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Joined: Mar 2003
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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
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Joined: Nov 2000
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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. 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
Don(resqcapt19)
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Joined: Jul 2007
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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.
"Live Awesome!" - Kevin Carosa
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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.
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Joined: Jul 2007
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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.
"Live Awesome!" - Kevin Carosa
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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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