eq gr

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stan a beller

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i have to run 670' to a motor 3ph 240v 32.2 a.what is size of ground wire? The wire size is #1 cu.#1 is the ungrounded cond.

[ February 26, 2006, 05:44 PM: Message edited by: stan a beller ]
 
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stan a beller

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Re: eq gr

moderators note: This post was edited by the poster. It had said that the wire was #2.

[ February 26, 2006, 06:09 PM: Message edited by: don_resqcapt19 ]
 
don_resqcapt19

don_resqcapt19

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Re: eq gr

First, I think that #2 is too small. That will have more than 7% voltage drop. The calculation for the minimum size EGC is based on the ratio of the area of the conductor that would normally used for the 68 amp load and the increased size that was actually used. The EGC has to be increased by the same ratio. The code required conductor size for a 68 amp load is #4 and the EGC would be a #8. The #4 has 41,740 circular mils and the #2 has 66,360. 66360/41740 =1.59
This is the factor that we must increase the EGC by. A #8 has an area of 16,510 circular mils. 16,510(1.59) = 26,248.3 circular mils. This is the smallest EGC that is permitted for this installation. A #6 has 26,240 circular mils making it too small, so a #4 with 41,740 circular mils is the code required EGC.
Don
 
1793

1793

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Louisville, Kentucky
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Inspector
Re: eq gr

I would like to know the reference for the EGC for the #4.

Table 310.16. For the #4 we use the 60 degree column for 70 amps, where do we go for the EGC?
 
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peter

Senior Member
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San Diego
Re: eq gr

Don.
Great. He is 8.3 circular mils short of being eligible for using #6. This is .0003163% short [which I don't think is really that much].
According to my figures, this will cost him an additional $153.68.
Perhaps he could use #5?
~Peter
 
bob

bob

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Re: eq gr

i have to run 670' to a motor 3ph 240v 32.2 a.what is size of ground wire? The wire size is #1 cu.#1 is the ungrounded cond.
Click to expand...
The post says 32 amps and Don is using 68 amps.
Stan
Which is it?
If its 32 amps #1 is ok for VD. If its 68 amps
then you may need to go to 1/0.
Due to the distance involved I do not think you can use the values in the table directly. If the load is 32 amps and breaker is 70 amps you would need #1 cu phase conductors and #4 EGC to ensure that the breaker will trip. Is the circuit in metal conduit?

[ February 26, 2006, 11:44 PM: Message edited by: bob ]
 
don_resqcapt19

don_resqcapt19

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Re: eq gr

I'll try it again based on the changed info provided by the poster. First with a 32 amp motor load the smallest permitted circuit conductors would be #10, and the EGC from Table 250.122 for a 32 amp overload device would also be #10. Any time you increase the circuit conductors above the minimum size required for that load by the code, then 250.122(B) requires that the EGC be increased in size proportionately according to circular mil area of the ungrounded conductors. In this case, since both the circuit conductors and the EGC are permitted to be #10s, then when you increase the circuit conductors to #1, you must also increase the EGC to #1.
Don
 
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bphgravity

Senior Member
Location
Florida
Re: eq gr

This is one of the flaws with 250.122(B). The EGC probably doesn't have to be full sized to remain effective however this section requirement is all we have to go by.
 
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pierre

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Location
Westchester County, New York
Re: eq gr

Bryan
I do not necessarily agree with your statement. I am not an engineer, I am saying this based on my experience in the field.

The note to Table 250.122 tells us that "Where necessary to comply with 250.4(A)(5), the equipment grounding conductor shall be sized larger than given in this table".


I have seen many times in the field when there was a ground fault of some type, and the overcurrent device did not open. This would be one time when erring on the side of safety may not hurt.
 
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bphgravity

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Location
Florida
Re: eq gr

Don't get me wrong Pierre, I am the first one to bring up that note when a question is posted regarding the EGC. I'm just saying that 250.122(B) doesn't work for all cases.

Again, this is the problem with this section and table. On one hand, it is too restrictive for smaller circuits yet may not be appropriate for circuits with high available fault current.

The code does a poor job providing guidance for these situations, and many electricians haven't even heard of withstand ratings. I know I didn't until I came to this Forum...
 
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stan a beller

Member
Re: eq gr

sorry about the mess up guys. My second try at an entry.i came up with #1 for a egc but didnt like the idea that my edc was the same size as my ungrounded cond..i talked to an elec. inspector, he said a #10 would do using an exemption in 430 for motors but that didnt settle right with me.
 
don_resqcapt19

don_resqcapt19

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Illinois
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retired electrician
Re: eq gr

Stan,
While 250.122(D) does permit you to size the EGC based on the rating of the motor overload protection, but only when the motor ciruit overcurrent device consists of an instantaneous trip circuit breaker or a motor short-circuit protector. Even in that case, 250.122(B) applies and any time you increase the size of the circuit conductors for circuit rated 30 amps or less, you will have to use the same size conductor for the EGC as for the circuit conductors.
Don
 
bob

bob

Senior Member
Location
Alabama
Re: eq gr

I came up with #1 for a egc but didnt like the idea that my edc was the same size as my ungrounded cond..i talked to an elec. inspector, he said a #10 would do using an exemption in 430 for motors but that didnt settle right with me.
Click to expand...
What exception is he quoting? Regardless of the exception #10 as an egc is not the correct size.
Table 250.122 should be used for a distance of about 100 ft. Any circuit longer should be checked to ensure that there is sufficient fault current to cause the breaker to operate as fast as possible. What is fast enough? 1/2 to 1 cycle.
Since this motor has a FLA of 32 amps, wouldn't
the breaker be sized at 70 amps and require a #8 egc to start with?
 
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pierre

Senior Member
Location
Westchester County, New York
Re: eq gr

Bob

"Table 250.122 should be used for a distance of about 100 ft. Any circuit longer should be checked to ensure that there is sufficient fault current to cause the breaker to operate as fast as possible. What is fast enough? 1/2 to 1 cycle."

Not that I disagree with this, but where can we substantiate this? Also do you know of any tables that can be referenced? Would this not create a ton of havoc between inspectors and ECs?


I believe that the EG is the most important facet of most any circuits we install, but there is not enough guidance to help us to determine in the field what is and is not effective.
 
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