250.24B1 Question

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biggladant

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I am paralleling 4/0 for a service that needs 450 amps. It is 240 3-phase Delta. It is mostly 3-phase motor loads but it does have a possible nuetral load of 85 amps. My question is can I size the neutral from 250.66 or becuase of 250.24B1 the last sentance do I have to have the same ampacity as the 4/0's?
 
Re: 250.24B1 Question

Table 250.66 is for the grounding electrode conductor which is installed at you main switch and runs to the grounding electrode. See 250.50 for information.
250.24B1 is a new requirement. If you bring the grounded conductor(neutral) to the main switch
it must be sized according to 250.24B 1 thru 3.

You said that you need 450 amps. What is the size of your main breaker?
 
Re: 250.24B1 Question

250.24B1 says my grounded conductor "shall not be smaller than the required grounding electrode conductor specified in table 250.66" I guess my concern is the last sentance in 250.24B1--the way I read it is that my grounded conductor will have to be the same size as my ungrounded conductors.Am I reading this Correct???

My service is 240V 3-Phase Delta- 3 ungrounded and 1 grounded, of course I will add my EGC at the Main. My Main is a 500 amp breaker, this is motor loads with a 12 S.P panel for some lights and recepts.
 
Re: 250.24B1 Question

biggladant,
You do not have a 3 phase, 3 wire delta. You have a 3 phase, 4 wire delta so the last sentance in 250.24(B)(1) does not apply. The grounded conductor must be the larger of what is required by 220.22 or 250.24(B)(1). In this case the wire size required for an 85 amp neutral load is smaller than what is required by Table 250.66 based on parallel 4/0 service conductors.
don
 
Re: 250.24B1 Question

Thanks Don
I guess I was confused about the 3-phase 3 and 4 wire part. So what is a 3 wire delta system? Is it 3-phase without a grounded conductor ran with it?
 
Re: 250.24B1 Question

Not trying to answer for Don, but yes, a three wire delta is a "Phase to Phase" system only.

On the other hand a "Three Wire Delta System" could refer to a "Corner Grounded System" which would involve a "Grounded Conductor" but not for a reduced voltage (1/2 winding voltage) to ground.

A "Corner Grounded Delta" would still see "Phase to Phase" voltage i.e. 240, 480, etc... from any two points.

Roger

[ July 29, 2004, 08:24 PM: Message edited by: roger ]
 
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