# Grounding Conductor Sizing



## Conduit Phil (Nov 19, 2009)

How does an undersized grounding conductor work? Take an 8-2 NM cable for example, it only uses a #10 ground. Why doesn't it need a full size ground? They used to make smaller gauge NM with undersized ground (14-2 with #16 ground), but that's not allowed anymore. Why does it work with higher ampacity circuits?

Thanks


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## jwjrw (Jan 14, 2010)

A number 10 ground is good for 60 amps


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## Conduit Phil (Nov 19, 2009)

jwjrw said:


> A number 10 ground is good for 60 amps


Right... So why can't we use #10 as an ungrounded conductor at 60 amps?


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## grounditplease (Jun 13, 2010)

Because it's an ungrounded conductor.........


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## jwjrw (Jan 14, 2010)

Conduit Phil said:


> Right... So why can't we use #10 as an ungrounded conductor at 60 amps?


 

That is two different situations. Conductor ampacity is sized off 310.16 which is basically how many amps that size conductor can carry without hurting the insulation. A grounding conductor is not supposed to carry current so it is sized differently.


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## walkerj (May 13, 2007)

The grounding conductor does not carry current during normal conditions.

It's minimum size is dictated by it's ability to clear a fault.


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## Conduit Phil (Nov 19, 2009)

So it's safe to have a #10 carry 60 amps for a very short time. Or is that over simplifying it? Isn't fault current sometimes well above that?


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## Bob Badger (Apr 19, 2009)

It has to do with heat and time.

The 10 AWG EGC will trip a 60 amp breaker faster than the 10 AWG heats up to dangerous levels.

Keep in mind the current rating a of wire is always about heat, a 14 AWG copper will carry almost 200 amps before the copper melts but of course the insulation would be long gone by that point.

Check out Table 310.21 it shows the ampacity of bare 8 AWG as 98 amps but the conductor temperature could be close to 176 F.


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## Bob Badger (Apr 19, 2009)

Conduit Phil said:


> So it's safe to have a #10 carry 60 amps for a very short time. Or is that over simplifying it? Isn't fault current sometimes well above that?


If a 60 amp circuit has a fault to the EGC the current during that fault will be in the 100s of amps until the breaker opens so even the circuit conductors will be 'overloaded' for a short time.


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## Conduit Phil (Nov 19, 2009)

Ok. This is just in general for circuits right? Arent there special rules for hot tubs etc that require full size ground? or maybe I'm think that it has to be insulated?


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## JohnR (Apr 12, 2010)

yeah a #8 ground even if the pump is fed only with a #12 :laughing: go figure!


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## Bob Badger (Apr 19, 2009)

Conduit Phil said:


> Arent there special rules for hot tubs etc that require full size ground?



The NEC does not require a full size EGC for hot tubs but if the hot tub instructions require it (and I have read some do) than 110.3(B) would kick in and you would have to follow the institutions.


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## danickstr (Mar 21, 2010)

the purpose of a big ground wire on pool stuff is more for prevention of physical damage, I believe. Harder to kick a #8 and have it break or come loose.


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## Conduit Phil (Nov 19, 2009)

Ground on hot tub etc is required to insulated in conduit is it not? What's the purpose of that?


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## bender1993 (May 13, 2009)

corrosion


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## goose134 (Nov 12, 2007)

The specs on the MRI machines we installed called for TWO grounding conductors the same size as phase conductors. One isolated and one equipment ground. We went to terminate and the equipment only had one place to land. So, one came out.


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## grounditplease (Jun 13, 2010)

The grounded conductor MUST be sized the same as the ungrounded "hot" conductors in MOST systems to be able to potentualy carry the maximim unbalanced load.......


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## bobelectric (Feb 24, 2007)

Read the N.E.C.!


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## RIVETER (Sep 26, 2009)

Conduit Phil said:


> Right... So why can't we use #10 as an ungrounded conductor at 60 amps?


All copper conductors are protected for their insulation. #10 copper will certainly carry 60 amps with no problem but that level of current will, over time, deteriorate the insulation . In the event of a short circuit the "current" will only be there for a short time and the cause of the heat will be gone if all is wired correctly.


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## Bob Badger (Apr 19, 2009)

grounditplease said:


> The grounded conductor MUST be sized the same as the ungrounded "hot" conductors in MOST systems to be able to potentualy carry the maximim unbalanced load.......


That is fine except we are talking about the ground*ing* conductor not the ground*ed* conductor.


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## RIVETER (Sep 26, 2009)

Bob Badger said:


> That is fine except we are talking about the ground*ing* conductor not the ground*ed* conductor.


Good catch.


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## Bob Badger (Apr 19, 2009)

RIVETER said:


> All copper conductors are protected for their insulation. #10 copper will certainly carry 60 amps with no problem but that level of current will, over time, deteriorate the insulation . In the event of a short circuit the "current" will only be there for a short time and the cause of the heat will be gone if all is wired correctly.


I agree with you.

I have read 14 AWG copper will not become a fuse until you get to around 200 amps. Of course voltage drop would be a real issue and forget about keeping insulation on it at that current level.:laughing:


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## RIVETER (Sep 26, 2009)

Bob Badger said:


> I agree with you.
> 
> I have read 14 AWG copper will not become a fuse until you get to around 200 amps. Of course voltage drop would be a real issue and forget about keeping insulation on it at that current level.:laughing:


That very well could be true. I've been told that #12 bare could carry the 200 amps before melting but it was told to me by those who did not do it. I think #14 will carry it too if it is not made of recycled copper with a few ball bearings mixed into it.
And I get the pun about the CURRENT level...that's good.:thumbsup:


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