undersized neutral?

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jdr

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A 2000A, 480/277 wye service is being built. they want to size the
neutral at 3/0. there is only 3ph load. I don't think this is a good way to go.
Is this legal?
 
The service grounded conductor need only be sized for the load it is expected to carry as per 220.61 and not less than the required grounding electrode conductor per 250.24(C)(1).
 
3/0 does not meet the 12.5% rule in 250.24(C)(1), however I would expect that the service is made up of prallel runs and 1/0 is the smallest permitted parallel conductor so I expect that a 1/0 in each raceway would do the job.
Don
 
jdr said:
there is only 3ph load.
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If that is true, then there is no need to run a neutral at all. Call it a 480 volt service, and not a 480/277, and save yourself a wire.
 
charlie b said:
If that is true, then there is no need to run a neutral at all. Call it a 480 volt service, and not a 480/277, and save yourself a wire.
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No, 250.24(C) requires it to be brought to the servce equipment. From there you run it only if needed.
 
Ragin Cajun said:
A 2000A service needs ~ 7 - 500MCM/phase for a total of 3500mcm. Per table 250.66 this is >1100 MCM so the grounded conductor should be 3/0?
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Why 7 sets? 2000amps/380amps=5.26 or 6 sets.

No, 250.24(C) requires it to be brought to the servce equipment. From there you run it only if needed.
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There is such a thing as a 3 wire Delta service which could be used if there are no 277 volt loads requiring a neutral.
 
charlie b said:
If that is true, then there is no need to run a neutral at all. Call it a 480 volt service, and not a 480/277, and save yourself a wire.
Click to expand...

Check with the power co and see if they will 'unbond' the neutral at the transformer or replace it with a delta one.

250.24(B) 2002
 
Ragin,
A 2000A service needs ~ 7 - 500MCM/phase for a total of 3500mcm. Per table 250.66 this is >1100 MCM so the grounded conductor should be 3/0?
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You have to read all of the section.
(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.
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Don
 
Pierre,
I did the math and figured it at 2/0 per raceway...calculations are not my strength. Can you show me where I went wrong?
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2000 amps will take 6 sets ot 500kcmil or 5 sets of 600kcmil. In either case the total is 3000 kcmil per phase. 12.5% of 3000 kcmil is 375 kcmil. 1/0 is 105.6 kcmil so 5 1/0 will have more than the required area.
Don
 
infinity said:
Why 7 sets? 2000amps/380amps=5.26 or 6 sets.
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I knew that, DUH. My fingers can't seem to hit the right keys.



infinity said:
There is such a thing as a 3 wire Delta service which could be used if there are no 277 volt loads requiring a neutral.
Click to expand...

Delta services - ungrounded - have too many issues. Hate them things!
 
don_resqcapt19 said:
Ragin,

You have to read all of the section.

Don
Click to expand...

Leave it to a Monday. I usually run the same size neutral anyway to keep things simple, unless it's a smaller service. I am doing one now that's 8-500MCM/phase.
8 x 500 = 4000 x 12.5% = 500MCM.

Have another that's 10 500 MCM. 10 x 500 = 5000 x 12.5% = 625. Good thing we don't have to have the neutral greater than the phase conductors. Oops, better not let the wire manufacturers hear that, they will want to change it. Yeh, I'm cynical.

It's Tuesday, today, maybe I'll get things right!

BTW, I sure appreciate this forum!!


RC
 
RC,
In both of the last two cases, a 1/0 in each raceway is more than the required 12.5%. 310.4 is the only reason that you need a 1/0 in each raceway.
Don
 
don_resqcapt19 said:
RC,
In both of the last two cases, a 1/0 in each raceway is more than the required 12.5%. 310.4 is the only reason that you need a 1/0 in each raceway.
Don
Click to expand...

Ok, what did I miss? Are we looking at 12.5% of the total mcm per phase or one conductor?
 
RC,
You only need 12.5% of the total of the largest set of phase conductors. If you have 10 sets of 500kcmil, you have a total of 5000kcmil. 12.5% of that is 625kcmil. That is the total of all 10 grounded conductors. If it wasn't for the 1/0 minimum size rule in 310.4 you could install 10 #2s and be in compliance with the 12.5% rule.
Don
 
don_resqcapt19 said:
RC,
You only need 12.5% of the total of the largest set of phase conductors. If you have 10 sets of 500kcmil, you have a total of 5000kcmil. 12.5% of that is 625kcmil. That is the total of all 10 grounded conductors. If it wasn't for the 1/0 minimum size rule in 310.4 you could install 10 #2s and be in compliance with the 12.5% rule.
Don
Click to expand...

Thank you. I guess with my thick skull and the convoluted NEC language that "interpretation" never made it through the bone mass.


THANKS!

RC
 
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