Parallel Conductors - extreme example thought experiment

[bryandjen]

We are doing some big parallel feeders and I got to thinking about something hypothetical.

Say you have a 5000 amp feeder breaker and you decide, for whatever reason, to run 34 parallel sets of conductors with #1/0 ungrounded conductors in each of the 34 parallel PVC conduits.

250.122(F)(1)(b) says the EGC in each parallel conduit must be sized per table 250.122 based on the setting of the OCPD. 700kcmil copper EGC in this example.

However, 250.122(A) says the EGC shall not be required to be larger than the circuit conductors supplying the equipment.

Which one wins?

Logically, the (F)(1)(b) rule is there because the EGC must be able to carry enough current to make the breaker trip. But in my extreme example, if there is a phase to EGC fault INSIDE of one of the conduits (I’ve seen it happen), I wouldn’t think that a 1/0 conductor would potentially not carry enough to make a 5000a breaker trip. Yes, the fault current would flow in all of the phase conductors via the parallel connection point, but the same is true for the EGC.

Thoughts?

 

[infinity]

The circuit conductors are 34* whatever size they are. Also the EGC has to be increased proportionally to the increase in conductor size. Without doing the math I would guess that the 700 kcmil is too small.

Welcome to the Forum.

 

[wwhitney]

The circuit conductors are 34* whatever size they are. Also the EGC has to be increased proportionally to the increase in conductor size. Without doing the math

1/0 Cu has a 75C ampacity of 150A, and 150A * 34 = 5100A, so the conductors have not been increased in size.

Cheers, Wayne

 

[infinity]

1/0 Cu has a 75C ampacity of 150A, and 150A * 34 = 5100A, so the conductors have not been increased in size.

Cheers, Wayne

Ah I misread the ungrounded part. The OP can ignore my 250.122(B) reference. Then 34-700 kcmil EGC's would be required.

 

[bryandjen]

Thank you for the replies. I guess I'm trying to wrap my head around why a ungrounded conductor could be as small as a 1/0 in my example but the EGC would have to be 700kcmil. Would they both not be relied upon to carry enought current to facilitate the operation of the overcurrent protective device under a ground fault condition?

 

[infinity]

Thank you for the replies. I guess I'm trying to wrap my head around why a ungrounded conductor could be as small as a 1/0 in my example but the EGC would have to be 700kcmil. Would they both not be relied upon to carry enought current to facilitate the operation of the overcurrent protective device under a ground fault condition?

Many of us have been asking the same question for decades. You'll get some wild theories but IMO concrete data saying that it is necessary is mostly imaginary.

 

[don_resqcapt19]

CMP 5 might act on this issue for the 2026 code, but without some documented testing, I don't think the rule will be changed.

 

[wwhitney]

Here's my attempt to rationalize the rules on EGC sizing:

One of the jobs of the EGC is to clear a fault from an ungrounded conductor on a grounded supply system. In a fault scenario, the EGC is going to be in series with the ungrounded conductor and so their impedances will add. That means that given the available fault current at the source end of a circuit, and given the size and hence trip curve of the OCPD protecting that circuit, there will be a maximum total impedance that is permissible.

Now conductor size determines impedance per unit length, and so length is a clear scale factor here. To continue the discussion we just need to assume some sort of typical or maybe 95% percentile length.

Then the ungrounded conductors will be sized based on the OCPD so that the I²R heating of the conductor doesn't damage its insulation. The EGC doesn't have this concern, as it only carries current during faults, not normal operation. The result is that the EGC can be smaller than the ungrounded conductors. That is most of the impedance "budget" goes to the EGC, since operational concerns already require the ungrounded conductor to be low impedance.

In this picture, an obvious issue is when, for a given OCPD size, the ungrounded conductor(s) have higher than expected impedance. One example of this is when the conductors are upsized for voltage drop. That's a sign that the conductor length is greater than the typical assumed value, which will increase the impedance of both the ungrounded conductors and the EGC. So if you need to bring down the ungrounded conductor impedance by upsizing it, you also need to bring down the EGC impedance by upsizing it.

Another case where ungrounded conductor impedance goes up without a corresponding reduction in OCPD size is the case of parallel conductors and the scenario of one of the parallel ungrounded conductors faulting to one of the EGC conductors. Now we've increased the ungrounded conductor impedance by a factor of 2 or 8 or whatever (not precisely, as ampacity is sublinear in conductor cross section, but same idea). If we also increased the EGC impedance by a similar factor, that would just compound the problem. So we aren't allowed to downsize the EGC relative to a single conductor installation. In fact we should be happy the NEC doesn't require us to increase the EGC size when we decrease the ungrounded conductor size by using parallel sets.

This is a direct effect of a single OCPD protecting our parallel sets jointly. If the NEC allowed us to parallel OCPD, so that each conductor in the parallel set was protected at its ampacity, then it would make perfect sense that each EGC could be sized based on this lower individual OCPD size. But paralleling OCPD is prohibited.

Cheers, Wayne

 

[don_resqcapt19]

Yet, we can size supply side bonding jumpers for parallel installations based on the area of the ungrounded conductors in each raceway.

 

[wwhitney]

Yet, we can size supply side bonding jumpers for parallel installations based on the area of the ungrounded conductors in each raceway.

At first this does not make any sense within the framework I described.

However, for an SDS, other than the outdoor case 240.21(C)(4), and the 240.4(F) case where the primary OCPD protects the secondary conductors, the secondary conductors will be limited to 25' or less. That means the supply side bonding jumpers will also generally be limited to 25' or less. That is presumably a lot shorter than the "expected" length for other parallel installations. So the fairly short length will reduce both the ungrounded conductor and SSBJ impedances, at least somewhat counterbalancing the impedance increase we get from dividing into parallel sets.

Obviously still not completely consistent, but then again this is the NEC. : - )

Cheers, Wayne

 

[don_resqcapt19]

At first this does not make any sense within the framework I described.

However, for an SDS, other than the outdoor case 240.21(C)(4), and the 240.4(F) case where the primary OCPD protects the secondary conductors, the secondary conductors will be limited to 25' or less. That means the supply side bonding jumpers will also generally be limited to 25' or less. That is presumably a lot shorter than the "expected" length for other parallel installations. So the fairly short length will reduce both the ungrounded conductor and SSBJ impedances, at least somewhat counterbalancing the impedance increase we get from dividing into parallel sets.

Obviously still not completely consistent, but then again this is the NEC. : - )

Cheers, Wayne

But the lack of closely coordinated overcurrent protection makes up for that, especially where the primary OCPD is 250%

 

[bryandjen]

I did a little digging in some older ROP/ROC's and found my answer in 5-195 Log #554 NEC-P05 in the 2013 ROP. Apparently, the CMP would NOT require the EGC in my example to be 700kcmil but rather the "General" rule in 250.122(A) stating it doesn't have to be larger than the circuit conductors. I really think they need to take the verbiage “…but in no case shall they be required to be larger than the circuit conductors supplying the equipment” out of first level subdivision (A) General and put it after the main title heading of 250.122. That would be a base rule for all instances. I would think that a first level subdivision "General" would be superseeded by the following specific subdivisions. Note that the comment by LEVASSEUR, P references change 5-201a. It shows "accept", but that is not the current verbage in 250.122(F) so it got show down at a later stage for some reason. Looks like I have some more digging to do, but gonna hang my hat on the substantion verbage in this ROP:

 

[wwhitney]

I did a little digging in some older ROP/ROC's and found my answer in 5-195 Log #554 NEC-P05 in the 2013 ROP. Apparently, the CMP would NOT require the EGC in my example to be 700kcmil but rather the "General" rule in 250.122(A) stating it doesn't have to be larger than the circuit conductors.

You are misreading the substantiation. It says "the panel reaffirms the size of the circuit conductors supplying the equipment referred to in 250.122(A) is the equivalent area of the circuit conductor in parallel." I.e. when making the 250.122(A) comparison, you multiple the area of each set by the number of sets before comparing to the EGC size.

1/0 Conductors are 105.6 kcmil, so your circuit conductors are 3,590 kcmil. 700 kcmil is smaller than 3,590 kcmils, so 250.122(A) provides no relief from the requirement for a 700 kcmil EGC for a 5000A OCPD.

Cheers, Wayne

 

[infinity]

1/0 Conductors are 105.6 kcmil, so your circuit conductors are 3,590 kcmil. 700 kcmil is smaller than 3,590 kcmils, so 250.122(A) provides no relief from the requirement for a 700 kcmil EGC for a 5000A OCPD

Correct, just take the cm size times the number of parallel sets that is the size of the circuit conductors. It's not the size of each parallel conductor it's the aggregate of all of the sets.

 

[bryandjen]

You are misreading the substantiation. It says "the panel reaffirms the size of the circuit conductors supplying the equipment referred to in 250.122(A) is the equivalent area of the circuit conductor in parallel." I.e. when making the 250.122(A) comparison, you multiple the area of each set by the number of sets before comparing to the EGC size.

1/0 Conductors are 105.6 kcmil, so your circuit conductors are 3,590 kcmil. 700 kcmil is smaller than 3,590 kcmils, so 250.122(A) provides no relief from the requirement for a 700 kcmil EGC for a 5000A OCPD.

Cheers, Wayne

Maybe, but you could also take the panel statement (I mistyped earlier by saying substantiation) "the phrase "in no case shall they be required to be larger than the circuit conductors" applies to the example included in the substantiation" further clarified by Levasseur's comment on affirmative which states "equavilant area of conductors installed in each conduit or cable", not the combined summed equavilant area of all phase conductors in separate parallel conduit or cable sets. My reasoning again is that both the EGC and circuit conductors in one conduit would carry the same amount of fault current with a fault inside of one PVC conduit.

 

[wwhitney]

Maybe, but you could also take the panel statement "the phrase "in no case shall they be required to be larger than the circuit conductors" applies to the example included in the substantiation"

That sentence is agnostic as to whether the size of the "circuit conductors" is the individual size of one member of a parallel set, or the combined area. The next sentence which I quoted clearly says it is the latter.

Thus this CMP statement in response to the PI does nothing to give you the result you are looking for. The scenario in the OP still requires 700 kcmil in each of the 34 parallel sets.

Cheers, Wayne

 

[infinity]

The bottom line is that there is no provision in Article 250 allowing one to parallel smaller EGC's to make a larger one. In the OP's example if only one EGC were required it would be a 700 kcmil. If 34 EGC's are required then each one needs to be 700 kcmil or larger.

 

[bryandjen]

Here's my attempt to rationalize the rules on EGC sizing:

One of the jobs...

Thank you so much for your very detailed reply! I would hope/think that there was some engineered testing done to come up with the conductors sizes in table 250.122 to be large enought to facilitate the operation of the OCPD. As such, it would to me, stand to reason that each paralleled circuit conductor should be no smaller than what table 250.122 allows for the EGC.

 

[wwhitney]

As such, it would to me, stand to reason that each paralleled circuit conductor should be no smaller than what table 250.122 allows for the EGC.

That would be a logical result of the reasoning I proposed. The fact the NEC doesn't require that shows either my framework or the NEC is incomplete. Or maybe I'm just trying to rationalize something that isn't fully rational. : - )

I guess it also suggests that if one is designing a long high ampacity feeder with many parallel sets, it is worth doing an analysis of the fault current if one individual conductor faults to one EGC near the far end of the feeder to see how the impedance would interact with the OCPD's trip curve.

It also makes me wonder whether safety would be improved if the NEC allowed the field paralleling of OCPD above a certain current rating.

Cheers, Wayne

 

[bryandjen]

That would be a logical result of the reasoning I proposed. The fact the NEC doesn't require that shows either my framework or the NEC is incomplete. Or maybe I'm just trying to rationalize something that isn't fully rational. : - )

I guess it also suggests that if one is designing a long high ampacity feeder with many parallel sets, it is worth doing an analysis of the fault current if one individual conductor faults to one EGC near the far end of the feeder to see how the impedance would interact with the OCPD's trip curve.

It also makes me wonder whether safety would be improved if the NEC allowed the field paralleling of OCPD above a certain current rating.

Cheers, Wayne

True. I think sometimes a person must use a little logic with some of the gray areas in the code. This is one example. Looking at Exhibit 250.45 diagram to 255.122(F) in the 2017 NEC Handbook (the newest one I have), their reasoning makes zero sense how the EGC could have more current flowing on it versus the shorted-out phase conductor as they could both (phase sets and EGC sets) be the same size and both connected in parallel at each end . The phase conductor(s) would still have to carry just as much fault current to trip the breaker as the EGC(s) does in that diagram. I read several other negative statements on the topic in the ROP/ROC’s, and they mostly basically say “prove it” to use a smaller EGC when paralleled. Well, that goes both ways. Prove to me how a 1/0 phase conductor will allow for a 5000a breaker to trip when faulted. If not, then make the phase conductors be at least the size of the EGC as per table 250.122.