310.15(b)(7) and derating

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electrofelon said:
Someone will correct me if I am wrong, but I do not believe you could meter these separately where there is only a common neutral available - you would need to meter at the point where there is an individual neutral for each metered circuit.
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I'm not sure, but I would think theoretically you only need the neutral for a voltage reference, so you know all three pairwise voltages. And then measuring the currents on the two ungrounded conductors would suffice. Since KCL provides a relationship on the currents the neutral current would be determined.

But I don't know how actual meters work. And if the metering is at the load end of the feeder, it wouldn't matter.

Cheers, Wayne
 
wwhitney said:
I'm not sure, but I would think theoretically you only need the neutral for a voltage reference, so you know all three pairwise voltages. And then measuring the currents on the two ungrounded conductors would suffice. Since KCL provides a relationship on the currents the neutral current would be determined.

But I don't know how actual meters work. And if the metering is at the load end of the feeder, it wouldn't matter.

Cheers, Wayne
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Yeah I think you are right that it would be okay. I was thinking you would need to measure the current on the neutral but forgot it is just the voltage the meter needs.
 
electrofelon said:
Okay well how about weasel your way around that rule by coming up with a combination that is not shown in the tables so you have to calculate it? How about using say copper xhhw for the neutral?
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Maybe I can put a strand of cotton in the conduit, surely I would have to include the strand of cotton in conduit fill. Anything can be a conductor if provided high enough voltage right? :ROFLMAO:
 
electrofelon said:
But really I was dead serious about that idea. How can you use the table if there is no table for your combination?
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Good loophole, to be fair although combination is not shown, they do list of all approximate areas of the conductors and max total fill for all conduits.

electrofelon said:
Tainted:. What are your thoughts on the common neutral idea, or the reduced neutral idea? You haven't commented on those, you seem dead set on individual feeders with full neutrals.
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It's absolutely a great idea but at the same time I want to avoid it. If something is wrong with the neutral or if there needs to be maintenance done or servicing, all the apartments sharing the neutral will be affected.
 
Tainted said:
It's absolutely a great idea but at the same time I want to avoid it. If something is wrong with the neutral or if there needs to be maintenance done or servicing, all the apartments sharing the neutral will be affected.
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The only solution to that would be a single feeder disconnect to all shared-neutral units.
 
Tainted said:
Good loophole, to be fair although combination is not shown, they do list of all approximate areas of the conductors and max total fill for all conduits.
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True, they do show the inflated conductor diameter which still screws you over. Only other thing I can think of is try to find compact or compressed copper conductors, but I am not sure anyone makes them. Priority wire and cable calls their copper xhhw conductors compact but I'm not sure if they are truly compact. if you believe the spec sheet then you could use table 5A for compact conductors.


It's absolutely a great idea but at the same time I want to avoid it. If something is wrong with the neutral or if there needs to be maintenance done or servicing, all the apartments sharing the neutral will be affected.
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I really don't think you're concerns are justified. What are the chances of "maintenance or servicing" needing to be done on one of the common neutrals? You can say that about anything. What is the difference between having a single service or distribution panel feeding multiple units? It's the same thing if the service or distribution panel or feeder needed servicing. Besides you have multiple feeders in the same conduit if something goes wrong or needs rerouting or servicing they all need to go down anyway. I think you are overthinking this.

Maybe show the person writing the checks the cost comparison for the shared aluminum neutral and see what he thinks?
 
electrofelon said:
True, they do show the inflated conductor diameter which still screws you over. Only other thing I can think of is try to find compact or compressed copper conductors, but I am not sure anyone makes them. Priority wire and cable calls their copper xhhw conductors compact but I'm not sure if they are truly compact. if you believe the spec sheet then you could use table 5A for compact conductors.




I really don't think you're concerns are justified. What are the chances of "maintenance or servicing" needing to be done on one of the common neutrals? You can say that about anything. What is the difference between having a single service or distribution panel feeding multiple units? It's the same thing if the service or distribution panel or feeder needed servicing. Besides you have multiple feeders in the same conduit if something goes wrong or needs rerouting or servicing they all need to go down anyway. I think you are overthinking this.

Maybe show the person writing the checks the cost comparison for the shared aluminum neutral and see what he thinks?
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I've been trying to look online everywhere for compact copper THWN-2 conductors and couldn't even find them. I want would like everyone to put a bounty on this one lol.

electrofelon said:
.... Or calculate the neutral load and drop the neutral down a size if you can. I'm really trying to get you a solution here tainted but you're not playing ball!
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It's a solution for sure, and I am definitely considering it, but I would also like to think about it a bit before actually doing it. I'm playing the ball just warming up first.
 
I think compact conductors are a way for aluminum to compete with copper for conduit sizing.

Making strands of different shapes must cost more with aluminum, and even more with copper.
 
wwhitney said:
So when the 3 runs separate you set a box and use a 4-way Polaris connector to split the neutral 3 ways? Even if just one feeder splits off from the other two, you could split the neutral 3-ways there and just run separate neutrals with those other two.

Cheers, Wayne
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I would be more likely to use something that taps the main run without cutting it as much as possible, or at least an irreversible splice method if reducing the run after the tap. Somebody that has no business doing so will otherwise open the neutral under load someday.
 
electrofelon said:
It just occurred to me that the only potential problem I can see with the common neutral method is from a metering standpoint. If this was 120/240 single phase we could still meter the three separately but not if it's 120/208 3 phase. Submetering may be a possibility.
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5 jaw meters typically used don't measure neutral current, they just have a voltage reference to the neutral. Shouldn't effect the reading to have a common neutral.
 
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