250.122(b) sizing ground

Pinnie

Pinnie

Humble Disciple
Location
Ohio
Occupation
Commercial Electrician
As per this code rule, if any ungrounded conductors are increased in size for any reason, other than as required at 310.15 (B) or 310.15 (C) wire type equipment grounding conductors, if installed, shall be increased in size proportionately to the increased and circular area of the ungrounded conductors.

Exception: equipment, grounding conductors shall be permitted to be sized by qualified person to provide an effective ground fault current path in accordance with 250.4 (A)(5) or (B)(4)

My question: given the allowed exception, what is the easiest way to determine if a egc is of sufficient size. After watching a Ryan Jackson video on the subject, 5 times the current rating on a breaker and over is enough current to trip the breaker in sufficient time.

So how would you determine the correct egc size in a scenario where you’ve upsized for voltage drop ( let’s say pole lights in a parking lot).

20 amp pole lights
Upsized ungrounded conductors to 8 awg
Lets say 500’ to the farthest light.
How would you size the ground and what formula if any would you use?
 
Pinnie said:
As per this code rule, if any ungrounded conductors are increased in size for any reason, other than as required at 310.15 (B) or 310.15 (C) wire type equipment grounding conductors, if installed, shall be increased in size proportionately to the increased and circular area of the ungrounded conductors.

Exception: equipment, grounding conductors shall be permitted to be sized by qualified person to provide an effective ground fault current path in accordance with 250.4 (A)(5) or (B)(4)

My question: given the allowed exception, what is the easiest way to determine if a egc is of sufficient size. After watching a Ryan Jackson video on the subject, 5 times the current rating on a breaker and over is enough current to trip the breaker in sufficient time.

So how would you determine the correct egc size in a scenario where you’ve upsized for voltage drop ( let’s say pole lights in a parking lot).

20 amp pole lights
Upsized ungrounded conductors to 8 awg
Lets say 500’ to the farthest light.
How would you size the ground and what formula if any would you use?
Click to expand...
I don't know. I've never thought very much on the subject. I just use whatever ground size I would use for ungrounded conductors used at their normal ampacity. So if I use #8 on a 50 amp circuit with a #10 ground, I would use #10 as the ground if I upside a 20 amp circuit to #8 for voltage drop.
 
CoolWill said:
I don't know. I've never thought very much on the subject. I just use whatever ground size I would use for ungrounded conductors used at their normal ampacity. So if I use #8 on a 50 amp circuit with a #10 ground, I would use #10 as the ground if I upside a 20 amp circuit to #8 for voltage drop.
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This is what I thought until I watched the video I mentioned and read this code. If you upsize for any reason other that ampacity corrections (ambient temperature and CCC amount) then you need to upsize your egc in proportionally.

Ryan explains it better than I can
 
Divide the circular mil of the up sized ungrounded conductor by the circular mil of the ungrounded conductor before up sizing, and multiply by the circular mil of the EGC before any increase, to get the minimum circular mil area of the new EGC needed.
 
Pinnie said:
20 amp pole lights
Upsized ungrounded conductors to 8 awg
Lets say 500’ to the farthest light.
How would you size the ground and what formula if any would you use?
Click to expand...
1 to 1 ratio so #8 EGC that is unless you have a metal raceway that qualifies as an EGC. If so then #12.
 
Pinnie said:
So how would you determine the correct egc size in a scenario where you’ve upsized for voltage drop ( let’s say pole lights in a parking lot).

20 amp pole lights
Upsized ungrounded conductors to 8 awg
Lets say 500’ to the farthest light.
How would you size the ground and what formula if any would you use?
Click to expand...
For that one its easy. No formula. 20A circuit uses a 12AWG ungrounded so if you move up to 8AWG you make your EGC 8AWG
 
Pinnie said:
This is what I thought until I watched the video I mentioned and read this code. If you upsize for any reason other that ampacity corrections (ambient temperature and CCC amount) then you need to upsize your egc in proportionally.

Ryan explains it better than I can
Click to expand...
Oh yeah, I do recall talking about this in a CEU course years ago. It's technically true, and it's written the way it is to cover the bases, but logically, like I stated in my post above, using the size of ground that would normally be used on the circuit at its ampacity is sufficient.
 
This is the stupidest rule in the book (as I'll mention every time it comes up). The rule really should only apply to voltage drop and let AHJs be the jusge of whether that is the reason for upsizing.

People will have to use their judgement, but in my opinion the exception should allow a certified or licensed electrician to determine that within a typical single family dwelling, all EGCs may be sized to table 250.122 without ever upsizing, period. Just my opinion that I'm going with until an AHJ calls me out on it, which hasn't happened yet.

For really long circuits where voltage drop in a fault may be an issue, or for commercial/industrial situations with higher fault current, I would probably ask an engineer.
 
solarken said:
Divide the circular mil of the up sized ungrounded conductor by the circular mil of the ungrounded conductor before up sizing, and multiply by the circular mil of the EGC before any increase, to get the minimum circular mil area of the new EGC needed.
Click to expand...
Correct but the exception allows us to size based on our own calculations
 
infinity said:
1 to 1 ratio so #8 EGC that is unless you have a metal raceway that qualifies as an EGC. If so then #12.
Click to expand...
That is true but how do you calculate for the exception. My thought was calculate resistance of the wire over the distance, then determine how many amps at 120v
 
solarken said:
Divide the circular mil of the up sized ungrounded conductor by the circular mil of the ungrounded conductor before up sizing, and multiply by the circular mil of the EGC before any increase, to get the minimum circular mil area of the new EGC needed.
Click to expand...
This is precisely correct for calculating the proportional upsizing of the ground. But exception allows us (qualified persons) to size it a long as its an effective ground fault path.
 
Pinnie said:
That is true but how do you calculate for the exception. My thought was calculate resistance of the wire over the distance, then determine how many amps at 120v
Click to expand...
Forget the resistance and the distance. You calculate for the exception by applying simple logic. If the raceway without a wire type EGC is suitable as the EGC then if you install a wire type EGC there is no reason to upsize it. The minimum size from T250.122 is all that's required.
 
infinity said:
Forget the resistance and the distance. You calculate for the exception by applying simple logic. If the raceway without a wire type EGC is suitable as the EGC then if you install a wire type EGC there is no reason to upsize it. The minimum size from T250.122 is all that's required.
Click to expand...
What about for pvc?
 
The exception is a step in the right direction because it permits a qualified person to avoid up sizing the EGC in the various paradoxical cases where it offers no benefit.

The exception is poorly written because the necessary requirements are not specified. Just how good does a current path need to be to count as 'effective'. Do you need the breaker to trip in kinda quickly, or must it trip in the instantaneous range? What if you have a GFCI, does that make a fault current of 20mA 'effective'?

With that limit in mind, I think we can identify cases that clearly fit the exception:

1) If the circuit with the maximum rated breaker and its normal EGC would function with normal voltage drop, then reducing the breaker rating will always meet the requirements of the exception. E.g. If a circuit with #6 NM is acceptable with a 50A breaker, then it will be acceptable with a 20A breaker.

2) In the case of a redundant wire EGC where you have an effective fault path without the wire, then you meet the exception (unless code requires two redundant EGCs)

Regarding calculating circuit resistance and fault current, this sounds to me like the correct general approach, but doesn't work because there isn't a metric for how much fault current is required.
 
winnie said:
Regarding calculating circuit resistance and fault current, this sounds to me like the correct general approach, but doesn't work because there isn't a metric for how much fault current is required.
Click to expand...
This is where I think the “qualified person” comes in. If it’s not too much to ask he talks about how inverse time breakers trip and shows a helpful graph at 22:30 in the video I linked.
 
Thanks for the time pointer into the video.

The software Ryan links does _exactly_ the calculation necessary, evaluating wire resistance combined with the source available fault current to determine the maximum circuit length that will still permit the target ground fault current to flow.

The graph he shows is a common trip curve for a small breaker, and it is exactly what you need to understand to pick the target ground fault current.

Say you have a 20A circuit for a gate opener. You upsize the circuit conductors to 6 awg for voltage drop. Now you want to size the EGC per the exception.
1) Get the breaker trip curve from the manufacturer. They are all similar, but you need the one that matches your breaker.
2) Look at the trip curve graph, and select the 'rating multiplier' necessary to get the desired trip time.
3) Enter the wire information (circuit conductor AWG and guessed EGC AWG), the breaker rating, and the multiplier number into the software.
4) The software will calculate the maximum allowed length.
5) Adjust the EGC AWG until the maximum allowed circuit length matches the actual circuit length.

1742394075523.png

So far so good, once you have a well selected 'current multiplier', you have a procedure that will give you the necessary EGC size.

Where I disagree with Ryan is selecting that current multiplier. Nothing in the code says how fast the breaker needs to trip for an 'effective' ground fault path, so Ryan has no basis other than his common sense for selecting that 5x multiplier. One key feature about trip curves: they all have a 'tolerance range'. If you look at the vertical line for 5x current, you will see that the breaker trips somewhere between 0.005 and 4 seconds.

If you want to _guarantee_ that the breaker trips faster than 0.02 seconds, you need to use a 10x multiplier. And if the breaker has a 'D' trip curve (they sneak into that diagram _two different_ breaker trip curves, type 'C' and type 'D'), and want to ensure a 0.02 second trip, you need a 20x multiplier. But since the code doesn't specify the required trip time, a qualified individual might say 'tripping in 10 seconds is fast enough' and get away with a 4x multiplier.

Or, as I previously mentioned, if the breaker has some sort of ground fault detection, perhaps you don't a high fault current at all. If you have a 20A breaker that trips in 1/20 second in the event of a 50mA ground fault, are you permitted to use a multiplier of 0.0025? (50mA = 0.0025 * 20)

If you have a strong basis for selecting the breaker multiplier (or breaker required trip time), then you will have everything necessary to use this procedure. But right now IMHO the only basis for selecting the breaker multiplier is Ryan's good sense.

-Jonathan
 
winnie said:
If you have a strong basis for selecting the breaker multiplier (or breaker required trip time), then you will have everything necessary to use this procedure. But right now IMHO the only basis for selecting the breaker multiplier is Ryan's good sense.

-Jonathan
Click to expand...
I agree with everything you said. In my opinion in my example running a #8 ground seems excessive, but I also want the breaker to trip instantly. I thought since I know the distance and the wire resistance I can calculate the amps with I=V/R. But I’m pretty sure there’s more nuance to AC then ohms law can handle. Thus why I’m here😂. Ryan’s program I’m sure works great but I’d rather have the formula for the field.
 
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