Low Voltage Aluminum Cabling (less than 600V)

[mshields]

Your help in resolving a difference of opinion.

For a 4000A feeder, it's my contention that you'd need to use 12 - 600kcmil per phase and a 600kcmil ground in each of the 12 conduits.

A colleague is telling me that you can't do this because by Table 250.122, you need to use an 800kcmil ground (which is no good from a pratical standpoint). I believe the verbage contained in 250.122, (A) would indicate that a 600kcmil grounding conductor would be acceptable. i.e. "but shall not be required to be larger than the circuit conductors supplying the equipment". My colleagues contention is that this does not apply to parallel conductor applications - I don't see why not.

What say you?

 

[petersonra]

it seems to me that a more practical approach might be a higher voltage and smaller conductors.

 

[Johnmcca]

Mike your colleagues are correct read 250.122(F)(1)

Wouldn't your circuit conductors be the equivalent of 7200kcm if you want to use 250.122(A)?

 

[mshields]

Damn - I hate it when that happens

Damn - I hate it when that happens

But thanks very much - I just read it and will concede the point.

 

[steve66]

This is one area where I think the code makes no sense at all. You can't run a 600KCM ground wire in each conduit, but if it's metal conduit, you can eliminate the ground wire altogether.

250.122(F)(2) seems a more reasonable approach if you meet the conditions listed. Use a GFPE with a setting of 300A or less.

Or you could use copper conductors.

 

[infinity]

This is one area where I think the code makes no sense at all. You can't run a 600KCM ground wire in each conduit, but if it's metal conduit, you can eliminate the ground wire altogether.

250.122(F)(2) seems a more reasonable approach if you meet the conditions listed. Use a GFPE with a setting of 300A or less.

Or you could use copper conductors.

250.122(F)(2) is only for cables not raceways.

 

[don_resqcapt19]

Steve,

250.122(F)(2) seems a more reasonable approach if you meet the conditions listed. Use a GFPE with a setting of 300A or less.

Have you found a GFPE that meets the code requirements? As far as I know, they don't exist.

250.122(F)(2)(3) The ground-fault protection is listed for the purpose of protecting the equipment grounding conductor.

Don

 

[steve66]

I stand corrected - (F)(2) must be a waste of ink. Thanks Trevor and Don.

I still don't like the idea that you can eliminate the grounding conductor. How can "no grounding conductor" be safer than a 500KCM grounding conductor?

 

[hugosauc2]

If your 4000A service comes directly from a transformer then a ground is not required in the sets of service entrance conduits. Your ground should be established at the service which would require a 3/0 cu or 250mcm al grounding electrode conductor

 

[petersonra]

Your help in resolving a difference of opinion.

For a 4000A feeder, it's my contention that you'd need to use 12 - 600kcmil per phase and a 600kcmil ground in each of the 12 conduits.

A colleague is telling me that you can't do this because by Table 250.122, you need to use an 800kcmil ground (which is no good from a pratical standpoint). I believe the verbage contained in 250.122, (A) would indicate that a 600kcmil grounding conductor would be acceptable. i.e. "but shall not be required to be larger than the circuit conductors supplying the equipment". My colleagues contention is that this does not apply to parallel conductor applications - I don't see why not.

What say you?

250.122(F) Conductors in Parallel. Where conductors are run in
parallel in multiple raceways or cables as permitted in
310.4, the equipment grounding conductors, where used,
shall be run in parallel in each raceway or cable. One of the
methods in 250.122(F)(1) or (2) shall be used to ensure the
equipment grounding conductors are protected.
(1) Each parallel equipment grounding conductor shall be
sized on the basis of the ampere rating of the overcurrent
device protecting the circuit conductors in the raceway or
cable in accordance with Table 250.122.

310.4 Conductors in Parallel. Aluminum, copper-clad
aluminum, or copper conductors of size 1/0 AWG and
larger, comprising each phase, neutral, or grounded circuit
conductor, shall be permitted to be connected in parallel
(electrically joined at both ends to form a single conductor).

 

[Rockyd]

Mshields,

hope this helps -

Straight out of the NEC handbook-


Where wire-type grounding conductors are installed in multiple raceways or cables used to enclose conductors in parallel, a full sized equipment grounding conductor selected from table 250.122 based on the size of the OCPD device protecting the paralleled circuit is required in each racewayor cable.
The full sized equipment grounding conductor is required to prevent overloading and possible burnout of the conductor should a ground fault occur along one of the parallel branches. The installation conditions for paralleled conductors prescribed in 310.14 result in proportional distribution of the current-time duty among the the several paralleled conductors downstream of the paralleled set of conductors.
Exibit 250.47 shows a parallel arrangement with two nonmetallic conduits installed underground. For clarity, a one-line diagram with equipment grounding conductors is shown. A grond fault with equipment grounding conductors is shown. A ground fault at the enclosure will cause the equipment grounding conductor it the top conduit to carry more than the proportionate share of fualt current. Note that the fault is fed by two different conductors of the same phase, one from the the left, and one from the right. The shortest aned lowest-impedance path to ground from the supply panelboard is through the equipment grounding conductor in the top conduit. The grounding path from the fault through the bottom conductor is no longer of higher impedance. therefore, the equipment grounding conductor in each raceway must be capable of carrying a major portion of the fault current without burning open.


Straight out of the NEC handbook-


Where wire-type grounding conductors are installed in multiple raceways or cables used to enclose conductors in parallel, a full sized equipment grounding conductor selected from table 250.122 based on the size of the OCPD device protecting the paralleled circuit is required in each racewayor cable.
The full sized equipment grounding conductor is required to prevent overloading and possible burnout of the conductor should a ground fault occur along one of the parallel branches. The installation conditions for paralleled conductors prescribed in 310.14 result in proportional distribution of the current-time duty among the the several paralleled conductors downstream of the paralleled set of conductors.
Exibit 250.47 shows a parallel arrangement with two nonmetallic conduits installed underground. For clarity, a one-line diagram with equipment grounding conductors is shown. A grond fault with equipment grounding conductors is shown. A ground fault at the enclosure will cause the equipment grounding conductor it the top conduit to carry more than the proportionate share of fualt current. Note that the fault is fed by two different conductors of the same phase, one from the the left, and one from the right. The shortest aned lowest-impedance path to ground from the supply panelboard is through the equipment grounding conductor in the top conduit. The grounding path from the fault through the bottom conductor is no longer of higher impedance. therefore, the equipment grounding conductor in each raceway must be capable of carrying a major portion of the fault current without burning open.

[i}The drawing wouldn't transfer...it's two metal boxes with PVC inbetween, parallel raceways stacked. The top one gets a fault at entry of the hub. Demo's large portion of fault current travelling on one equipment grounding conductor. [/i]




This pretty well clears up the expanation as to why thegrounding conductor is upsized in parallel conduits or cables.



This pretty well clears up the expanation as to why the grounding conductor is upsized in parallel conduits or cables.

 

[Rockyd]

Steve66,

I stand corrected - (F)(2) must be a waste of ink. Thanks Trevor and Don.


The handbook has full explanation -

Basically it says that because cable doesn't contain "a super conductor" is stuck with the size that it is. 250.122(F)(2)(3)

The ground-fault protection is listed for the purpose of protecting the equipment grounding conductor.

My speculation is that the cable manufacturers had to have a way to parallel cable in a safe manner. This was the most efficient for this code cycle. It is new, and I want to see what the 2008 is going to do with it. Plenty of heads to discuss it, hard to secure a 2/3 vote in a fog.

 

[jtester]

I stand corrected - (F)(2) must be a waste of ink. Thanks Trevor and Don.

I still don't like the idea that you can eliminate the grounding conductor. How can "no grounding conductor" be safer than a 500KCM grounding conductor?

Steve66

I believe that if you really follow code, you will find instances wherre conduit is not an adequate ground. 250.(4)(A)(5) requires an effective ground fault current path. EMT certainly has limitations on the distance it can be run when providing ground fault circuiting for larger breakers. I imagine that even RGS would when used in very large circuits.

A conscientious designer will take that into consideration when determining the need for a separate ground wire.

Jim T

 

[don_resqcapt19]

Jim,
You are correct that both the wire and conduit versions of the EGC have limitations as far as length. In many small circuits, the conduit provides an effective fault clearing path over a longer distance than does an EGC sized per Table 250.122. There is a program that was developed by Gerogia Tech, under contract from the Steel Tube Institute that does these types of calculations. You can download a free copy here.
Don