Upsizing the Equipment Grounding Conductor

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rbarnett's picture
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Joined: 2014-03-04

250.122 says you have to proportionately increase the circular mill size of the wire-type equipment grounding conductor when you increase the size of the ungrounded conductors (such as for voltage drop). I often get a blank stare when I bring this up. Is this something a lot of us just don't think about, or is there a rule of thumb somebody is using? 

DougSimpson's picture
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Joined: 2014-01-12

I would hope that the required sizes are based on good engineered principles to adequately clear a fault. Experience tells me that solid continuity of the EGC is equally important as maybe size. Let's not waste precious resources for sake of quick profits.

daewizard's picture
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Joined: 2012-12-30

The NEC handbook has an example of how to perform this calculation called the proportion calculation. You have to use the circular mils of the conductors found in NEC chapter 9, Table 8, conductor properties. There are misconceptions to the rule. For circuits above 30 amps this calculation is required. For 30 amp circuits or lower, the equipment grounding conductor works out to be the same increased size of the ungrounded circuit conductor every time.
This is very important that the concept is understood. If the equipment grounding conductor is not properly increased in size, it acts as a resistance in the circuit, and the overcurrent device does not operate properly during a fault event.
Example: a 100 amp ungrounded circuit conductor gets upsized to 1/0 (105,600 circular mils). A 100 amp EGC is #8 (16,510 CMILS) with #3 (52,620 CMILS) circuit conductors. the calculation is as follows: the result of 105,600 (1/0) divided by 52,620 (#3) is multiplied by 16,510 (#8) equals 33,133 CMILS (#4=41,740 CMILS) minimum for the equipment grounding conductor. #6=26,240 CMILS. The handbook has this calculation verbatim.
The misconception is that a 1/0 would typically be protected by a 150 amp OCPD, which a #6 EGC would be appropriate per table 250.122, which by example is not correct for this application per 250.122(B).

rbarnett's picture
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Joined: 2014-03-04

Both good explanations of how to help maintain that effective ground fault current path. Yes the 30 amps and lower circuits is interesting. Thanks for the detailed explanation. Now I know!

vperov's picture
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Joined: 2014-03-07

For long circuits (100 ft and more), we need to increase the size of the ungrounded conductors to have the nominal voltage for the load (for example: 120V +/- 5%), because the wires have the resistance. But the current is the same in any point of this circuit (Electrical Engineering class in college). The size of EGC shall be in accordance with Table 250.122, in accordance with ampere rating!
Therefore, Section 250.122, (B) is useless.
It's strange, why some of the Electrical Engineers do not remember basic rules.
Best Regards,
Vladimir Perov, P.E.

journeymanjoe's picture
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Joined: 2014-02-19

[quote=vperov]For long circuits (100 ft and more), we need to increase the size of the ungrounded conductors to have the nominal voltage for the load (for example: 120V +/- 5%), because the wires have the resistance. But the current is the same in any point of this circuit (Electrical Engineering class in college). The size of EGC shall be in accordance with Table 250.122, in accordance with ampere rating!
Therefore, Section 250.122, (B) is useless.
It's strange, why some of the Electrical Engineers do not remember basic rules.
Best Regards,
Vladimir Perov, P.E.[/quote]

???? So, you are saying that a 30A circuit during a fault condition would remain at 30A? Please explain for us non-college educated people. Your statements are fragmented and hard to follow.

mc5w's picture
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Joined: 2013-10-09

Voltage drop in the equipment ground during a ground fault needs to be reasonable to minimize the amount of electric shock exposure during a ground fault.

Also, increasing the ungrounded conductor size increases the available ground fault current and the equipment ground needs to be increased to avoid heating that could melt or otherwise damage the insulation of adjacent wires. My experience is that sheath and aluminum "bonding strip" of steel armored type AC cable will cause the insulation of the wires to melt during a ground fault. Hence, there are some instances where I have used the red copper wire for an equipment ground like in Germany prior to 1968.

If voltage drop in the equipment ground of a solidly grounded system cannot be kept reasonable during a ground fault, MSHA requires that surface mining systems be resistance grounded instead of solidly grounded.

dpiper's picture
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Joined: 2014-05-28

Please note the Table 250.122 is the "Minimum" size requirement.

If your using 3/C cables with a ground the size increase is inherent.

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