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Splitting large ampacity need across multiple smaller conductors  pros/cons (?)
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I have a VD calculator that does not include 1000 MCM conductors. But at 750 MCM (not enough ampacity, I know), I get a VD result of 8 volts. Certainly, with 1000 MCM, the VD would be lower. So I do not confirm your calculated values of voltage drop. With a set of three 250 MCM conductors, I get about the same result of 8 volts.
As to derating, you get to use the 90C column as the base value for ampacity. Three sets of 250 MCM conductors would derate from 290, not from 255. So the total ampacity would be 3 times 290 times 70%, or 609 amps.
Is there an option of running separate conduits? You could do this job with two sets of 300 MCM conductors.Charles E. Beck, P.E., Seattle
Comments based on 2017 NEC unless otherwise noted.

Is Parallel Less Expensive?
When I run the numbers from the estimating guides, 1000 kcmil wire is usually the highest installed cost in $ per amp, even including the cost of running added parallel conduit runs. As a rule, I parallel anything over 500 kcmil.
I never trust the estimating guides and rely on the electrician's who actually do the work. The feedback I get is the ease of pulling 2  350's saves costs over pulling 1000's.
Another way of looking at it is weight= labor. Heavier wire & larger conduit means more weight = more labor = more costs. The smaller, parallel wire runs usually have less pounds per foot of circuit.
I'm interested in hearing what the people who work with it every day prefer.Bob Wilson
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The IR drop in the 1000 kcmil conductor will be lower than that of 3x 250 kcmil conductors.
However once you get to the bottom of table 9, the reactive impedance of the conductors is of similar magnitude to the resistance.
It is plausible to me that the IZ of 3 x 250 kcmil would be lower than the IZ of a single 1000 kcmil. However I would not trust the table 9 values for Z to apply to your situation. The actual reactive impedance will depend upon the geometry of the conductors in the conduit, and with 9 conductors the geometry will certainly be different from what table 9 assumes.
Jon
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Thanks for the quick reply.
This isn't for an actual job, my question is just theoretical. My background is EE but I've been in the software industry for a long time now, and am pretty ignorant on this realworld stuff. Why can I go with the 90C column even if the wiring is rated at 75C?
I've played with various VD calculators and, again speaking from a newbie point of view, many seem flawed when dealing with large diameter conductors. It seems the "skin effect" and power factor are not taken into account... and these two factors seem to really take hold on VD calculations when the conductor size grows large. Most seem to just go with a "k=12.9" factor for copper. I tried the calculation listed in NEC Table 9 Note #2 for large diameter conductors and that is what yielded the higher VD value in the posting.
One online calculator that seems to to avoid this kfactor limitation and go with VD calculation based upon actual reactance / AC resistance / PF factors is the "Southwire" one at
http://www.southwire.com/voltagedropcalculator.jsp
Anyway, I need to sort this out, and your response is much appreciated!
thanks again
Nate.
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Originally posted by rcwilsonAnother way of looking at it is weight= labor. Heavier wire & larger conduit means more weight = more labor = more costs. The smaller, parallel wire runs usually have less pounds per foot of circuit.
I'm interested in hearing what the people who work with it every day prefer.
Same basic issue just scaled down.
http://forums.mikeholt.com/showthread.php?t=96803"I dont have a girlfriend, but I know this girl who would get mad if she heard me say that." Mitch Hedberg RIP
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Originally posted by nateholtWhy can I go with the 90C column even if the wiring is rated at 75C?
If, and only if, the insulation system is rated for 90C, then you can use the 90C column of Table 310.16 for the purposes of derating. This applies both to derating for ambient temperatures over 30C and derating for more than three current carrying conductors in a raceway. The place in the NEC that says you can do this is the last sentence of 110.14(C).Charles E. Beck, P.E., Seattle
Comments based on 2017 NEC unless otherwise noted.
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Anyone / Everyone...
A few more questions about splitting up a large ampacity need into multiple conductors.
1. What is "rule of thumb" wiresize breakpoint where you would consider going with multiple conductors?
2. What is the maximum number of conductors that you would split across? For example, let's say you have ampacity and voltage drop that calculates out to 11000MCM, 2350MCM, 3250MCM or 54/0 for each phase. Would you ever split into 5 conductors? What is the "normal" max you would normally go?
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I'm not sure "rule of thumb" is a good term. I would say that the size for large ampacities would be 250 to 500 MCM with as many runs as required for the total ampacity. I also do not try to spec greater than 500 MCM based on feedback I got from the guys in the field. I use separate conduit also if I can. Other feedback was that setting up multiple spools adds to the difficulty.
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Originally posted by nateholtThanks for the reply. And if you run multiple conduits to accommodate the splitparallel conductors, you would make sure that each conduit carries conductor representatives of all three phases (i.e. you wouldn't run one phase per conduit), right?
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