grounded conductor

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joetrim

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I am running a subpanel off of a mdp. the mdp is 1200 amps 480/277 volts. I'm running a 3- 500's which are good for 380 amps as the ungrounded conductors and (1) 1/0 for the grounding conductor. My grounded conductor(neutral) load is about 100 amps. Whats the minimum size grounded conductor I can run and where would I find it in the NEC?
 

Dennis Alwon

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385*3= 1125???? Article 240.4 gives us no realease for over 800 amps,

You can calculate the neutral to the sub panel and just use whatever size it needs. Thus if you have 200 amps then you only need 3/0 copper
 

joetrim

Member
grounded conductor

Would I go off of 250.30 8a? My grounding conductor is supposed to be a 1/0 per table 250.66. So would I keep my neutral the same? Remember I'm feeding a 400 amp panel board off of the MDP with 3-500mcm ungrounded conductors and 1- 1/0 grounding conductor. I'm trying to figure out what neutral(grounded conductor) I have to put in since I have a lighting load of only about 100 amps.
 

jetlag

Senior Member
The neutral only carries the unbalanced load from the panel, it will never even be anywhere near the 100 amps. , but there is a limit how small the ahj is going to accept. He probably wont argue with 3 sizes under the grounded conductors.
 

Dennis Alwon

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First off we are talking a sub panel and not the main service- big difference. If you run a 200 amp panel that has all electric heating and one circuit for a service outlet, then you do not need to run anything other than what is calculated for the neutral. In this case it could be a #12 for the service recep.

In most situation the neutral tends to be sized the same as the ungrounded conductors.
 

Dennis Alwon

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The neutral only carries the unbalanced load from the panel, it will never even be anywhere near the 100 amps. , but there is a limit how small the ahj is going to accept. He probably wont argue with 3 sizes under the grounded conductors.


I don't care what the AHJ wants, I want a reference to turn it done. I have done a panel that only had electric heat and I had no neutral.
 

jetlag

Senior Member
Would I go off of 250.30 8a? My grounding conductor is supposed to be a 1/0 per table 250.66. So would I keep my neutral the same? Remember I'm feeding a 400 amp panel board off of the MDP with 3-500mcm ungrounded conductors and 1- 1/0 grounding conductor. I'm trying to figure out what neutral(grounded conductor) I have to put in since I have a lighting load of only about 100 amps.

The most lighting load you could ever have on the neutral is the amount on the leg with the most lights on it if you turned off the lights on the other legs. As soon as you turn lights on the other legs they will start to balance the neutral.
 

joetrim

Member
grounded conductor

This is a sub panel thats 400 amp rated. Most of the loads are 480 volt motor loads with no neutral. I have approximately 60 amps of 277 volt lighting which requires a neutral. I wanted to size my neutral atleast for 100 amps. The 3 ungrounded conductors will be 500 mcm. I need to know where I size the neutral. Do I size it the same size or bigger than the grounding conductor? Do I get my grounding conductor from 250.122 or 250.66. My breaker size will be a 3 pole 350 amp breaker. If I size my grounding conductor off of 250.66 it is a 1/0, if I size it off of 250.122 its a #3.
 

Dave58er

Senior Member
Location
Dearborn, MI
The neutral only carries the unbalanced load from the panel, it will never even be anywhere near the 100 amps. , but there is a limit how small the ahj is going to accept. He probably wont argue with 3 sizes under the grounded conductors.

I thought the neutral was the grounded conductor. :)
 

Dave58er

Senior Member
Location
Dearborn, MI
So my grounding conductor would be a #3. What would my grounded conductor be and where is the code reference?

It seems to me 215.2 (A) (1) would have you size the neutral by 250.122. (#3 Cu.)

I get the impression this is going to be challenged though. :)
 
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elohr46

Senior Member
Location
square one
This is a sub panel thats 400 amp rated. Most of the loads are 480 volt motor loads with no neutral. I have approximately 60 amps of 277 volt lighting which requires a neutral. I wanted to size my neutral atleast for 100 amps. The 3 ungrounded conductors will be 500 mcm. I need to know where I size the neutral. Do I size it the same size or bigger than the grounding conductor? Do I get my grounding conductor from 250.122 or 250.66. My breaker size will be a 3 pole 350 amp breaker. If I size my grounding conductor off of 250.66 it is a 1/0, if I size it off of 250.122 its a #3.

see article 220.61, 2008 nec
 

jetlag

Senior Member
2008 nec handbook

2008 nec handbook

So my grounding conductor would be a #3. What would my grounded conductor be and where is the code reference?

If you have or can get a 2008 nec handbook , study example D3a . according to that a 100 a neutral (grounded conductor) is the most they can make you use.. but I can tell you if you had exactly 20 a lighting load on each ungrounded conductor the neutral load would be near zero. They have to assume that someone may put all the lighting loads on the same leg. I am refering to the examples in the back of the handbook.
 
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kbsparky

Senior Member
Location
Delmarva, USA
First off we are talking a sub panel and not the main service- big difference. If you run a 200 amp panel that has all electric heating and one circuit for a service outlet, then you do not need to run anything other than what is calculated for the neutral. In this case it could be a #12 for the service recep. ....

Except that you have to use at least the same size as your egc in such a scenario. Which would be a minimum #6 for a 200 Amp panel.
 

Dennis Alwon

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Chapel Hill, NC
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Wow, a lot has gone on since I went into oblivion yeaterday.

Where does 215.2(A)(2) come into effect here. It states

(2) Ampacity Relative to Service Conductors. The feeder conductor ampacity shall not be less than that of the service conductors where the feeder conductors carry the total load supplied by service conductors with an ampacity of 55 amperes or less.

Article 250.122 is for sizing the EGC or equip. grounding conductor.

Table 260.66 is for bonding jumpers and GEC.

I agree with Elohr46 statement

see article 220.61, 2008 nec
 

Dennis Alwon

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Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Except that you have to use at least the same size as your egc in such a scenario. Which would be a minimum #6 for a 200 Amp panel.
You may be correct but where is that written?

It does not appear correct because if I have an egc in emt and have no neutral I don't have to pull a white wire
 
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