Do I need a bigger ground wire?
- Thread starter mlnk
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petersonra
Senior Member
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- Northern illinois
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- engineer
You can leave the neutral wire as is.
The EGC has to be increased in size IMO.
The EGC has to be increased in size IMO.
- Location
- New Jersey
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- Journeyman Electrician
You don't need any "ground wires" but you do need an EGC. 
The EGC needs to be increased in size by the same proportion as the ungrounded conductors which went from #4/0 to 350 kcmil. Once you figure out that ratio you would increase the EGC accordingly.
The EGC needs to be increased in size by the same proportion as the ungrounded conductors which went from #4/0 to 350 kcmil. Once you figure out that ratio you would increase the EGC accordingly.
Looking at 250.122 the EGC is sized based on the circuit breaker. Example: for 200 amp, #2 Alum is required.
It is not based on the size of the wire used for the ungrounded conductors.
So even if I used 500 mcm with a 200 amp circuit breaker, I think I could still use #2 grounding. Do you all agree?
As for the neutral, do I need to show calculations for reducing the size of the neutral, or does voltage drop not apply to the neutral?
It is not based on the size of the wire used for the ungrounded conductors.
So even if I used 500 mcm with a 200 amp circuit breaker, I think I could still use #2 grounding. Do you all agree?
As for the neutral, do I need to show calculations for reducing the size of the neutral, or does voltage drop not apply to the neutral?
winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
Look at the next part of the section.
The _first_ step is selecting an EGC based on the size of the breaker.
_Next_ if the ungrounded conductors are increased in size (relative to what they would normally be on that breaker) then you must also increase the size of the EGC.
We have had huge threads about just what the normal size conductor is, but if we take your opening post at face value you would normally have a 4/0 conductor on the breaker, but you are using a 350 kcmil conductor instead. So
step 1: find the egc based on the breaker
step 2: look up its area
step 3: multiply this by the area ratio from 350 kcmil to 4/0 awg,
step 4: find a conductor with this (or larger) new area
-Jon
The _first_ step is selecting an EGC based on the size of the breaker.
_Next_ if the ungrounded conductors are increased in size (relative to what they would normally be on that breaker) then you must also increase the size of the EGC.
We have had huge threads about just what the normal size conductor is, but if we take your opening post at face value you would normally have a 4/0 conductor on the breaker, but you are using a 350 kcmil conductor instead. So
step 1: find the egc based on the breaker
step 2: look up its area
step 3: multiply this by the area ratio from 350 kcmil to 4/0 awg,
step 4: find a conductor with this (or larger) new area
-Jon
I stated the question incorrectly.
I have to run wire from house to barn and well. It will be a 200 amp circuit with aluminum wire 4/0 4/0 4/0 and #2 EGC.
I have some 350 MCM wire which I will use for the two hot wires. Since it is larger wire it will work even better and I assume that I can leave the other wires the same. Will this be OK with the inspector?
I have to run wire from house to barn and well. It will be a 200 amp circuit with aluminum wire 4/0 4/0 4/0 and #2 EGC.
I have some 350 MCM wire which I will use for the two hot wires. Since it is larger wire it will work even better and I assume that I can leave the other wires the same. Will this be OK with the inspector?
- Location
- New Jersey
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- Journeyman Electrician
kwired
Electron manager
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- NE Nebraska
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- EC
How much neutral current is expected? Can be smaller than 4/0 if neutral is well balanced and never sees much load. Can never be smaller than minimum required EGC though on a feeder or min SSBJ for service conductor.
Unfortunately, even if you are using larger conductor just because you have a spool of it around you still have to increase the EGC size. The idea is that the EGC is sized to the OCPD rating and the assumed conductor size that the OCPD would normally be paired with. Using a larger conductor reduces the resistance in the circuit which will increase the fault current the EGC has to carry and the EGC can act as a choke point for the ground fault current increasing the time it takes for the OCPD to operate. Using a larger EGC will account for this.
kwired
Electron manager
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- NE Nebraska
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It is not a perfect rule either, it is the best they could come up with for a one size fits all type of rule. Some installations it makes sense, others it is overkill.
Some installations where conductor isn't "upsized" but is a long run, you will have much slower response of OCPD if there is a short circuit or ground fault even though you are in compliance with conductor sizing - resistance of conductor is going to be current limiting.
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Expanding a little on what kewired said:
For what it is worth, the NEC would be perfectly happy with the ungrounded conductors not being oversized in the first place, which would make the trip time even worse.
Another argument is that the voltage divider effect before the OCPD trips will make the momentary voltage on grounded surfaces higher than it would be if the EGC is also oversized. But since it would be at least one half of the line voltage anyway, I do not see that as particularly important.
If you take as given that a size increase for VD will only be done if the VD would otherwise be more than 5%, then the need to upsize the EGC to maintain low trip time does make sense.
If you upsize to reduce 2% VD to 1% VD, then the motivation to upsize the EGC iscbmnot nearly as strong.
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For what it is worth, the NEC would be perfectly happy with the ungrounded conductors not being oversized in the first place, which would make the trip time even worse.
Another argument is that the voltage divider effect before the OCPD trips will make the momentary voltage on grounded surfaces higher than it would be if the EGC is also oversized. But since it would be at least one half of the line voltage anyway, I do not see that as particularly important.
If you take as given that a size increase for VD will only be done if the VD would otherwise be more than 5%, then the need to upsize the EGC to maintain low trip time does make sense.
If you upsize to reduce 2% VD to 1% VD, then the motivation to upsize the EGC iscbmnot nearly as strong.
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kwired
Electron manager
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- NE Nebraska
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- EC
I see long runs on irrigation installation where even if EGC is compliant with the rules, there is still enough resistance that fault current is limited. Seen several instances where fuses never blow where they would have if long circuit lengths weren't involved. But this is often further complicated by the fact that 480/277 can sustain an arc fairly easily and that arc is also current limiting. Bolted faults should still blow the fuse, just may take a little longer than if there wasn't long conductor run.
True and you touch on a point that almost everyone overlooks. I've had some interesting conversations over the note at the bottom of table 250.122. The table provides the minimum EGC size conductor, which may not be the size of the conductor that will do the job. The note says, "Where necessary to comply with 250.4(A)(5) or (B)(4), the equipment grounding conductor shall be sized larger than given in this table." Section 250.4 gives the general requirements the fault current path has to meet, and if the EGC size given in table 250.122 will not meet the requirements, such as due to a long run, the conductor is supposed to be upsized. But the NEC does not provide any rule of thumb, plug and chug, way to calculate the size so most people just take the size from 250.122 and call it a day. I can't really blame anyone for that either since calculating the correct upsize can be complex. I'm not sure what the sizes given in table 250.122 are even based on.
The #4 GEC per table 250.122 has been increased in cm by factor of 1.65 to #1
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