reduced neutral: reuse existing bus duct with 50% neutral?

[jcarter052496]

I'm working on a job that is an existing industrial plant (circa 1950) converted to offices. There is an existing 120/208V wye bus duct running throught the plant that we'd like to reuse. The nameplate says "1350A, 600 volts or less, 3 PH-4W, neutral capacity 675A (337A per bar, 2 bars)."

The original distribution system was demolished. A different engineering firm designed to feed this duct with a 1200A circuit breaker. I am extending a circuit from this buss duct to a 1200A distribution panel. Obviously, I am concerned about the undersized neutral (675A/1200A=56%).

The only code section I could find is 220.61 (2008) for calculating the feeder neutral. It says size the neutral per "The maximum unbalanced load shall be the maximum net calculated load between the neutral and any one ungrounded conductor". This confuses me since the 3-phase loads should have 0 neutral current. Also, at 1200A that's a lot of circuits to track down and try to figure out the actual neutral load.

My plan is to discuss the issue with the engineer that did the "line side" design and possibly reduce the feeder breaker from 1200A to 800A, similarly, reduce my "load side" distribution panel to 800A. Then the system would have an 84% neutral as opposed to a 56%.

Any thoughts?

 

[augie47]

I may be off base, but IMO, 220.61(B)(1) sounds like it would no apply for offices and (B)(2) would allow a 70% demand on the load over 200 amps.
For a 1200 amp buss you would need a neutral capacity of 900 amps. For 1000, 760 amps, etc,
(If I'm dong my math correctly).
Looks like you are more in line for an 800 amp OCP.

 

[Jraef]

Aside from the code specific issues, modern offices now include a LOT of non-linear loads from things with electronic power supplies, which is just about everything now. Electronic ballasts for the lighting, power supplies for PCs and servers, even the HVAC systems almost always have VFDs now. This will mean heavy harmonic currents. Full sized or even over sized neutrals are sometimes in order, but I think that for the most part, a reduced neutral is a now bad idea for any office complex. Just my opinion though.

 

[jcarter052496]

Thanks. I was thinking about the "downsize to 800A option". That would make the neutral at 84% (675A/800A) as opposed to 56% (675A/1200A). I don't believe that would be a problem especially since the actual load will probably not surpass 675A.