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Posted By: Yoopersup 3000 277/480 V Y Neutral size? - 02/13/08 01:51 AM
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
Posted By: frenchelectrican Re: 3000 277/480 V Y Neutral size? - 02/13/08 02:03 AM
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

Posted By: Yoopersup Re: 3000 277/480 V Y Neutral size? - 02/13/08 02:27 AM
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
Posted By: frenchelectrican Re: 3000 277/480 V Y Neutral size? - 02/13/08 03:34 AM
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
Posted By: sparkyinak Re: 3000 277/480 V Y Neutral size? - 02/13/08 05:14 AM
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.
Posted By: SteveFehr Re: 3000 277/480 V Y Neutral size? - 02/13/08 12:03 PM
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
Posted By: Yoopersup Re: 3000 277/480 V Y Neutral size? - 02/13/08 04:56 PM
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.
Posted By: SteveFehr Re: 3000 277/480 V Y Neutral size? - 02/13/08 05:26 PM
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.
Posted By: Yoopersup Re: 3000 277/480 V Y Neutral size? - 02/13/08 07:49 PM
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
Posted By: resqcapt19 Re: 3000 277/480 V Y Neutral size? - 02/16/08 03:05 PM
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
Posted By: Yoopersup Re: 3000 277/480 V Y Neutral size? - 02/16/08 06:53 PM
Don
Gotta be at least a 3/0.
250.66 says a service this size requires a 3/0 ground Minumin.
250.24 (C) 1 says Neutral cannot be smaller then grounding electrode conductor per 250.66.
Thats where I came up with the 3/0.
1/0 is Min. size under 310.4 for Paralling conductors.
Yoopers
Posted By: resqcapt19 Re: 3000 277/480 V Y Neutral size? - 02/17/08 12:02 AM
Yoopersup,
The 3/0 does not apply here as you have have service entrance conductors that total more than 1100kcmil. It has to be 12.5% of the total if the phase conductors are in a single raceway or cable tray. When the grounded conductor is run in parallel conduits it is sized by Table 250.66 based on the phase conductor in each raceway. In this case you need a 1/0 in each raceway unless the Article 220 load calculation requires a larger grounded conductor.
Quote
250.24(C)(1) Routing and Sizing This conductor shall be routed with the phase conductors and shall not be smaller than the required grounding electrode conductor specified in Table 250.66 but shall not be required to be larger than the largest ungrounded service-entrance phase conductor. In addition, for service-entrance phase conductors larger than 1100 kcmil copper or 1750 kcmil aluminum, the grounded conductor shall not be smaller than 12 1/ 2 percent of the area of the largest service-entrance phase conductor. The grounded conductor of a 3-phase, 3-wire delta service shall have an ampacity not less than that of the ungrounded conductors.
(2) Parallel Conductors Where the service-entrance phase conductors are installed in parallel, the size of the grounded conductor shall be based on the total circular mil area of the parallel conductors as indicated in this section. Where installed in two or more raceways, the size of the grounded conductor in each raceway shall be based on the size of the ungrounded service-entrance conductor in the raceway but not smaller than 1/0 AWG.

Don
Posted By: sparkyinak Re: 3000 277/480 V Y Neutral size? - 02/17/08 02:19 AM
Keep in mind that just because the NEC says you can use a smaller neutral means that you can. The loads will have a say in it. This is why it has been recommend in several posts to consult the EE who did the design. Wavering from the stamped design can opens Pandora's box with grief.
Posted By: Yoopersup Re: 3000 277/480 V Y Neutral size? - 02/17/08 03:46 AM
Don
Never read it that way but you might have a point there! cause 8 1/0"s have a lot more current carrying capacity then one 3/0 and the Neutral Must be more conductive then the Grounding electrioid conductor .

Sparky Beleive me even 8 1/0's are wayyyyyy over sized for a Neutral with 500 amps Possible on it.



Posted By: sparkyinak Re: 3000 277/480 V Y Neutral size? - 02/17/08 10:56 PM
I fully agree. I have seen hulking neutrals carrying just a few amps because the system is balanced that day. It may not be balanced tomorrow.

I am not saying that any of the proposed suggestions will not work. I am saying is giving the size of the service, it tells me that an EE did the design. It has been years since I worked in Michigan and a design over $100K (possibly local ordinance) required an EE stamp. By veering from the design basically voids the stamp just like voiding a product listing. If something to were to happen due to the change, the EE will not be resposible but whomever made and allowed the change will.

I have no clue on what the service is powering so I could or would not chime in on what size of neutral to use. For example, what kind of harmonic loads and sizes are involved? Whatever it is, it is big. I would not want to be the one to give the go ahead now to use a smaller neutral and have something go wrong with it years later because of an unknown. I would not want to be anywhere near the facility if it were to lose its neutral.

I would not have any problem using a smaller neutral if there is real value to the project, but not without the EE's blessing. I do not use reduced neutrals on a dwelling service. The cost to size it is not covered in the savings of using a smaller wire.
Posted By: Yoopersup Re: 3000 277/480 V Y Neutral size? - 02/18/08 04:50 AM
Sparkyinak
When you get into services this big most times theres hardly any Load on the Neutral sometimes the Neutrals installed only because its a WYE system and its required for Grounding Purposes only. The loads are mostly all three phase motors or Transforers 480 to 120/208 volt.When your into smaller services I agree the Neutral ect is of concern.
Yoopers
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