grounding conductor sizing

[tim morris]

NEC 250.122 reqires ground conductors to be increased in size proportionatly wih phase condutors. I assume the reason for this is to provide the same additional capacity to the ground conductor to allow for voltage drop as you are allowwing for the phase conductors.

The question is if the phase conductors for a 30 ampre 277 volt roadway lighting circuit needed to be #4 wire and the corresponding ground conductor was a #8 to meet the 3% voltage drop limitations, and for other reasons that did not relate to the current or voltage capacity of the conductors(ie the contractor had larger wire available) the phase conductors installed were #1/0, would the ground need to be increased just because a larger than necessary phase coonductor was used?

If the point of the code is to insure all conductors are adequatly sized then as long as the ground is sized for the connected load and length of circuit, the increase in phase conductors is irrevelent. The fact that you can meet all code requirements using the #8 ground if the phase wires are downsized would seem to make this a clear issue.

 

[bphgravity]

Re: grounding conductor sizing

I brought up a similar point in an earlier post regarding this issue. The best way to understand the reason for this is to understand that the grounding conductors have nothing to do with normal current carrying operations. What it does do is facilitate to clear line to ground faults. With the decrease in total circuit impedance from the increased ungrounded conductors, a much greater fault-current carrying capabilty now exists. To accomodate this, the fault path must also have less impedance. An increase in conductor size allows for this.

 

[don_resqcapt19]

Re: grounding conductor sizing

Tim,
If you need #4 for the hots on a 30 amp circuit, you also need #4 for the equipment grounding conductor. This is because Table 250.122 requires a full size EGC for a 30 amp circuit. That is Table 310.16 requires a #10 for the 30 amp OCPD and Table 250.122 requires a #10 for a 30 amp OCPD. Any increase in size of a hot conductor for a 30 amp circuit requires the same increase for the EGC. If you are using the 2002 code, you must install a 1/0 EGC with the 1/0 phase conductors on the 30 amp circuit. Under the '99 code you only increased the EGC if the hots were increased to compensate for voltage drop. When the inspector would say that the EGC had to be increased in size, the contractor would say, "I didn't increase the hot wire size because of voltage drop, it was just the size wire that I had on the truck, so I don't have to increase the EGC size". The change in the '02 code was made to close this "loophole".
Don

 

[charlie b]

Re: grounding conductor sizing

Originally posted by tim morris: I assume the reason for this is to provide the same additional capacity to the ground conductor to allow for voltage drop?.

I think not. For starters, in an ideal world, the ground conductor never carries current. This has nothing to do with harmonic currents or phase imbalances. The function of a ground conductor comes into play only if there is a fault. At such a time, the only concern is enabling the OCPD to trip; voltage drop is not a concern.

I used to think the reason for using a larger EGC (in proportion to the increase in the size of the ungrounded conductors) is to prevent a future design change from inadvertently creating a safety hazard. I'll think this through again, in light of Bryan's answer. But here's my (previous) reasoning:

For today?s work, you use a phase conductor that is larger than is necessary for today?s load. Next year, another person buys the property, does some remodeling that increases the load, sees that the cable is good for more current, and changes the breaker to a larger size. If the original EGC was sized on the basis of the original OCPD, it will be too small for the new OCPD. The new occupant might not see the need (or might not want to spend the money) to pull a different EGC. So we make sure that today?s installation has an EGC that would be large enough to support a larger OCPD (i.e., the largest OCPD allowed for the selected phase conductors).

I may be way off base here. I can?t think of any other code requirement that goes beyond what is needed for today, and is based on preventing a future person from making an inadvertent error. Any other ideas?

 

[bennie]

Re: grounding conductor sizing

Think loop impedance... The UK requires loop impedance tests at the final inspection of an installation. This test includes the source impedance.

A LG fault in the US is 120 or 277 volts. A LL fault on a 240 and 480 volt system is line voltage.

Without an accurate impedance test, the ability to create enough current for opening the overcurrent device in a short time is problematic.

This is the reason for a GFP assist on wye 480 systems over 1000 amps. A LG fault burns away the contact area before the overcurrent device operates.

 

[dereckbc]

Re: grounding conductor sizing

Tim, I agree with Don. If you are going to use a #4 AWG conductor on a 30-amp breaker your EGC will be required to be a #4 AWG. Read 250.122(B), it is an increase in proportionately according to the CM of the ungrounded conductor. The ratio of a #10 to #4 is 4.02.

IMHO the reason for increasing the size of the EGC is straightforward. The increased length increases the overall loop impedance of the circuit. A minimum fault current of 6 times or higher is needed to operate an OPPD efficiently. Therefore, a 30-amp breaker operating at 240 volts could only have a maximum loop impedance of 1.3 ohms. This includes the series impedance of the breaker, ungrounded conductor, and EGC.

 

[tim morris]

Re: grounding conductor sizing

Wait a minute. I have a problem with things that don't make logical sense ( I know we are talking about coodes here but...). First, according to your interpretions I would need a full size ground on this circuit, ie #4 conductor means #4ground on a 20 amp breaker when I could use a 75 amp breaker for #4 phase conductors and use a #8 ground. how does that work?

In addition, in response to Don's comment that you need a full sized ground conductor to satisfy 250.122 the example in the NEC Handbook for this section gives an example which states that for a circuit using 350 kcmil conductors (increased for VD from 250 kcmil) the required ground conductor would be increased from #4 to #2. Ihis implies that the only time the ground would be full sized would be for circuits where the original conductors are for 30 amperes and under.

It doesn't make sense that you can use smaller ground wires for higher amperage circuits but you need to use full size or dis-proportionatly larger ground conductors on long circuits. It would seem like the relationships should be the same.

Thoughts?

 

[bennie]

Re: grounding conductor sizing

Look at Table 8, property of conductors. The change in resistance accelerates out of proportion from the larger conductors to the small ones.

[ April 22, 2003, 02:23 PM: Message edited by: bennie ]

 

[bennie]

Re: grounding conductor sizing

When metal conduit is the equipment ground conductor, you have to live with what you've got, and hope for the best. Like the second ground rod procedure.