A/C wire size.

[Elect117]

What is "in" in the equation? Is that supposed to be "ln" as in natural log?

 

[jaggedben]

As Wayne pointed out, it is more than just the fault current return that matters. It is also the clearing time of the OCPD. I did some messed up math and then deleted my post because I really didn't like how many assumptions I made to explain it.

For a simple fault current calc, you take the available full load current from the source and divide that by the impedance. As the impedance gets smaller in larger cables, that value will go up. As that value increases, the OCPD's Current Vs Time graph, which hasn't changed, will probably clear faster. But the fault current the EGC has to carry back to the source in order for the OCPD to clear is higher. EGCs are sized to be large enough to return the fault current. A simple fault should be a fast high current value that gets interrupted very quickly. The insulation shouldn't fail prior to clearing. The ampacity of the wire needs to be large enough to carry it back or it could end up increasing the clearing time, which puts more strain on the insulation, and can lead to a failure. But the NEC table 250.122 groups OCPD devices from 20A to 60A in one wire size, which leads me to believe that the wire size is tested to be large enough for the available fault current's return back to the source for any load between 20A to 60A.


P.S. I am not saying anything about NM #8 vs #10. I was just explaining why EGC wire size would reasonably increase when the CCCs wire size increases. I have no idea if a simple change from #10 to #8 would make a difference, but in an instance where you used 1000kmcil instead of #10, one could argue it will.

I'm not convinced that any of this matters in the real world at short distances and low amperage OCPDs (meaning under, say 600A). Every OCPD trip curve I've looked at basically starts flatlining well below the thousands of amps that would be drawn at a bolted (0 ohms) fault given, say, 50ft of circuit length. Meaning, it's going to trip very fast regardless, fast enough that the ampacity of the EGC won't matter. Also meaning, in the real world at shorter distances, the nature of the fault (how many ohms) will dwarf the size of the wires as a relative factor that determines when/if the OCPD trips. The code rules should be based on real safety issues, things that override the safety factors that are assumed in the code rules, not small differences that are of merely theoretical interest.

 

[TX+ MASTER#4544]

Remember half of people are bellow average.

After realizing half of electricians are below average was when I had to explain that a hardwired hot tub still needs gfi here in Oregon where they don't require gfi on normal dedicated circuits and that back in WA multiple competitors didn't know that you had to size overcurrent to the hvac max ampacity not just match the wire like 30a is good for everything even from inspectors that I'm gonna just do me when silly stuff comes u

 

[fishin' electrician]

Imo 250.122 (B) dosn't apply to NM cable, a wire type equipment ground if installed, could only be referring to a raceway type installation.
You do not install a wire type equipment ground in a cable the manufacture does, so the if installed has to be something other than a cable type wiring mythod

No one likes getting around the code more than me, but it doesn't say that you physically installed it, just that it is installed.

 

[electrofelon]

No one likes getting around the code more than me, but it doesn't say that you physically installed it, just that it is installed.

The argument is of course that only applies to egcs that are individual and under the control of the installer. It kinda makes sense to me that the EGC in cables would be exempt, I mean they exempt raceways when used as egc's, there is no required to bump up to IMC from EMT or from RMC from IMC.

 

[infinity]

It kinda makes sense to me that the EGC in cables would be exempt, I mean they exempt raceways when used as egc's, there is no required to bump up to IMC from EMT or from RMC from IMC.

It would make no sense to have 250.122(B) in the code and not have it apply to cables. In fact it should be just the opposite, metal raceways that do qualify as an EGC does not need to apply 250.122(B).

 

[electrofelon]

It would make no sense to have 250.122(B) in the code and not have it apply to cables.

why not? The theoretical issue 122(B) is trying to address would apply to raceway type EGC's but we can ignore the issue if that is the case, so couldnt the same be said of the EGC in cables?

 

[jaggedben]

Metal raceways are naturally upsized when one makes the conductors larger.

 

[infinity]

why not? The theoretical issue 122(B) is trying to address would apply to raceway type EGC's but we can ignore the issue if that is the case, so couldnt the same be said of the EGC in cables?

Not really. If a metal raceway qualifies as an EGC why would it matter if a wire type EGC were installed or not? If it's not needed to clear a fault then why would it need to be increased in size?

The opposite is true for cables when the ungrounded conductors are increased in size for voltage drop. The EGC within the cable could end up being too small to clear a fault.

Jaggedben brings up a good point about metal raceways, typcially when the conductors are increased in size the raceway will also be increased in size making the EGC (the raceway itself) larger too.

 

[electrofelon]

Not really. If a metal raceway qualifies as an EGC why would it matter if a wire type EGC were installed or not? If it's not needed to clear a fault then why would it need to be increased in size?

Not sure what that has to do with anything, not do I have an answer on that, you will have to take that up with the CMP's and ask why they still require a full size EGC (along with the increased in size stuff) if the raceway is an acceptable EGC.

I am just trying to figure out what purpose the "Where installed" phrase has. 122(B) would make perfect sense without it.

 

[wwhitney]

I am just trying to figure out what purpose the "Where installed" phrase has.

The "if installed" phrase in 250.122(B) is just to acknowledge that there may not be any wire type EGC. Just like 300.3(B) has the phrase "where used" in relation to the grounded conductor.

122(B) would make perfect sense without it.

Nah, if that phrase were missing, then we'd be arguing about whether 250.122(B) requires a wire type EGC whenever the ungrounded conductors are increased in size. : - )

Cheers, Wayne

 

[Elect117]

I'm not convinced that any of this matters in the real world at short distances and low amperage OCPDs (meaning under, say 600A). Every OCPD trip curve I've looked at basically starts flatlining well below the thousands of amps that would be drawn at a bolted (0 ohms) fault given, say, 50ft of circuit length. Meaning, it's going to trip very fast regardless, fast enough that the ampacity of the EGC won't matter. Also meaning, in the real world at shorter distances, the nature of the fault (how many ohms) will dwarf the size of the wires as a relative factor that determines when/if the OCPD trips. The code rules should be based on real safety issues, things that override the safety factors that are assumed in the code rules, not small differences that are of merely theoretical interest.

"EGCs should be sized to provide adequate fault current to insure operation of the circuit protective device. The NEC table for minimum size equipment grounding conductor should be recognized for what it has stated, namely the minimum size conductor that may be used, with no endorsement of adequacy. The conductor must have the capacity to safely conduct any fault current imposed upon it and to have sufficiently low impedance to limit the voltage to ground and to facilitate the operation of the overcurrent device even if the necessary conductor size is larger than given in the table." - IEEE STD 142-2007 (Thank you Mr. Tortuga lol)

You have to consider the long time, short time, and instantaneous ratings when looking at the trip curve. You want to the EGC to last long enough to trip even on ground faults falling in the short time rating. Not to mention, that the instantaneous rating isn't "instantaneous" and still has physical parts that need to operate that can take time as well. A couple of cycles might seem fast, but it can be long enough to fuse the EGC or destroy insulation.

I think we agree, but I don't want there to be a impression that the impedance is theoretical or not a "real world" consideration. It very much matters when considering ground faults and the current on the EGC. You buy bolted fault rated equipment because that is the worst case. It is misleading to claim that a bolted fault will always be the case. And to assume the NEC's size is appropriate to withstand it at the short time pick up. Instead of saying the "EGC needs to be large enough to return the fault current", I could have said, the EGC needs to be large enough to withstand the fault current long enough for the device to clear it.

To bring it back to the topic, the upsizing of a current carrying conductor and subsequently not upsizing the EGC, especially with devices less than 100A, and services rated for residential, will most likely not have an effect on it's ability to withstand the fault. In NM cable or in non-metallic conduit, one should especially consider the impact a ground fault could have on the EGC considering there is no redundant path. In NM cable specifically, due to the nature of installation, it can be difficult to replace and a fault could fuse or destroy the EGC in it during a sustained fault between 5x to 10x the rated current of the OCPD.

 

[wwhitney]

You buy bolted fault rated equipment because that is the worst case. It is misleading to claim that a bolted fault will always be the case.

Worst case in terms of peak current, but not necessarily worst case in terms of conductor/EGC damage, as your later comments seem to imply?

In NM cable specifically, due to the nature of installation, it can be difficult to replace and a fault could fuse or destroy the EGC in it during a sustained fault between 5x to 10x the rated current of the OCPD.

Are you indicating that as you compare the incident energy curve and the conductor damage curve, for small conductors (NM is #2 or smaller) the region of the curve in which the incident energy is likely to first exceed the conductor damage curve is in the 5x-10x rated current region?

It seems to me that if we accept the premise that the Table 250.122 sized EGC (the minimum size) is adequate for very short circuits (a few feet), then upsizing the ungrounded conductors in and of itself can't possibly cause the EGC to be too small to withstand a fault. In that the resulting total circuit impedance would be equal to the circuit impedance from a shorter circuit with normal sized ungrounded conductors and the same size EGC. And that therefore 250.122(B) uses upsizing the ungrounded conductor solely as a proxy for excessive circuit length that may mean too much impedance with a normal sized EGC, even when the impedance is reduced by upsizing the ungrounded conductor.

Or am I mistaken, and we can construct a reasonable example where a normal sized EGC depends on a minimum impedance of the ungrounded conductors in order to be adequate during a fault?

Cheers, Wayne

 

[jaggedben]

You have to consider the long time, short time, and instantaneous ratings when looking at the trip curve. You want to the EGC to last long enough to trip even on ground faults falling in the short time rating. Not to mention, that the instantaneous rating isn't "instantaneous" and still has physical parts that need to operate that can take time as well. A couple of cycles might seem fast, but it can be long enough to fuse the EGC or destroy insulation.

To be clear, for short circuit lengths (conservatively, under 100ft) I just don't believe you. If you want to convince me otherwise, show me a trip curve spec for a commonly used OCPD where an upsize in wire would make a meaningful difference to the OCPDs operation. Like, the difference in trip time at the higher and lower amps would be significantly greater than the difference between the min and max trip times at either amp value. So far this discussion is all talk and no specs or math.

 

[Elect117]

Worst case in terms of peak current, but not necessarily worst case in terms of conductor/EGC damage, as your later comments seem to imply?

Worst case in terms of peak current value. But yes, not necessarily worst case experienced by the conductors and EGC. That worst case depends on the OCPD. I was commenting on the assumption that ignoring the impedance or the variance is the lowest fault value and highest fault value and their respective clearing times is short sighted.

Are you indicating that as you compare the incident energy curve and the conductor damage curve, for small conductors (NM is #2 or smaller) the region of the curve in which the incident energy is likely to first exceed the conductor damage curve is in the 5x-10x rated current region?

Using the information in the IEEE paper, the short time rating of the OCPD and fault current ranging from 5x to 10x and higher can have longer clearing times than a couple of milliseconds. The time it takes to clear, the current value it is experiencing, and the rating of the conductor's insulation are what we are using to make sizing considerations. Including other circumstances like redundant path of metallic conduit.

It seems to me that if we accept the premise that the Table 250.122 sized EGC (the minimum size) is adequate for very short circuits (a few feet), then upsizing the ungrounded conductors in and of itself can't possibly cause the EGC to be too small to withstand a fault. In that the resulting total circuit impedance would be equal to the circuit impedance from a shorter circuit with normal sized ungrounded conductors and the same size EGC. And that therefore 250.122(B) uses upsizing the ungrounded conductor solely as a proxy for excessive circuit length that may mean too much impedance with a normal sized EGC, even when the impedance is reduced by upsizing the ungrounded conductor.

I agree that 250.122(B) is only related to circuit length creating a higher impedance and can great a voltage to ground rise. I do still believe that there are other considerations to be made when sizing the EGC that are mostly ignored by 250.122. As an example, the dual element ratings or where ratings can be modified on a breaker should exist in it's own right. Currently, it only exists under motors.

Or am I mistaken, and we can construct a reasonable example where a normal sized EGC depends on a minimum impedance of the ungrounded conductors in order to be adequate during a fault?

If I had the time I would. But there is an example in the IEEE paper. And to clarify it is the impedance's effect on the fault current. But yes, there could be a time where it is applicable to consider upsizing the EGC based on more than just table250.122's guidance and ignoring voltage drop as a reason to increase.

To be clear, for short circuit lengths (conservatively, under 100ft) I just don't believe you. If you want to convince me otherwise, show me a trip curve spec for a commonly used OCPD where an upsize in wire would make a meaningful difference to the OCPDs operation. Like, the difference in trip time at the higher and lower amps would be significantly greater than the difference between the min and max trip times at either amp value. So far this discussion is all talk and no specs or math.

You don't have to believe me. My points have been in more or less in agreement to both you and Wayne. I added asterisks to the conversation because just sizing the EGC to the minimum isn't always enough. You don't have to take my word for it, you can take IEEE's word. Or don't. You should consider the change in the fault current value, if the wire is designed to handle it, and if the breaker will protect all that.

If you want to limit the conversation to what I mentioned above, to smaller breakers in a smaller single phase environment. Then yes. If you would like, there was a post by Wayne where he brough up the specifications of a 30A breaker from square d. On that trip curve there is a big difference between 1x the rated current (overload), 1-9x the rated current (Short time), and 10x-100x the rated current. But you will also notice that the different breaker sizes have very similar current values. The instantaneous pick up, while being a different "multiple of (or times) the rated current" is a similar numerical value. Meaning, you could assume that the same size EGC would be adequate for all the sizes on the graph. To test that, you could do a short circuit study (requires quite a bit of information and time) or also use the calculation posed by Tortuga / IEEE paper and verify for the different time and current values, we are not exceeding the wire's ability to hand it. I am not going to do all of the difference values, times, and compare them to the math in the IEEE paper (I would also have to make assumptions about the wire). You are more than welcome to do the math for us if you have the time. It is fourth of july tomorrow and I got other things on my mind lol.

QO230 - Mini circuit breaker, QO, 30A, 2 pole, 120/240VAC, 10kA, plug in | Schneider Electric USA

Schneider Electric USA. QO230 - Mini circuit breaker, QO, 30A, 2 pole, 120/240VAC, 10kA, plug in.

www.se.com

 

[jaggedben]

...

You don't have to believe me. My points have been in more or less in agreement to both you and Wayne. I added asterisks to the conversation because just sizing the EGC to the minimum isn't always enough. You don't have to take my word for it, you can take IEEE's word. Or don't. You should consider the change in the fault current value, if the wire is designed to handle it, and if the breaker will protect all that.

If you want to limit the conversation to what I mentioned above, to smaller breakers in a smaller single phase environment. Then yes. If you would like, there was a post by Wayne where he brough up the specifications of a 30A breaker from square d. On that trip curve there is a big difference between 1x the rated current (overload), 1-9x the rated current (Short time), and 10x-100x the rated current. But you will also notice that the different breaker sizes have very similar current values. The instantaneous pick up, while being a different "multiple of (or times) the rated current" is a similar numerical value. Meaning, you could assume that the same size EGC would be adequate for all the sizes on the graph. To test that, you could do a short circuit study (requires quite a bit of information and time) or also use the calculation posed by Tortuga / IEEE paper and verify for the different time and current values, we are not exceeding the wire's ability to hand it. I am not going to do all of the difference values, times, and compare them to the math in the IEEE paper (I would also have to make assumptions about the wire). You are more than welcome to do the math for us if you have the time. It is fourth of july tomorrow and I got other things on my mind lol.

QO230 - Mini circuit breaker, QO, 30A, 2 pole, 120/240VAC, 10kA, plug in | Schneider Electric USA

Schneider Electric USA. QO230 - Mini circuit breaker, QO, 30A, 2 pole, 120/240VAC, 10kA, plug in.

www.se.com

Here is the trip curve for a QO230, for those interested, not found at the link you gave. But as I alluded to above, I'm already familiar with the typical trip curves for such breakers, and the statements I made above were with them in mind.

The instantaneous trip is less than a cycle and I submit (from experience) that no EGC sized to 250.122 for 200A or less will be damaged in that time at 600V or less. The short time current doesn't come into play in a bolted fault at short distances, and that's the heart of my point. If the fault is what is limiting the amount of current to 1-9X the breaker rating then the size of the ungrounded conductors is irrelevant, because that's not what puts the fault current in the short-time window. That EGC is either going to withstand the fault in the trip time or not, regardless of whether the ungrounded conductors are upsized. In fact, to the extent that a larger ungrounded conductor reduces the trip time by any amount, it safeguards the EGC proportionally without the EGC needing to be upsized to withstand the current for a longer time. That's the beauty of the trip curve; the higher the current, the less time my EGC needs to withstand it. It's a self correcting problem.

The notion that there is scenario, with a properly functioning molded case circuit breaker and without an unusually long wiring distance, wherein an upsized ungrounded conductor increases the likelihood of damage to the EGC, is not supported by looking at the trip curves.

There's also that larger point that, no, it is not realistic for me to consider all this for every circuit I install in the regular course of my residential and small commercial work, and so Table 250.122 should be reliable.

 

[david]

I just do not see a wire type equipment ground if installed as enforceable language when it comes to manufactured cables.
Your not installing the equipment ground in a cable, your bonding what the manufacture provided.
I also see installers wire tieing an equipment ground to the outside sheath of nm cable to satisfy a requirement to increase the circular mil area of the equipment ground if necessary.

And also can see electricians switching to ac cable over Mc cable as much as possible. And perhaps ac cable over nm cable if this keeps becoming an issue for them

 

[jaggedben]

I just do not see a wire type equipment ground if installed as enforceable language when it comes to manufactured cables.
Your not installing the equipment ground in a cable, your bonding what the manufacture provided.

Whether you install a cable with wire type ground or a raceway with wire type ground you are installing a wire type ground. I think your argument is weak. There is also no physics or safety justification for it.

I also see installers wire tieing an equipment ground to the outside sheath of nm cable to satisfy a requirement to increase the circular mil area of the equipment ground if necessary.

That would be a violation of something in Article 300.

(We solar guys used to wire tie a bare solid 8awg to the outside of NM as a GEC for micro-inverters back in the day when a GEC was needed, but technically those were different conductors, an EGC and a GEC.)

 

[electrofelon]

. There is also no physics or safety justification for it.

Well not sure if that is a valid argument when debating NEC requirements

 

[david]

Whether you install a cable with wire type ground or a raceway with wire type ground you are installing a wire type ground. I think your argument is weak. There is also no physics or safety justification for it.



That would be a violation of something in Article 300.

(We solar guys used to wire tie a bare solid 8awg to the outside of NM as a GEC for micro-inverters back in the day when a GEC was needed, but technically those were different conductors, an EGC and a GEC.)

I agree that it is not perhaps what is intended it could have said used instead. But I don't see it week in saying this cable is manufactured with an equipment ground already installed in the cable

What would be violated in Article 300 there is no rule that disallow Parallel equipment grounds.

And when it comes to installing an equipment ground there is language in effect saying other effective means of installing the equipment ground with the supply conductors

But you may be referring to something else

Ps not really that important to debate since the rule will be applied to cables