Voltage Drop and the EGC

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George Stolz

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Windsor, CO NEC: 2017
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Service Manager
Hypothetical: Suppose someone had never heard that when upsizing conductors for voltage drop, you must upsize the equipment grounding conductor proportionately according to 250.122(B).

Can you explain the reasoning, importance, and/or need to upsize the EGC?
Does the importance change when using a metallic raceway?

For example, let's say a 20A circuit fed with 10 AWG CU.
Or a 20A circuit fed with 6 AWG CU. Or whatever you please. :)

Edit: Let us proceed as though "code is code" is not the end-all answer. ;)

I'm going to see if I can let this one roll with no more input from me, but that's never worked before. :D
 
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mdshunk

Senior Member
Location
Right here.
I don't know what happens. I suspect that there might be enough voltage drop on a fault that the breaker may take many, many more cycles to clear that fault. Less voltage at the ground bar equals less fault current observed by the breaker.

It is a common code violation that I see fairly regularly. Parking lot lights are popular offenders. Maybe a 20 amp circuit with #6 hots and a #10 ground.
 

JohnJ0906

Senior Member
Location
Baltimore, MD
As I understand, voltage drop is due to the resitance of the wire. If this is the case, then you would need to reduce the resistance on the egc as well to facilitate the breaker tripping.
Actually, I think I understand better than I can express it in words. A little help anyone?
 

Rockyd

Senior Member
Location
Nevada
Occupation
Retired after 40 years as an electrician.
George,

Let bring in a factor called the "Q" factor....

it fits in like this -
VD=Q x K x I x D
CM​

See reference to earlier code cycles for table 8 chapter 9. Not because of code that it is so, but because these charts allow us to calculate [size=+2]R[/size]t and [size=+2]X[/size]t. This is used in regard to the conduit surrounding the conductor(s) and total impedance to the current flow.
 
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Rockyd

Senior Member
Location
Nevada
Occupation
Retired after 40 years as an electrician.
George,

I knew I was close on my answer...here's Mike's comments from years ago (stilll true today). -

Q as a Multiplier

For ac dircuits, the dc Resistance Constant (K) must be adjusted for the effects of eddy current and skin effect. The Q-Factor multiplier is calculated by dividing the ac resistance (chapter9 table 9) by the dc resistance (chapter9 table 8.) as listed in the NECode. Eddy currents and skin effect are insignificant for conductors No. 1/0 and smaller and their effects can be ignored.

Scary part? This is from a Mike Holt book 1993 - getting older...but I still remember somethings...(CRS shops me at times)

So as can be seen, "secret" forces conspire to offer evidence as to why the GRC needs to be increased due to inductive reactance effect when using metal pipe. Preferred for results PVC, then Aluminum, then Steel, in regard to fighting Q factor build up.

Here is a q factor table -

Size COPPER ALUMINUM
AVG ... PVC.......AL.......STEEL... PVC.......AL..... STEEL
MCM - Conduit Conduit Conduit Conduit Conduit Conduit
2/0 -- 1.0341 1.0341 1.0341... 1.0062 1.0062 1.0062
3/0 -- 1.0052 1.0704 1.0313... 1.0317 1.0317 1.0317
4/0 -- 1.0197 1.1019 1.0362... 1.0000 1.0000 1.0000

250 -- 1.0097 1.1068 1.0485... 1.0035 1.0626 1.0153
300 -- 1.0256 1.1422 1.0489... 1.0042 1.0749 1.0184
350 -- 1.0354 1.1717 1.0627... 1.0083 1.0909 1.0413

400 -- 1.0280 1.1838 1.0903... 1.0201 1.1153 1.0397
500 -- 1.0465 1.2403 1.1240... 1.0142 1.1321 1.0613
600 -- 1.0748 1.3084 1.1682... 1.0198 1.1615 1.0765

Funny how as time rolls on, how the current thought of things, migrates from electrical issue to electrical issue. Electrical isn't so much different than anything else...except we are playing in a relatively new field with lots of room for new thought and discovery.
 
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jtester

Senior Member
Location
Las Cruces N.M.
georgestolz said:
Hypothetical: Suppose someone had never heard that when upsizing conductors for voltage drop, you must upsize the equipment grounding conductor proportionately according to 250.122(B).

Can you explain the reasoning, importance, and/or need to upsize the EGC?
Does the importance change when using a metallic raceway?

For example, let's say a 20A circuit fed with 10 AWG CU.
Or a 20A circuit fed with 6 AWG CU. Or whatever you please. :)

Marc and John nailed it.

If you have excessive voltage drop, you may have too little fault current to make a breaker operate effectively. The lower the voltage in a circuit the lower the current. remember I=V/z.

By increasing the phase wire you increase the voltage delivered to the load. By increasing the EGC, you increase the ground fault current that the breaker sees.

You would have to increase the conduit size only if your circuit was at its maximum length using the conduit as an egc. Most people don't even think about limitations of conduit as an EGC, but they are there just as surely as the green wire limitations.

Jim T
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Is providing for sufficiently low fault current impedance the only factor here?

Would it be as safe to provide ground fault protection rather than to increase the size of the EGC, in other words would it be as safe to decrease the fault current required to open the breaker rather than to decrease the impedance for fault current? (I am not asking if this would meet current code; I am asking if this would be as safe as what current code requires.)

-Jon
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
winnie said:
Is providing for sufficiently low fault current impedance the only factor here?

Would it be as safe to provide ground fault protection rather than to increase the size of the EGC, in other words would it be as safe to decrease the fault current required to open the breaker rather than to decrease the impedance for fault current? (I am not asking if this would meet current code; I am asking if this would be as safe as what current code requires.)

-Jon

it would probably be safer, but not code.
 

haskindm

Senior Member
Location
Maryland
GFCI does NOT replace the need for grounding. GFCI protects against electrocution. Gronding protects against shocks and allows a conventional circuit breaker to trip quickly during a fault condition.
 

Bob NH

Senior Member
mdshunk said:
I don't know what happens. I suspect that there might be enough voltage drop on a fault that the breaker may take many, many more cycles to clear that fault. Less voltage at the ground bar equals less fault current observed by the breaker.

It is a common code violation that I see fairly regularly. Parking lot lights are popular offenders. Maybe a 20 amp circuit with #6 hots and a #10 ground.
Table 250.122 permits a #10 ground as long as the OCPD does not exceed 60 Amps. Since the #6 is consistent with a 60 Amp OCPD, it seems to be consistent with 250.122(B) to use a #10 EGC. What have I missed in the code?
 

mdshunk

Senior Member
Location
Right here.
Bob NH said:
What have I missed in the code?
You missed 250.122(B), where it requires the ground to be upsized proportionally according to circular mil area. For typical 15, 20, 25, and 30 amp circuits, this will effectively make the EGC the same size as the phase conductors, no matter what gauge you upsize to. For 35 amp and larger circuits, you need to do some simple math. Forget about the 250.122 table when you're upsizing conductors to compensate for VD.
 

Bob NH

Senior Member
I am still at a loss to understand the rationale.

If I have a 48 amp continuous load, which requires #6 AWG copper THHN with three in a raceway, with 60 amp OCPD, then I am allowed per Table 250.122 to use 10 AWG copper for the EGC.

But if I have a 16 amp load and 20 amp breaker, and put in a #6 AWG THHN to reduce voltage drop, then I am required to put in #6 AWG EGC!!??

I confess that I don't understand what technical rationale requires a larger EGC when I use a smaller ODPD.

Are there any references that would provide the rationale for requiring a larger EGC when the required OCPD is smaller?
 

JohnJ0906

Senior Member
Location
Baltimore, MD
The grounding path is supposed to be low resistance. If the length of circuit causes voltage drop in the current-carrying-conductors(due to increased resistance of the wire itself), don't you think it would also increase resitance in the EGC, slowing or preventing the tripping of the OCPD?
 

JohnJ0906

Senior Member
Location
Baltimore, MD
amps=volt/resistance. In a ground fault, I want the amps high to trip the breaker as fast as possible. the more I increase resistance (in the case of a EGC the only resistance is the wire itself) the lower the amps. The conductor sizes in table 250.122 dont take long distances into consideration, IN THE SAME WAY that table 310-16 doesn't. So I have to adjust BOTH
 

mdshunk

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Location
Right here.
I'm remembering a post many years back (2 BBS systems ago, I think), that somebody posted an example of a certain distance of #8 that was hard shorted to the #12 EGC at the far end, but would take something like 2 minutes (according to a standard trip curve) to trip a typical plug on 20 amp breaker. I've been hunting that post off and on all night, and can't seem to locate it. I'm not enough of a math guy to re-create it.


[This to me brings up a problem. Since upsizing the hots for voltage drop is optional, I don't have to upsize the EGC if I don't upsize the hots. I can create a hazard (in terms of fault clearing time) on a long run if I don't upsize the hots, and it would be permissable. ]
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
John from Baltimore has the rational. You are not putting #6 conductors on a 20A breaker just for giggles; you have to have some _reason_ for doing so.

That reason is generally voltage drop. If the voltage drop is sufficient to warrant increasing the size of the circuit conductors, then presumably you have the same voltage drop issues in the EGC, and thus you should oversize this as well.

However IMHO this is not a piece of code that is particularly specific to the problem that it is supposed to address. It is easily 'worked around' if you want to 'value engineer', and it can create code violations when there is no real safety issue. There are no explicit voltage drop requirements, EGC impedance requirements, or OCPD clearance time requirements.

You could legally install a circuit with 50A OCPD, run it on #8 conductors with a #10 EGC, and supply a 16A load. You could not legally use 20A OCPD to supply this same circuit. IMHO this makes the less safe installation code compliant.

On the flip side, say you wish to 'reuse' some conductors already in place for an application that requires a lower rated OCPD. You want to use an otherwise unused range circuit to feed general purpose receptacles, for example. You have #8 conductors with a #10 EGC, where #12 conductors would be totally acceptable. The only reason for oversizing the conductors is that they just happen to already be in the walls. Bingo: code violation.

-Jon
 

don_resqcapt19

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Staff member
Location
Illinois
Occupation
retired electrician
Bob,
There is no technical rationale for this issue. It is just a result of the oversized equipment grounding condcutors that are required by 250.122 for 15, 20 and 30 amp circuits. To write a rule to cover these wire sizes would complicate this issue even more that it is now.
Don
 

mdshunk

Senior Member
Location
Right here.
don_resqcapt19 said:
...To write a rule to cover these wire sizes would complicate this issue even more that it is now.

Something in me wants to save this quote for reference 20 or 30 years from now. I'm thinking that guys were saying the same thing about various issues 100 years ago when the code was nothing more than a little booklet.
 

Rockyd

Senior Member
Location
Nevada
Occupation
Retired after 40 years as an electrician.
George,

See 110.10 Circuit Impedance and Other Characteristics

The OCPD devices, the total impedance, the component short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit protective devices used to clear a fault to do so without excessive damage to the electrical components of the circuit. This fault shall be assumed to be either between two or more of the circuit conductors or between any circuit conductor and the grounding conductor or enclosed metal raceway. Listed products applied in accordance with their listing shall be considered to meet the requirements of this section.

This section identifies the grounding conductor as the last line of defense to a ground fault or short circuit. If the ground were not capable of the worst case scenario, and were to burn open because of high resistance (say as in undersized and overly sharp bend at a corner), the next thing to burn down is what it was designed to protect.
 
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Bob NH

Senior Member
don_resqcapt19 said:
Bob,
There is no technical rationale for this issue. It is just a result of the oversized equipment grounding condcutors that are required by 250.122 for 15, 20 and 30 amp circuits. To write a rule to cover these wire sizes would complicate this issue even more that it is now.
Don

The OCPD and ungrounded conductors are selected to ensure that the conductors are protected under all conditions of load. It would be simple to say, and the result would be safe, if 250.122(B) were revised to say:

(B) Increased in Size. Where ungrounded conductors are increased in size, equipment grounding conductors, where installed, shall be increased in size to correspond to the requirements of Table 250.122 for the maximum size overcurrent protection that would be permitted to be installed for the larger ungrounded conductors.

I think that interpretation could be permitted by the existing 250.122(B), based on the "shall be increased in size proportionately according to the circular mil area of the ungrounded conductors" phrase of that section.

That interpretation would result in an EGC that would protect the circuit in the event that the maximum size OCPD for the conductor is ever installed in the circuit. I believe that interpretation is consistent with the requirement of the code that the EGC size be "increased in size proportionately".
 
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