CABLING

Dear Sirs,

I am having few queries on cabling for a particular UPS systems. In the system, a 40 kva UPS system is to feed scada system equipment along with some other instrumentation.

1. Under what condition there will be higher current in the neutral and how much the its magnitude will be.

2. In our case, will there be 1.8 times phase current in the neutral conductor.

3. In the UPS output circuit, if there is 1.8 times the phase current, how to size the output cables.

4. If we can use 2 separate cables: 1) 3 core cable for phase
2) single core cable ( of higher size as per current) for neutral

What are important factors while deciding it. Any reference document can be used.


Regards


RISHI KUMAR SINGH

rishi29s@indiatimes.com

 

[busbar]

…3 core cable for phase / single core cable (of higher size as per current) for neutral…

If I understand your description correctly, I don’t think it's a good idea to separate the neutral conductor from the phase conductors, regardless of the magnitude of neutral current. The increased reactance (especially with harmonic currents) will only make voltage regulation worse. I don’t think the practice is permitted from a ANSI-code perspective.

Although the analysis changes somewhat if the condition is calculated in the design phase or found in the field after startup, it would be time and money well spent to determine that cause of neutral-conductor overcurrent. A 40kVA system doesn’t seem all that large to be able to narrow in on the root problem. A common cause of this complaint is multiple poorly designed switchmode power supplies served in a phase-to-neutral fashion. If the problem is not otherwise curable, consider retrofit at the load-distribution point (downstream of the UPS output) where the 3ø source and 1ø loads join, using a special-purpose zigzag autotransformer intended for neutral-current reduction.

It may pay to determine that the UPS can safely handle the excess neutral current. The potential for damage and retrofit mop-up may be greater than a cable-overload problem.

 

[jbartos]

Suggestion to the original posting marked by ///\\I am having few queries on cabling for a particular UPS systems. In the system, a 40 kva UPS system is to feed scada system equipment along with some other instrumentation.
1. Under what condition there will be higher current in the neutral and how much the its magnitude will be.
///The nonlinear loads (motor drives, computer, induction ovens, ballasts, etc.) produce harmonics and some higher currents that potentially can flow in the neutral. The current magnitude is often considered twice the line current.\\2. In our case, will there be 1.8 times phase current in the neutral conductor.
///It may potentially be even higher. The manufacturers are offering panels with the neutral bus doubled in the cross-sectional area.\\3. In the UPS output circuit, if there is 1.8 times the phase current, how to size the output cables.
///If one cable, then the conductor sizes can be doubled. If individual conductors, the neutral conductor can have double cross-sectional area.\\4. If we can use 2 separate cables: 1) 3 core cable for phase
///It depends on their installation. If not in separate metalic conduits, yes.\\\ 2) single core cable ( of higher size as per current) for neutral
///Depending on the installation. See my comment above.\\
What are important factors while deciding it. Any reference document can be used.

References:
1. Manufacturers catalogs, e.g.

http://www.squared.com

2. There may be a cable manufacturer that offers 3conductor smaller and one bigger for the neutral harmonic applications.
3. Electrical Safety Codes, e.g. NFPA 70-2002 National Electric Code

http://www.nfpa.org

4. Industry Standards, e.g. IEEE

http://www.ieee.org