Egc sizing

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wwhitney said:
While the above is true, as pointed out in some installs the table sized EGC may not be adequate to clear the fault, whatever the OCPD size.

So a rule that uses the condition "if the ungrounded conductors are upsized due to VD" as a proxy for "situations where the EGC is really long and the table size may not be big enough, whatever size the OCPD is" has a technical substantiation.

However, if upsizing for VD is not a requirement, then relying on that proxy fails to satisfy the "practical safeguarding" intention of the NEC, since with the larger OCPD the EGC would be allowed to be downsized.

Cheers, Wayne
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Djelite said:
This section def. needs some tweeking
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Yes it does.

Upsizing for VD is almost never required by NEC alone, is only a suggestion in an informational note.

This is also sort of a one size fits all approach to something that doesn't always need to be that way.
 
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This issue is why the change in 250.122 that was accepted for the fist draft of the 2020 code should have made it into the code. It would have based the EGC on the size of the ungrounded conductors and not the size of the OCPD.
 
don_resqcapt19 said:
This issue is why the change in 250.122 that was accepted for the fist draft of the 2020 code should have made it into the code. It would have based the EGC on the size of the ungrounded conductors and not the size of the OCPD.
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Right on
 
don_resqcapt19 said:
This issue is why the change in 250.122 that was accepted for the fist draft of the 2020 code should have made it into the code. It would have based the EGC on the size of the ungrounded conductors and not the size of the OCPD.
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How should it be handled with parallel conductors in separate raceway/cables?
 
Parallel run additive . The ocpd reflects the size for both parallel conductors so should the egc. Sizing the egc to the ocpd is silly
 
Djelite said:
Parallel run additive . The ocpd reflects the size for both parallel conductors so should the egc. Sizing the egc to the ocpd is silly
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But now factor in the fact the OCPD might be two to eight times the ampacity of the ungrounded conductors. I kind of agree with you, but where is the line drawn for when you can have EGC smaller than the individual ungrounded conductors vs at least being same size as ungrounded conductors with parallel sets?

6 AWG EGC might be sufficient for single set of 3/0 ungrounded conductors,

Now run four sets but have a fault in raceway of one set - is it still sufficient size to reliably allow operation of OCPD? Might be sufficient for four of them to carry a fault occurring at the far end after the paralleling segment of said run. I don't think the EGC should ever need to be more than the size of the ungrounded conductor it is run with - even in parallel segments of a circuit.
 
kwired said:
How should it be handled with parallel conductors in separate raceway/cables?
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That is another one where the NEC makes no sense...the EGC for parallel installations must be full sized in each raceway or cable, but the supply side bonding jumper, a worse case as compared to an EGC, is sized based on the ungrounded conductors in each raceway or cable for parallel installations.
 
kwired said:
But now factor in the fact the OCPD might be two to eight times the ampacity of the ungrounded conductors. I kind of agree with you, but where is the line drawn for when you can have EGC smaller than the individual ungrounded conductors vs at least being same size as ungrounded conductors with parallel sets?

6 AWG EGC might be sufficient for single set of 3/0 ungrounded conductors,

Now run four sets but have a fault in raceway of one set - is it still sufficient size to reliably allow operation of OCPD? Might be sufficient for four of them to carry a fault occurring at the far end after the paralleling segment of said run. I don't think the EGC should ever need to be more than the size of the ungrounded conductor it is run with - even in parallel segments of a circuit.
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Had the read a few time to get your point. Interesting outlook on it.
If I understand you point on this the EGC size same as ungrounded in the same raceway. The conductors all the same size would not have an impact on clearing fault or type of fault.

In your example of four sets of 3/0 @ 75 C and cu conductors 200 amp.
250.122 , 800 amp 1/0 cu in each race way.

So your thought is where the ratio called for an increase that it would not need be larger than the ungrounded. I'll run that number.
 
Tulsa Electrician said:
Had the read a few time to get your point. Interesting outlook on it.
If I understand you point on this the EGC size same as ungrounded in the same raceway. The conductors all the same size would not have an impact on clearing fault or type of fault.

In your example of four sets of 3/0 @ 75 C and cu conductors 200 amp.
250.122 , 800 amp 1/0 cu in each race way.

So your thought is where the ratio called for an increase that it would not need be larger than the ungrounded. I'll run that number.
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Yes, I didn't look to see what was actually needed (now) but knew it would be more than #6 but less than 3/0.

If you ran four #6 EGC's the parallel effect would give you almost same thing as 1/0, which probably is fine for safely clearing a fault that occurrs beyond the parallel sets. So the question becomes what minimum size can be allowed in each individual set that is still capable of safely clearing the OCPD in an individual set? Hard to set a one size fits all rule when you can have just two parallel sets or maybe twelve parallel sets and a pretty high OCPD setting on those smaller conductors should a fault occur in just one set of the parallel run.
 
I would not necessary agree with you on that when you have a line to ground fault. One reason why is the there tied together at both end forming a single conductor path. Not saying single conductor, Form a Single Conductor Path. So the line to ground fault would be feed to the fault from both ends.
Thus the wire type EGC in that raceway must carry this total amount of fault until the OCD opens. What is key is open time and Conductor Withstand rating.

Another way to look at maybe is the impact on the faulted conductors in that raceway. We already know that set is no good and has to be replaced. what about this effect on the other three sets. To small of an EGC could damage the other three sets. I say could because we do know the open time and the withstand rating of the EGC. I have seen this happen and the wire MFG would not cover cost of replacement of the second set due to the EGC was undersized and damaged the conductors in that raceway. Long story however relevant. Long story short the OCD opened due to an install Error leading to a ground fault in the first set, It did its job. what happened next was the EC said Um and turned up the break to max blaming the trip on setting. It was not a load issue it was a ground fault on that 480 volt feeder. The EGC was sized as you mentioned ignoring the engineering. He thought he knew better than the professional that designed it and went with sizing the EGC from the OCD for each run. It was up sized for voltage drop from the MDP underground to a remote panel in the same building, pretty typ. They simply nicked the line conductor and warped it up with a scotch pad/ tape and finished pulling, It was dug up at the fault. The withstand of the EGC was not is spec and melted to the other conductors and damaged them. We know because we dug them up and cut section of raceways out per engineers request. This was not our install. I was called in because the local EC needed help. By the time it all went bad the EC that did the job was out of warrenty and said not his problem so the building owner called out local EC. I recommended before we proceed call first EC and then up the chain of command after noteing conductor size did not match plans and the breaker had been re set and adjusted after the Gear provider had done final settings.

Based on that I can see why it does have to larger than the underground conductors. The example you listed if on a 200 amp OCD would be 4 to 1 and than the not larger rule applies. I say bad design. This would be in case of a underground PVC run. Above ground EMT, IMC, RGC, I would be fine with it. I would also think about the available let through fault current and the open time of the OCD for a EGC with stand rating and make sure it will open with out damage. If I see a red flag I RFI engineering for a ruling.

SO what happens when some one works around the rule. I am sure you asking where is he going by know. What if we design a 2 to 1 for the same install. 400 amp OCD for the same load. Granted if a panel and it was rated for 225 it would now be 400 amp. I have seen this as well and recently. The engineer designed around the rule for a long run with a 180 amp load at the 2 to 1. The EGC works out right under 1/0, so instead of 3/0 you can use 1/0 EGC and be done per NEC. The time open works on the 1/0 for the withstand rating. Better yet I seen it designed with GRC underground and no wire type and EMT above ground with no wire type. I would be careful when you choose to go against what written unless you understand the outcome in a what if. Sometimes putting that green wire in can be a bad thing if not installed correctly.

Just my take on it anyway. May be all wet and need more schooling. An engineer may help with this question
Quick withstand rating on #6 and 1/0 @ 1/2 cycle
#6 =15,213 amps
1/0=61,223
Which one you want in that pipe with a let though of 42K
 
Well in parallel installations similar to egc bonding jumpers are sized per conduit based on largest conductor or ONE bonding jumper of the same size for all conduits. So imo more studies are needed and more importantly more clarification is needed. Otherwise we’re simply installers
 
infinity said:
Bonding jumpers for feeders in parallel raceways are sized according to the OCPD size ahead of the feeder not the size of the conductors within each raceway.
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And the ocpd is sized to the conductors. So the sizing issue still applies, correct?
 
horsegoer said:
I've always noticed that but never thought about why not?
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Because, unlike after the service main, there is nothing ahead of that point that needs to be independent bonded.

EGCs make sure that a fault to a bonded point makes the OCPD behave as if the fault was to a grounded conductor.

Ahead of the service, what better point to use as a low-impedance pathway for fault current than the neutral itself?

The grounded conductor is (re-)grounded at the service main, so it (re-)becomes the zero-volts reference point.
 
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