Increased Size of grounding conductor why ?

[ohmhead]

Well question art 250.122 (B) As shown in the example the step by step formula is fine i see how this is done .

Now my question is for what reason is this based on .

Why is this important to change the size of the grounding conductor if the breaker is sized at 300 amps and the wire is larger than needed but not due to voltage drop lets say you had a larger wire and used it no voltage drop issues .

What is the main reason for this section in the NEC ?

 

[necnotevenclose]

This link my help explain:

http://www.bomara.com/powervar/wp-neutral-ground-voltage.htm

 

[Dennis Alwon]

I think the main reason is because on long runs- that is usually when you upsize-- the impedence of a small conductor may be so great that it will not trip the breaker under some condition. Increase the size of the EGC will solve that problem.

 

[ActionDave]

I can't see that being the reason at all since the code allows a bigger breaker and a smaller EGC if the breaker is sized according to the max ampacity in 310.16.

The only reason that makes sense to me, posted by member here, is it makes it easier for inspectors.

 

[jumper]

I think the main reason is because on long runs- that is usually when you upsize-- the impedence of a small conductor may be so great that it will not trip the breaker under some condition. Increase the size of the EGC will solve that problem.

That is what I was told.

 

[broadgage]

I believe that the reason is to limit touch voltages during a fault.
Consider an installation with equal sized hot and ground conductors.
If a short circuit occurs between hot and ground, then before the fuse opens, a large current will flow through both the hot and ground wires.
This will result in anything connected to the ground wire being at half line voltage to earth, until the fuse blows, which might take some seconds.
Anyone touching a grounded appliance whilst standing outdoors or in contact with a water pipe etc would receive a shock at half line voltage until the fuse opens. The shock voltage would be about 140 volts on a 277/480 volt system.

Now consider a similar installation, but with the ground wire of one third the size of the hot(s)
The voltage drop during a fault will now be about one qaurter in the hot conductor, and about three qaurters in the ground conductor.
On a 277 volt system, the shock voltage would be about 210 volts.

This is clearly potentialy more dangerous, especialy as the fault will persist for longer due to the reduced fault current causing the fuse or breaker to take longer to open.
A 210 volt shock for 5 seconds is more likely to kill than a 140 volt shock for 2 seconds.

IMHO the requirement to upsize the ground to match the hots is pointless on 120 volt circuits, but is still required by code.
On higher voltages such as 277/480, then it reduces the risks of dangerous shock.

 

[ptonsparky]

I can't see that being the reason at all since the code allows a bigger breaker and a smaller EGC if the breaker is sized according to the max ampacity in 310.16.

The only reason that makes sense to me, posted by member here, is it makes it easier for inspectors.

I agree. IMO they haven't figured out a way to word a change that would make sense.

 

[infinity]

I agree. IMO they haven't figured out a way to word a change that would make sense.

I agree too. It seems that every possible scenario cannot be covered by simple wording therefore the simplest method is to require the EGC to be up sized when the ungrounded circuit conductors are up sized for any reason.

Why else would you need a #10 EGC for a 20 amp circuit in EMT with #10 Circuit conductors? Pull in a #12 EGC and you're NG, pull out the #12 and you're good.

 

[Dennis Alwon]

Think of distance which is the main reason for upsizing the conductors. I would bet that a 22 gauge EGC would trip a 20 amp circuit from 5' however run that circuit 200 feet and leave the egc at 22 gauge and I bet it won't trip the breaker.

 

[raider1]

Another thing to keep in mind is that when you increase the ungrounded conductors you also increase the available fault current (decreased impedance) and therefore if the EGC is not also increased it may not be large enough to handle the new available fault current long enough to clear the fault without being destroyed.

This question was asked at the North West section meeting during a class that I taught on voltage drop. A senior engineer from UL and a CPM from CMP5 both talked about the reason that 250.122(B) exists is due to clearing faults when the ungrounded conductors are upsized and the EGC remains unchanged.

Chris

 

[ohmhead]

Well thanks for the lesson i understand why now .

This basically means any feeder or branch circuit in my case feeders could be a issue if the breaker amps are lower then the wire table 250 .122 means nothing anymore .