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07-27-2011, 07:32 PM   #1
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Ground Wire Size Question

Hello,
I'm not an electrician but I have done some wiring in my time. Where I am currently working we are installing a machine that required three phase 480 we ran three 1 AWG conductors and one 8 Awg conductor as the ground wire, the lenght is appox 150 ft. I question the ground wire size as any engineers I have worked with in the past ran a ground wire of equal size. When I questioned the person in charge about the size, he showed me in an old book he used that 8 Awg was a minimum for a ground when ruinning 1 Awg wire. I'd like to here what the truth is.

Thank You
Geezer

07-27-2011, 07:41 PM   #2

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The equipment ground is sized according to the overcurrent device protecting the circuit.

For a 100A circuit a #8 ground is appropriate and typical. NEC Table T250.122

I have to say, any electrician who sizes the equipment ground equal to the circuit conductors (on larger than 30A circuits) is wasting money.

07-27-2011, 08:13 PM   #3
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Quote:
 Originally Posted by Speedy Petey The equipment ground is sized according to the overcurrent device protecting the circuit. For a 100A circuit a #8 ground is appropriate and typical. NEC Table T250.122 I have to say, any electrician who sizes the equipment ground equal to the circuit conductors (on larger than 30A circuits) is wasting money.
Speedy, but doesn't #8 need protection where #6 doesn't so #6 is easier to do??

07-27-2011, 08:51 PM   #4
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Quote:
 Originally Posted by a7ecorsair Speedy, but doesn't #8 need protection where #6 doesn't so #6 is easier to do??
Sounds like a circuit in conduit which would be protected, yes?

 The Following User Says Thank You to brric For This Useful Post: Speedy Petey (07-27-2011)
07-27-2011, 08:52 PM   #5

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Quote:
 Originally Posted by a7ecorsair Speedy, but doesn't #8 need protection where #6 doesn't so #6 is easier to do??
We are talking about equipment grounding conductors. Not grounding electrode conductors.

 07-27-2011, 09:12 PM #6 I=E/R     Join Date: May 2010 Location: Minnesota Posts: 2,052 Rewards Points: 1,000 OK, I'll have to pay closer attention to detail. That 100 amp example sucked me right in to a panel
07-27-2011, 09:18 PM   #7
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Quote:
 Originally Posted by a7ecorsair OK, I'll have to pay closer attention to detail. That 100 amp example sucked me right in to a panel
Don't you just hate when that happens?

 07-27-2011, 09:24 PM #8 Electrical Contractor     Join Date: Sep 2008 Location: Delmarva Posts: 3,368 Rewards Points: 2,000 Speedy, you have failed to consider that the #1 conductors have been upsized from a #3, for a 100 Amp circuit. Normally, a #8 is allowable for a 100 Amp circuit, per table 250.122. But if you consider the requirement of 250.122(B), you will have to upsize the EGC by a proportional amount. A #1 conductor is 59% larger than a #3. A #8 conductor is 16510 circular mils. 59% of that is 9741 circular mils. The total of those numbers is 26251 circular mils. A #6 conductor is 26240 circular mils, close enough for me, but a real stickler (are you paying attention, electures?) might require one to upsize again to a #4. __________________ -KB Life is uncertain -- eat dessert first!!
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07-27-2011, 09:41 PM   #9
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Quote:
 Originally Posted by kbsparky Speedy, you have failed to consider that the #1 conductors have been upsized from a #3, for a 100 Amp circuit. Normally, a #8 is allowable for a 100 Amp circuit, per table 250.122. But if you consider the requirement of 250.122(B), you will have to upsize the EGC by a proportional amount. A #1 conductor is 59% larger than a #3. A #8 conductor is 16510 circular mils. 59% of that is 9741 circular mils. The total of those numbers is 26251 circular mils. A #6 conductor is 26240 circular mils, close enough for me, but a real stickler (are you paying attention, electures?) might require one to upsize again to a #4.
I don't see any mention of circuit ampacity in the OP.

 The Following 2 Users Say Thank You to brric For This Useful Post: electures (07-28-2011), Speedy Petey (07-28-2011)
 07-27-2011, 10:05 PM #10 Electrical Contractor     Join Date: Sep 2008 Location: Delmarva Posts: 3,368 Rewards Points: 2,000 Maybe not, but my response was directed to Speedy, who used 100 Amp in his example. IF the OP is dealing with a circuit that exceeds 100 Amp, then a #6 EGC would be the minimum size required, adding more if upsizing was involved. __________________ -KB Life is uncertain -- eat dessert first!!
07-28-2011, 09:45 AM   #11
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Quote:
 Originally Posted by Suprageezer Hello, I'm not an electrician but I have done some wiring in my time. Where I am currently working we are installing a machine that required three phase 480 we ran three 1 AWG conductors and one 8 Awg conductor as the ground wire, the lenght is appox 150 ft. I question the ground wire size as any engineers I have worked with in the past ran a ground wire of equal size. When I questioned the person in charge about the size, he showed me in an old book he used that 8 Awg was a minimum for a ground when ruinning 1 Awg wire. I'd like to here what the truth is. Thank You Geezer
In order to answer correctly the size and type of the OCPD protecting the circuit is required.
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07-28-2011, 12:58 PM   #12
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Quote:
 Originally Posted by kbsparky Speedy, you have failed to consider that the #1 conductors have been upsized from a #3, for a 100 Amp circuit. Normally, a #8 is allowable for a 100 Amp circuit, per table 250.122. But if you consider the requirement of 250.122(B), you will have to upsize the EGC by a proportional amount. A #1 conductor is 59% larger than a #3. A #8 conductor is 16510 circular mils. 59% of that is 9741 circular mils. The total of those numbers is 26251 circular mils. A #6 conductor is 26240 circular mils, close enough for me, but a real stickler (are you paying attention, electures?) might require one to upsize again to a #4.

90.1 Purpose.
(A) Practical Safeguarding.
The purpose of this Code is
the practical safeguarding of persons and property from
hazards arising from the use of electricity.

This Code contains provisions that are considered
necessary for safety. Compliance therewith and
proper maintenance results in an installation that is essentially
free from hazard but not necessarily efficient, convenient,
or adequate for good service or future expansion of

electrical use.

Before we travel down that road lets find out more about the machine and wiring. Don't want to get the "experts" excited yet.
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07-28-2011, 03:00 PM   #13

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Quote:
 Originally Posted by kbsparky Speedy, you have failed to consider that the #1 conductors have been upsized from a #3, for a 100 Amp circuit.
You are correct, but it is feasible to use #1 on a 100A circuit and it not be upsized. #1 @ 60 deg C is only 110A.
If there are 60 deg terminations somewhere #2 or #1 would be required, yet only a #8 ground.

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