPD and a 85 amp conductor on the load side of that OCPD why does the tap conductor need to be 90 amps?
jaggedben
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It doesn't? Am I missing something?
jumper
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Carultch
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Understanding why the feeder size can be inconsistent from the tap size for the exact same current, would require understanding of why we can use the "next size up rule" in the first place. Until then, it is "because the NEC says so", that we need full OCPD worth of amps on the tap, and we get to "round up" the amps to the OCPD on the feeder.
The "next size up rule" is the part that is counterintuitive. Not the need for at least the OCPD-rating-worth of amps on the tap. One would initially think that the OCPD has to be the "weak link", and thus that the conductor amps should meet or exceed it. 240.4(B) allows you to be more permissive than what this intuition would tell you.
Also counterintuitive is why single phase single/duplex dwellings get to have less ampacity than the service rating on the service conductors. Why the double standard for services vs feeders? I get the load diversity argument, but I would expect that to be accounted for in the algorithm for "adding up" building loads.
jaggedben
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Why not? Because we are making a legalistic argument that the language of 240.21 contradicts 204.4(B)? I admit that I haven't thought about it before and that the language is open to interpretation. (Although I would argue that for other than 10ft taps the language is open to still invoking 204.4(B).)
In that case I'm fairly confident that there is no good reason for the difference. Just code language that says what it says, and probably not intended to produce that result.
jumper
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