Equipment grounding conductor sizing for increased phase conductors

[clint2000]

We are doing a project and installing 400? of 300 mcm (voltage drop due to distance) fused at 150 amps at main gear and installing 200 amp rated panel board at other end. The engineer specified #6 ground wire to be installed in the same 3? conduit as the phase conductors. We asked him if the ground conductor should be larger per 250-122 Size of equipment grounding conductors- (B) ?Increased in size? when phase conductors are increased in size, (250-122(B)). The response was that it will work fine, no, do not increase the equipment grounding conductor. I have called the electrical inspector and anticipate his return call sometime late tomorrow. Is the electrical panel grounding conductor supposed to be increased in size proportionately according to the circular mil area of the ungrounded conductors? If so, how would one go about calculating that? Would it be deriving a percentage relationship between the 250-122 table of 3/0 circular mil ungrounded conductors to the #6 required grounding conductor size and then take that percentage and apply it to the 300 mcm and then look up the circular mil in the tables and choose the conductor that has that minimum value or would one assume the 300mcm for 285 amps if fused to rating and then look up table 250-122 and determine the proper size which would be a #4 conductor?
Thank you

 

[infinity]

If the feeder conductors were increased in size then the EGC must be proportionately increased in size also.

 

[C3PO]

The EGC needs to be increased.
You said it is fused at 150A so the "normal" size would be a 1/0 copper (75C) with a 6 EGC


So 300000/105600 = 2.84

6awg is 26,240 cmils so 26240 * 2.84 = 74,522

Your EGC needs to be a 1awg copper

 

[bob]

Would it be deriving a percentage relationship between the 250-122 table of 3/0 circular mil ungrounded conductors to the #6 required grounding conductor size and then take that percentage and apply it to the 300 mcm and then look up the circular mil in the tables and choose the conductor that has that minimum value or would one assume the 300mcm for 285 amps if fused to rating and then look up table 250-122 and determine the proper size which would be a #4 conductor?Thank you

This never seems to make sense. As you said if you fuse the 300 kcm at 300 amps you would need a #4 cu but because you fuse it at 150 or 200 amps you need a #1 Cu which is used for a 600 amp OC device. Why?
You need the Engr to take another look.

 

[Smart $]

This never seems to make sense. As you said if you fuse the 300 kcm at 300 amps you would need a #4 cu but because you fuse it at 150 or 200 amps you need a #1 Cu which is used for a 600 amp OC device. Why?

Because voltage drop occurs on the EGC too during a ground fault condition.

 

[bob]

Because voltage drop occurs on the EGC too during a ground fault condition.

I agree with your comment. However, if we have #300 kcm with a 300 amp breaker we use a #4 egc. If we use the same #300 kcm with a 200 amp breaker we are required to use #1 egc. The fault current may be the same for each installation. In fact they may be in the same panel.

 

[Smart $]

I agree with your comment. However, if we have #300 kcm with a 300 amp breaker we use a #4 egc. If we use the same #300 kcm with a 200 amp breaker we are required to use #1 egc. The fault current may be the same for each installation. In fact they may be in the same panel.

OK... now I'm seeing where you are going, and I agree it seems to and may well contradict the apparent logic.

The required minimum EGC size per Table 250.122 does not increase linearly as the OCPD rating increases. Yet when we upsize circuit conductors, the corresponding and required increase in EGC size is linear. The former, I believe, is based on short-term duration when a fault occurs, whereas the latter is not. Justified... I can't say. I don't fully grasp the concepts involved (...and uncertain if I will in the near future).

 

[rbalex]

Prior to the 2002 NEC, Section 250.122 and its predecessors clearly stated the rationale for increasing the EGC was where the ungrounded conductors of the circuit were increased specifically for voltage drop purposes.

A close reading of the Substantiation from the Proposal that made it a general rule indicates there was no genuine safety issue; it just made it easier to inspect.

This is simply another case where an engineer’s PE should be given much heavier weight. If an inspector had questioned a smaller EGC where larger ungrounded conductors were specified but voltage drop wasn’t the issue, a simple, “Yep – that’s what I wanted”, from the PE should have been sufficient. But of course, it isn’t now because it’s a specific, though technically unfounded, general rule.

 

[rbalex]

(Log #1750)
5- 264 - (250-122): Accept
RECOMMENDATION: Revise to read as follows:
250-122 (b) Increased in size Adjustment for Voltage Drop. Where ungrounded conductors are increased adjusted in size to compensate for voltage drop, equipment grounding conductors, where installed, shall be increased in size adjusted proportionately according to circular mil area of the ungrounded conductors.
SUBSTANTIATION: The current text is limited to voltage drop only and is subject to abuse and misinterpretation (e.g. it was done per the plans, not for voltage drop). The manufacturers [sic] directions often call for conductor to be increased in size, with no explanation for why the ungrounded conductors size is increased, with no corresponding requirement for the equipment grounding conductor to be increased.
PANEL ACTION: Accept.
NUMBER OF PANEL MEMBERS ELIGIBLE TO VOTE: 17
VOTE ON PANEL ACTION:
AFFIRMATIVE: 16
NEGATIVE: 1
EXPLANATION OF NEGATIVE:
TOOMER: The submitter did not provide sufficient substantiation.
[RBA Note: Just as in the original ROP document, underlined text is added, italics is deleted text.

Who says there is a ?corresponding requirement for the equipment grounding conductor to be increased.?? A manufacture could have had other reasons for specifying an increase for the ungrounded conductors, such as large inrush, surge, long starting currents, ?all of the above?, etc., none of which affect ground-fault current detection as voltage drop might. In reality, even voltage drop is rarely an issue for detecting ground-fault currents on circuits above 120V.

To me, I don?t think the CMP recognized the unintended consequences of accepting the Proposal.

 

[bob]

To me, I don?t think the CMP recognized the unintended consequences of accepting the Proposal.

I'll drink to that. Maybe someone will find time to submit a change.

 

[rcwilson]

Another unintended impact is having to special order multi-conductor power cables for tray use. The "standard" size EGC wires in larger tray cables are too small. They're large enough to meet "normal" use but not for this upsizing.

To comply, special order cables are needed if we upsize for voltage drop or ampacity derating. "Regular" cables are OK. What is really frustrating is that these power cables run in an aluminum cable tray that is bonded every 20 feet to a #2/0 copper ground wire that's running in the tray from the source ground bus to the end. Plus the tray by itself is usually a sufficient EGC. Rigid conduit runs down to the loads are also bonded to the tray and the #2/0. The EGC in the cable is bonded to the motor or other equipment.

(Lots of copper. I just wish our estimators had figured out the clients' requirements earlier.)

 

[don_resqcapt19]

Another unintended impact is having to special order multi-conductor power cables for tray use. The "standard" size EGC wires in larger tray cables are too small. They're large enough to meet "normal" use but not for this upsizing.
...

Actually I think that was part of the reason for the code rule...it had strong support from the conduit people. As I recall, and I could be wrong, this rule originally only applied to raceway and not cable installations. The raceway people called foul and got it to apply to both.

 

[charlie b]

Here is what I believe to be the intended process. Situations vary, so take this as one example only.

• Step One: Temporarily disregard any considerations of voltage drop, ambient temperature, number of conductors in a conduit, and all else.
• Step Two: Calculate the load.
• Step Three: Look at table 310.16, to determine the minimum size conductor that has sufficient ampacity for the calculated load.
• (This is not relevant to the present discussion, but here it is anyway.) Step Four: Select an OCPD that can protect the selected conductor.
• Step Five: Now start thinking about voltage drop, ambient temperature, number of conductors in a conduit, and the roll of cable you happen to have in your truck at the moment. If any of these considerations, or if the instructions of the voices in your head, of if any other factors cause you to install an ungrounded conductor size that is larger than that determined in step three, then you must increase the size of the EGC.

You will notice that I omitted Step Six, which would be when you explain the reason for Step Five to your apprentice. I can't offer any help there. It makes little sense to me. I really don't think that physics is on the side of the code authors on this one. I like the suggestion that the present code language is there to assist in inspections, rather than to enhance safety.

 

[Smart $]

I like the suggestion that the present code language is there to assist in inspections, rather than to enhance safety.[/SIZE][/FONT]

I think a better and easier to inspect [and safe] method would be to base the EGC minimum size as follows:

250.122 Size of Equipment Grounding Conductors.
...
(B) Increased in Size. Where ungrounded conductors are increased in size, equipment grounding conductors, where installed, shall be sized per Table 250.122 using the Table 310.16 ampacity of the ungrounded conductor having the greatest ampacity, in lieu of a lesser ocpd rating.​

 

[Pierre C Belarge]

Why not have the PE provide the calculations for voltage drop on the Effective Ground Fault Current Path to show why he picked whatever size EGC he has chosen for each circuit installed that does not follow the standard procedures?

If the calculations for upsized conductors and the EGC are shown to be effective (see the note at the bottom of Table 250.122), I would be satisfied.


If they do not want to go through the effort of providing supporting calculations to substantiate their design, they would follow the code.

This could be added to the future codes as a proposal in the form of an exception, or an added clause/condition.

 

[rbalex]

Don, I do seem recall a debate of the sort you mention, but I don't believe this was the subject matter. I already posted the original Proposal; the only Comment was to note that deleted text was indicated by italics.

If my fuzzy memory is correct, the debate was whether a wire-type EGC should be included with all circuits. It was accepted, and later rejected after MUCH Comment.

 

[rbalex]

I usually have no issue substantiating my designs; even though I don't think I should have to since it is my professional judgment that is a requisite to my license.

The problem here is that, since most faults, ground or otherwise, are not full bolted faults, there is no way to assure a minimum fault current would occur - even with an increased EGC; even where the ungrounded conductors are NOT increased.

As I mentioned, the problem occurs with OCPDs on single-phase circuits, 120V and below, where it is to respond to both short circuits and ground faults. It is virtually non-existent on higher voltages or where other ground fault detection means, such as GFCI or zero sequence detection is used.

 

[K8MHZ]

Here is what I believe to be the intended process. Situations vary, so take this as one example only.

• Step One: Temporarily disregard any considerations of voltage drop, ambient temperature, number of conductors in a conduit, and all else.
• Step Two: Calculate the load.
• Step Three: Look at table 310.16, to determine the minimum size conductor that has sufficient ampacity for the calculated load.
• (This is not relevant to the present discussion, but here it is anyway.) Step Four: Select an OCPD that can protect the selected conductor.
• Step Five: Now start thinking about voltage drop, ambient temperature, number of conductors in a conduit, and the roll of cable you happen to have in your truck at the moment. If any of these considerations, or if the instructions of the voices in your head, of if any other factors cause you to install an ungrounded conductor size that is larger than that determined in step three, then you must increase the size of the EGC.

You will notice that I omitted Step Six, which would be when you explain the reason for Step Five to your apprentice. I can't offer any help there. It makes little sense to me. I really don't think that physics is on the side of the code authors on this one. I like the suggestion that the present code language is there to assist in inspections, rather than to enhance safety.

In an attempt to rationalize the requirement using common sense, here is what I came up with:

The EGC is there to get the fault current back to the breaker so it will trip and clear the fault. Since voltage and amperage will drop together, an undersized EGC may present enough resistance to keep the fault current below the trip rating of the breaker.

Now, since we have mitigated the voltage drop to our load by increasing the size of our conductors we have also increased the available fault current (as compared to the original conductor size). Since there is now an increase of available fault current the EGC must be re-sized accordingly.

Now, as for the apprentice, if he is in school I think it should be suggested that his teacher go over this with him using their books. It may be that the teacher can't explain it and just may have to dig out some printed matter for an explanation. I was always jotting down things I saw at work when I was coming up and asking for explanations. Our teachers loved it and encouraged it. We spent many hours learning about real life situations in our class and it really paid off.

 

[K8MHZ]

Another way to look at it......

Let's say you have a 1200 foot long driveway and want to feed a light array at the end that draws 12 amps. It is on a single 20 amp circuit.

Let's also say you wanted to achieve a goal of a 3 percent voltage drop.

You find that you need a 1/0 hot and neutral to get 3.1 percent and go for it.

So.....do you depend on 1200 feet of 12 AWG to be your ECG?

Hopefully this extreme condition illustrates the need for the ECGs to be up-sized with the other conductors.

 

[charlie b]

. . . an undersized EGC may present enough resistance to keep the fault current below the trip rating of the breaker.

To say that a longer wire will decrease the amount of fault current is to speak an obvious truth. To infer that it will not be enough to trip a breaker is to go too far. A calculation could prove it one way or the other, but we are not allowed to use a calculation. Also, if there is no voltage drop concern, and no ambient temperature concern, and fewer than 3 CCCs in a conduit, and if the only reason you upsized the phase conductors is that you had a spare roll of it in your truck, I don't think there is a need to upsize the EGC. It would be hard for an inspector to make a call on such a basis. I say again, this is about making it easy for the inspector, not about safety.

Since there is now an increase of available fault current the EGC must be re-sized accordingly.

That is backwards. You have already increased the available fault current (by making the phase conductors bigger). Why should you think you need to increase the available fault current further, by also making the EGC bigger?