Confused about grounds in parallel conduits

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peter

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I am designing a chart to simplify feeder calculations. So far, I have come up with something like this:
1000amp load
1000 amp breaker
(2) sets of 3 900kcm hots and (2) sets of 500kcm neutral
OR
(3) sets of 400kcm and 4/0
OR
(4) sets of 4/0 and 3/0 "neutral"
OR
(5) sets of 3/0 @200amps and 2/0 grounded conductor.
But I cannot make head nor tail of 250-122(f)(1) which says 'base on Table 250-122' and then (a) says it doesn't need to be larger than the circuit conductor. Also, there is something about at least 12 1/2% of area of hots if they are [added together] more than 1100kcmil.
So what is the secret, Excel digestible, formula for the green ["equipment gtounding conductor"]?
~Peter
 
Re: Confused about grounds in parallel conduits

Peter,

Each conduit,in your 1000a feeder,will need a 2/0 copper or 4/0 alum. EGC. That's it.
 
Re: Confused about grounds in parallel conduits

Frank is correct. For your example, the EGC in each conduit is sized according to the OCPD listed in table 250.122.
 
Re: Confused about grounds in parallel conduits

Peter that is going to get complicated.

Have you considered derating issues?

How many current carrying conductors in each raceway?

Even with one 'set' per raceway you can have four current carrying conductors.

By the way, and this is just my opinion you might want to set a cap of 750 kcmil.

We generally do not use larger than 600 kcmil.

The larger you go the less 'current per copper' you get.

300 kcmil @ 75 C = 285

600 kcmil @ 75 C = 420

Two 300s will give you 570, 150 amps more than one 600.

Bob
 
Re: Confused about grounds in parallel conduits

Originally posted by iwire:
The larger you go the less 'current per copper' you get.
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That's something I get a kick out of.

The sixplexes I do get a 400 amp service. I generally put in two runs of 250 MCM AL.

When you ask someone who hasn't been down that road before, "How big do you think this would have to be for one run?" you always get the response, "500?"

It's fun to see their expression when you show them Table 310.16. :)
 
Re: Confused about grounds in parallel conduits

Originally posted by georgestolz:
Originally posted by iwire:
The larger you go the less 'current per copper' you get.
Click to expand...
That's something I get a kick out of.

The sixplexes I do get a 400 amp service. I generally put in two runs of 250 MCM AL.

When you ask someone who hasn't been down that road before, "How big do you think this would have to be for one run?" you always get the response, "500?"

It's fun to see their expression when you show them Table 310.16. :)
Click to expand...
Exactly. One of my suppliers called to ask me what size single conductors would be needed for a 400 amp residential service. He was trying to help a customer. I told him to tell the guy to parallel 250's. He said inspector would not allow parallel conductors. I said "huh" :( ? Then told him he probably should do a load calc and size for the load, not the 400. He hadn't yet done this. I don't know what he ended up doing, but I would have fought the decision that you can not parallel.
 
Re: Confused about grounds in parallel conduits

In my grand chart, at this point I am assuming that the continuous and non-continuous loads have already been calculated. That would be another Excel routine.
I don't see how derating would apply since I am using only three hots, one neutral and the ground in each conduit. My understanding is that in 3 phase, Y the neutral is not counted. In the smaller sizes, like 15 amp [alternative is 12] in say 3/4", derating does play a major factor.
As for the cap at 750000 square mil, @ $11.15/foot and 1 5/32" thick, this does seem to be something to avoid. But my chart will end up listing all options including weight per foot and cost per foot.
As for the inspector, is he crazy? What would he suggest for an 800 amp panel. 2,000,000 only carries 665 amps.
I think I now have the ground size settled. Just use 122 for each conduit. And I thank you.

Now I have a problem with sizing the neutral. I have spent all afternoon poring over 250-24(b)(2). It seems to be saying that the size of the grounded conductor in parallel is based on the size of the ungrounded conductor but doesn't say how. Right now, I am taking 200 off of the load, finding 70% of that and then adding the 200 back on. Then this is split up 2 or 3 or 4 ways for each conduit. Is this correct? [Not to be less that 1/0].
~Peter
 
Re: Confused about grounds in parallel conduits

Originally posted by throttlebody:
For residential, I would refer to 310-15(B)(6), not 310-16.
Click to expand...
Bob you can not use Table 310.15(B(6) for parallel conductors.
 
Re: Confused about grounds in parallel conduits

Originally posted by peter:
In my grand chart, at this point I am assuming that the continuous and non-continuous loads have already been calculated. That would be another Excel routine.
Click to expand...
As is required.

Originally posted by peter:
I don't see how derating would apply since I am using only three hots, one neutral and the ground in each conduit. My understanding is that in 3 phase, Y the neutral is not counted.
Click to expand...
It depends on what you are suppling,.

310.15(B)(4)(c)On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral shall therefore be considered a current-carrying conductor.
Click to expand...
Go here for a little more info.

You also can run more than one set in each raceway.

One set of 3/0 is good for a 200 amp feeder, are two sets of 3/0 run in one raceway good for 400 amps?


Originally posted by peter:
Now I have a problem with sizing the neutral.
Click to expand...
You should have trouble with that. :p

You have two choices, full size neutral or do load calcs for each individual application.

You need to know the load the neutral will carry before you can decide to downsize it.

It also makes a difference if it is a feeder or a service, if it is a service there are limits to how much you can reduce the neutral as it is also the ground fault path.

If it is a feeder the neutral only has to be cable of carrying the load.

[ February 01, 2006, 06:27 AM: Message edited by: iwire ]
 
Re: Confused about grounds in parallel conduits

The problem with making "charts" or handy references is that the code must cover all conceivable applications and conditions. It is very hard to make generalizations that apply in all circumstances. A wise man once told me "there are no shortcuts". The older I get and the more I learn the more I realize that he was correct.
 
Re: Confused about grounds in parallel conduits

I suggest that when you finish your handy reference tool, you add a bold, clear, and unambiguous statement to the effect that it is for assistance and information only, and is not to take the place of reading the real rules in the real code book.
 
Re: Confused about grounds in parallel conduits

Ryan, that would be 2,000,000 cmil, or 2,000 kcmil. But yes, that is a big conductor.
 
Re: Confused about grounds in parallel conduits

I once saw a set of plans that included such a table. I regret that I didn't have time to copy itbut it was part of the print. Evidently the designer simply put it there to avoid doing the same calculations over and over again. Actually, there were two such tables -- one for Y and the other for delta, but there didn't seem to be much difference.
Right now, I'm just confining it to THHN and EMT. It will include weight and cost per foot.
Iwire,
Thanks for alerting me to the difference between Service and Feeder. Where is this in the Code?
Perhaps in 250-24(b)(2)? or -122?

Since nobody objects to my 30% discount for the neutral, I'll assume I got that right. :)
My computer understands 2000000 much better than 2000ksmil. In fact, it converted #1/0 into Jan. '06.
~Peter
 
Re: Confused about grounds in parallel conduits

Peter,
Since nobody objects to my 30% discount for the neutral, I'll assume I got that right.
Click to expand...
You have to size the grounded conductor based on 220.61 or 250.24 which ever results in the largest size.
Don
 
Re: Confused about grounds in parallel conduits

:confused: California is, I think, still on the 1999 Code so that is what I have. What is 220.61?
~Peter
 
Re: Confused about grounds in parallel conduits

Originally posted by peter: California is, I think, still on the 1999 Code so that is what I have. What is 220.61?
Click to expand...
The 1999 version is 220-22.
 
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