Struggling with 1-phase. 1/3 neutral calculation

[Wreckmore]

The distribution system under consideration is a 3-phase, multi-point grounded-wye 13.8kV line-to-line, 7.96kV line-to-neutral. We are trying to evaluate if we can save some money on copper by using 1/3 neutral on single-phase taps. We are using Southwire 1/0 XLPE cable with for all these situations. We would like to produce a calculation to show if this is feasible or not. Let's say we have a fault on the primary of a 7.96kV/.240kV transformer. We want to evaluate possible cable damage at various interruption times. We cannot figure out a way to evaluate this in ETAP. Has anyone had any experience with this? Can I just replace the 1/0 with a smaller cable in ETAP and run the short circuit? If so, do you know how I could determine the equivalent cable size of a 1/3 of 1/0?

I performed a search to see if this type of thread already existed. If it does and I just happened to miss it, I would appreciate a link. I reviewed the thread on 1/3 neutrals in residential wiring but didn't come away with a clear idea on how to apply that information to this situation.

Thank you in advance,

Josh

 

[Ingenieur]

use circ mils for conductor equivalents

circ mils
1/0 105600
1/3 x 105600 = 50,200
#3 is 52,600
about as close as you'll get unless etap allows you to enter conductor dimensions

as far as etap, not sure, but it would seem configurable for a reduced neutral

imo this is governed by the NESC, not the NEC

you are talking about the MV primary side correct?

is this the cable? https://powergen.southwire.com/wp-content/uploads/2017/07/35kV-TRXLPE-MV-XLPE-Jacket.pdf
in 1/0 they make full and 2/3 N, only larger sizes have the 1/3 N

 

[Ingenieur]

after looking at it, it may be better to model the N as 5 or 6 parallel #14's

 

[mivey]

We are trying to evaluate if we can save some money on copper by using 1/3 neutral on single-phase taps.

Not a correct application of the cable.

The 1/3 neutral is designed for 3-phase lines, not single-phase. The full neutral is designed for single-phase lines.

Save money by using the 1/3 neutral on the 3-phase sections.

 

[mivey]

We cannot figure out a way to evaluate this in ETAP. Has anyone had any experience with this? Can I just replace the 1/0 with a smaller cable in ETAP and run the short circuit? If so, do you know how I could determine the equivalent cable size of a 1/3 of 1/0?

Not in ETAP but it should have a cable modeling tool like other software.

But again, 1/3 neutral designs are for 3-phase circuits, not single-phase. Neutral overload and excessive NEV could result from using it on a single-phase circuit.

Cable damage from faults could be evaluated using conductor damage times for an equivalent cable size. You could also ask the manufacturer to assist with this analysis as they have software to do it and know how. Your sales rep should be able to get you to the right people at the plant.

 

[mbrooke]

Yes- I second that. 1/3 neutral is for 3 phase runs. For a single phase go with 100% neutral URD.

 

[Wreckmore]

Thank you for the replies. The manufacturer's sales rep did give us a damage curve and based on the max. currents we would not be able to trip fast enough to keep the cable from turning into a very long fuse.

The idea behind using the 1/3 was to just take from the 1/0 reels we were already ordering instead of ordering minimums for these limited-in-number 1-phase padmounted applications. But the analysis looks like that won't be feasible anyways.

Again, thank you so much for your insights. It helps a great deal to clarify an issue I (as a second year engineer) had not encountered before.

Josh

 

[mbrooke]

Welcome


I don't know your scenario or inventory, but around here full neutral URD is just as common as 1/3 URD- then again my area has an extensive and elaborate single phase network in subdivisions which are in the many.

 

[ramsy]

The idea behind using the 1/3 was to just take from the 1/0 reels we were already ordering..

Still possible under engineering supervision, if you can split single-phase loads between multiwire-branch circuits. A large enough EGC or metalic raceway can share ground-fault path.