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Old 04-11-2017, 02:39 PM   #1
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Default 250.122(b)

Just a heads up, I'm doing EGC sizing calculations in school right now. Ran into a question that I'm having just a bit of an issue with. The question reads:

From the service panel, a feeder is run to a distribution panel in a remote part of an aircraft hangar. Although the 150-A rating of the feeder breaker indicates that 1/0 AWG conductors are required for the feeder, the plans require that 4/0 AWG conductors be installed and connected to the feeder breaker in the service panel. With the 4/0 AWG wire installed as specified, the minimum size EGC permitted to be run with the feeder conductors is ___ AWG.



Okay so because of the length of the run and voltage drop, they are going to have to up the size of the ungrounded conductors from 1/0AWG up to 4/0 AWG. Now if you read 250.122(B), it says that the EGC must be sized "proportionately" to the ungrounded conductor size. What EXACTLY does this mean? Does this mean my EGC will also be 4/0 AWG? (Which isn't one of the answer choices).
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Old 04-11-2017, 02:45 PM   #2
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Is this not something that you're supposed to discuss with your instructor ?

BTW, in the real world, such calculations are almost always performed by EEs and slipped into the prints.

Making you look inappropriately smart to your instructor is not time well spent.
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Old 04-11-2017, 02:48 PM   #3
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What it basically means is if the increase in the ungrounded conductors was 62 percent you must also increase the grounding conductor by the same percentage based on the original table 250.122. Therefore it is proportionate.
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Old 04-11-2017, 02:57 PM   #4
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Quote:
Originally Posted by telsa View Post
Is this not something that you're supposed to discuss with your instructor ?

BTW, in the real world, such calculations are almost always performed by EEs and slipped into the prints.

Making you look inappropriately smart to your instructor is not time well spent.
We just don't have enough time. Our homework consists of over 100 questions, and that is a fraction of what we need to read on a weekly basis. We have no where near enough time to go over things like this.

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Originally Posted by kg7879 View Post
What it basically means is if the increase in the ungrounded conductors was 62 percent you must also increase the grounding conductor by the same percentage based on the original table 250.122. Therefore it is proportionate.
Okay that is what I was thinking, thanks man! Will try and work it out and post a follow up reply.
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Old 04-11-2017, 03:01 PM   #5
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circular mil area of 1/0 = 105600
circular mil area of 4/0 = 211600

So the circular mil area doubled.

So.... So usually the EGC would be #6 wire because that's what 150 amps is good for.

#6 circular mil area = 26240 x 2 = 52480. Which would be the same as #3 wire. 3 would be my answer? Is this how I would do it
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Old 04-11-2017, 09:02 PM   #6
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Originally Posted by JasonCo View Post
circular mil area of 1/0 = 105600
circular mil area of 4/0 = 211600

So the circular mil area doubled.

So.... So usually the EGC would be #6 wire because that's what 150 amps is good for.

#6 circular mil area = 26240 x 2 = 52480. Which would be the same as #3 wire. 3 would be my answer? Is this how I would do it
Yep.
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Old 04-11-2017, 11:28 PM   #7
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If the conduit is metallic like EMT or GRC the code still has to be followed but I would follow the code and eliminate the equipment ground from it altogether and spend the money on utility stocks instead. But I bet that answer is not on the test.................
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