allowable ampacities

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wankster

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According to my '08 NEC, between 4-6 conductors in a conduit must have their ampacity reduced to 80%. The part that perplexes me is that I see tons of 200 amp 3 phase 4 wire service/sub panels run with 3/0 wire. 80% of 225 is only 180. How are they getting away with this? I've seen it on inspected jobs as recently as 2005.

Am I missing some exception somewhere, or is this newer code?
 
wankster said:
According to my '08 NEC, between 4-6 conductors in a conduit must have their ampacity reduced to 80%. The part that perplexes me is that I see tons of 200 amp 3 phase 4 wire service/sub panels run with 3/0 wire. 80% of 225 is only 180. How are they getting away with this? I've seen it on inspected jobs as recently as 2005.

Am I missing some exception somewhere, or is this newer code?
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How much current is on the neutral in a 3ph 4 wire service given a balanced load?
 
310.15(B)(4) only requires the neutral conductor being counted as current-carrying if the majority of the load is non-linear.
On a majority of the jobs I see that is not the case, therfore, by Code, you only have 3 c-c-conductors.
 
ray cyr said:
Remember... this is for "current carrying conductors", not just the number of conductors.
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Right, and on a 3 phase 4 wire system there are 4 wires carrying current, even if you have next to nothing on the nuetral. Sorry guys if I'm missing something, but I'm still learning.
 
wankster said:
Right, and on a 3 phase 4 wire system there are 4 wires carrying current, even if you have next to nothing on the nuetral. Sorry guys if I'm missing something, but I'm still learning.
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The neutral in this case is not a CCC even if it carries some current. The answer is in 310.15(B)(4):

310.15(B)(4) Neutral Conductor.
(a) A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(2)(a).
(b) In a 3-wire circuit consisting of two phase conductors and the neutral conductor of a 4-wire, 3-phase, wye-connected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors and shall be counted when applying the provisions of 310.15(B)(2)(a).
(c) On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral conductor shall therefore be considered a current-carrying conductor.
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Oops! Guess I missed that post. Cell phone browsing is a PITA!

That makes sense, though. I knew I was missing something. Thanks for the input.
 
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I agree with the replies. Even if it was a current carrying conductor couldn't you use 240.4(b) and 240.6 and go to the next standard size which is 200. I know that you would have to know some other things like actual load, any continous loads, ambient temperature.
 
CONDUIT said:
I agree with the replies. Even if it was a current carrying conductor couldn't you use 240.4(b) and 240.6 and go to the next standard size which is 200. I know that you would have to know some other things like actual load, any continous loads, ambient temperature.
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For this example 4-#3/0, yes you could if the calculated load was 180 amps or less.
 
wankster said:
. . . on a 3 phase 4 wire system there are 4 wires carrying current, even if you have next to nothing on the nuetral.
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It?s all about heat. Current flowing in a wire creates heat. Too much current causes too much heat, which can damage insulation.

To address your specific question, I once did a simple math problem from which I concluded the following: The amount of heat generated by three phase conductors with the load balanced and with no current flowing in the neutral is always greater (assuming the same total load) than the amount of heat generated in the three phase conductors plus the neutral, when the load is imbalanced and when the neutral therefore has current flowing. That is why it is not necessary to count the neutral as a current-carrying conductor, when all it does is to carry the imbalance current from the three phases.

I should mention that my little math problem assumed linear loads (i.e. no harmonics).
 
While were on the topic of non-linear loads, how would neutral current be effected by a 2 pole non linear load such as a 240v ballast?
 
wankster said:
. . . how would neutral current be effected by a 2 pole non linear load such as a 240v ballast?
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Since the load is not balanced, there will be neutral current. Since there is non-linear load on any of the phases, there will be harmonics on the neutral.
 
So even though there I no nuetral conductor in said 240 ballast, unbalanced current will be picked up by the nuetral at the transformer?


charlie b said:
Since the load is not balanced, there will be neutral current. Since there is non-linear load on any of the phases, there will be harmonics on the neutral.
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The neutral conductor that only carries the imbalance of the phase conductors DOES NOT count when applying the derating. Neither does the grounding conductor.
Reason is pretty obvious. Let's take a 240V single phase. if there's 100A on one phase and 0 on the other the neutral will have 100A, if we have 100A on both phases then the neutral will have 0. If we have 100 and 50 then the neutral will have 50, in which case both the 50A phase and the neutral will not heat up to their rating. So the heat will always be evenly distributed between the three conductors as if there were two.


wankster said:
According to my '08 NEC, between 4-6 conductors in a conduit must have their ampacity reduced to 80%. The part that perplexes me is that I see tons of 200 amp 3 phase 4 wire service/sub panels run with 3/0 wire. 80% of 225 is only 180. How are they getting away with this? I've seen it on inspected jobs as recently as 2005.

Am I missing some exception somewhere, or is this newer code?
Click to expand...
 
wankster said:
So even though there I no nuetral conductor in said 240 ballast, unbalanced current will be picked up by the nuetral at the transformer?
Click to expand...
No current on the neutral for a 240V Ballast (assuming a 240/120 3? 4-wire system).

I think charlie b missed that the example load was connected L-L.

Furthermore, I don't believe harmonics (from non-linear loads) are a neutral current issue for a 240/120 delta 4-wire system.
 
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