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Joined: Mar 2003
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3000 amp service 8 sets 500mcm 3 per conduit plus neutral.
most loads 3 phase hardly any single phase.min. neutral size .
3/0 or 12 1/2 % of phase conductors which ever larger Right.
Yoopers

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Yoopersup;

normally this will be done by EE[ Electrical Engineer ] and the EE will use the correct size for it but you have to watch the grounding size as well. and the netutral willbe solid or implanced [ resitored ] system ??

typically with the service like this large will have GFI system on it as well.

Merci, Marc



Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

Joined: Mar 2003
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230.95 Ground Fault on Equipment.
Exception comes into play (Industrial)
neutral conductor read 250.24c C (1)& (2)
grounding conductor 250.66
I feel it must be a 3/0 just want to confirm.
Yoopers

Joined: Feb 2003
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i did make a quick figures for netural it will be at least 500 KCM size.

i came up by add the total sum of line conductors then multi by 12.5 i came up about 510 so 500 is common size hope that should work but be on safe side run that by electrical inspector to verify it.

Merci, Marc


Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

Joined: Jul 2007
Posts: 1,335
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Judging by the size of the service, you are working of a EE stamped design. You would need to bounce your question off the EE. The NEC allows smaller neutral but you are dealing with some serious loads that could be harmonic in nature. You are better off by going with the drawing or get the EE to sign off on the use of a smaller neutral. If you were to "shoot from the hip" by installing a smaller the called for neutral, and something were to happen because of it, you, not the EE will be held accountable. The size of service tells me not to take the risk especially if it is to save few dollars on copper. Let the EE take the risk. Engineers are not totally useless. smile

A few projects ago, we had a contrator used a smaller neutral siting the NEC up one side and down the other and saying that we do this all the time and no one has called him on it before. I told him what I telling you and told them my office would accept it if the EE would sign off of it. They went through the motions and the EE stated in thoery the neutral could be smaller but it was not worth the added expense to crunch the additional info plus it could hinder future needs so he always goes with a 100% neutral so he would not sign off. The contractor had to re-pull the service wires at their expense.


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EE may even require 200% neutral because of harmonics currents generated by the VFRs, UPSs, electronic ballasts and other switched power supplies that comprise the bulk of building loads these days.

Very very very few cases where anything less than 100% neutral would be acceptable. And generally cheaper for the customer for the EC to just pull a 100% neutral it than spend the man-hours for the engineer to calculate the minimum safe size. Though, if there are extenuating circumstances- like the existing 2-miles worth of duct bank will only fit a 50% neutral- I'm sure the engineer can be obliged to break out the modelling software or throw an extra relay into the switchgear wink

Last edited by SteveFehr; 02/13/08 08:04 AM.
Joined: Mar 2003
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Total Neutral load well run about 500 amps MAX.
3/0 =200ampx8= 1600 amp Neutral.
480 volt Mtr loads 300hp several, 200 hp, 100, hp and several smaller well NOT effect the Neutral. all except lighting 277 is off 480/120/208 transformers. Code SAYS no larger then Required has to be installed.
So where are you getting theses Larger Neutral requirements???I can see putting in whats required But not a lot more.

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220.61(A) requires 100% neutrals. It's not so much what *is* on the neutral as what *can* be on the neutral. If you lose a phase, then what? L-N fault insufficient to trip the breaker, then what? 220.61(B) permits some reductions, but you still can't just ignore loads, there's just a demand factor placed on them, and stats flat out that non-linear loads cannot be reduced.

200% neutral comes into play with harmonics and is an engineering decision, not a code-based one. You can have a perfectly balanced load, yet still be putting 170% current onto the neutral simply due to the nature of the load. In fact, the FPN note 2 in 220.61(C) explicitely calls attention to this.

Last edited by SteveFehr; 02/13/08 01:38 PM.
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This ain't a House where 100% Neutral may apply .
Read 250.21 (c) 1 & 2 What does it say. Reduced neutrals are installed all over Most because a Y requires a Neutral even if its not going to be used for grounding purposes.
As far as Harmonics this is not a schoolfull of computors and data switching equipment.If I have 500 amp load on the Neutral and I have it rated for 1600 amps I can't see the problemeven 500 x 175 % which on the loads I have WAS possible the neutral load even then would max out at 875 well below the 1600 amps.

Yoopers

Joined: Nov 2000
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250.24(C)(2)applies to this installation. You install the larger of what is required by the Article 220 load calculation or 250.24(C)(2). 250.24(C)(2) would require a 1/0 in each of the 8 raceways and as long as the load calculation shows less than 1040 amps for the neutral you are good to go.
Don


Don(resqcapt19)
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