Adding feeders and neutrals to larger transformer

[vector1234]

We are in the process of replacing a 500 kva 480/277 transformer with a 2000 kva transformer. Originally, a 1500 kva transformer was going to be used. The ownner installed 4 sets of 4 conductor 750 KCMIL CU THWN-2 (MTW) conductors, only hooked up two sets for the 500 and left two curled up in the vault below the transformer. 4 sets would have worked for the 1500 kva transformer but not the 2000 kva unit, it needs 6 sets of phase conductors. The transformer essentially serves motor loads with very small if any neutral current, There are six 5" PVC underground conduits to the transformer (duct bank to vault under transformer from main switchgear). The feeder length is about 35 ft. (switchgear to transformer) Technically the existng 4 750 KCMIL neutral conductors are more than adequate, however, it looks like the code will require us to install two additional neutral conductors or two more sets of 4 750 kcmil conductors. This is a retrofit and this is where the code often breaks down. We did not design the original installation, if so we would have down sized the neutral conductor (3 750 kcmil phase, 1 ~500 kcmil neutral), in duct bank phase conductors can run separate from neutral conductors, but I do not think we can make this case. I would like to install just 2 additional sets of 3 750 kcmil in two conduits (ABC) and leave 4 conduits with (ABCN) but I do not think the code allows this. Basically we end up with 6 750 kcmil CU connected to a neutral carrying almost no current. Not an economical and/or practical arrangement. Any ideas? Thanks

 

[bob]

250.24(C) Requires that the neutral be brought to the main switch. 250.24(C1) requires the neutral be sized not smaller than shown in 250.66 .
A 2000 kva 277/480 volt FLa = 2408 amps/475 amps for 750 kcm = 5.06 ckts.
I would question the requirement to go to 6 ckts.
The 4 ckts are rated 1900 amps. If you think that is adequate for the load install a main breaker at that rating or just under the 1900 amps an let it go.
You are not required to provide the full ampacity of the transformer.

 

[augie47]

What is your calculated load and what are you supplying (main, mainS, or MLO) ?

 

[iwire]

I would question the requirement to go to 6 ckts.
The 4 ckts are rated 1900 amps. If you think that is adequate for the load install a main breaker at that rating or just under the 1900 amps an let it go.
You are not required to provide the full ampacity of the transformer.

I was thinking the same thing.

 

[bob]

I was thinking the same thing.

What is you opinion regarding the question of 5 or 6 ckts for the 2408 amps?

 

[augie47]

What is you opinion regarding the question of 5 or 6 ckts for the 2408 amps?

may i ?

Ignoring the possibility of duct bank engineering for a moment, it almost looks like a good application for the exception to 310.15(A)(2)

 

[vector1234]

number of feeders

number of feeders

250.24(C) Requires that the neutral be brought to the main switch. 250.24(C1) requires the neutral be sized not smaller than shown in 250.66 .
A 2000 kva 277/480 volt FLa = 2408 amps/475 amps for 750 kcm = 5.06 ckts.
I would question the requirement to go to 6 ckts.
The 4 ckts are rated 1900 amps. If you think that is adequate for the load install a main breaker at that rating or just under the 1900 amps an let it go.
You are not required to provide the full ampacity of the transformer.

yes 5 ckts is ok if we do not overload the transformer, this is a liquid filled transformer that may be operated in overload for short periods (days) and so we really need to deal with say 115% full load current, since the overload can extend for several days we increased the number of feeders....

 

[vector1234]

more on neutral issue

more on neutral issue

may i ?

Ignoring the possibility of duct bank engineering for a moment, it almost looks like a good application for the exception to 310.15(A)(2)

Thanks for the suggestion, I do not think it applies, but we looked at the problem again...

first more on the installation: We have two, 2000 kva transformers feeding switch gear (3200 amp bus rating), 3200 rated breaker set at long time trip of about 2560 amps (.8) but we may raise that setting. There is a tie breaker between teh two 3200 amp buses normally open, under administrative control this tie breaker can be closed and one transformer taken out of service so one transformer would carry all the load. In this case the load can move up to 2700 amps or so, that is why we put 6 conductors in.

back to the real issue, which is additional neutrals, one can look at Annex B Fig B.310-3 for example, (2005 code maybe some differences in 2008 but left that in another office), note that this figure points to Neutral configuration from 300.5(I) Exception 2. This allows neutral to be separate from the phase conductors, provided all 3 phases (ABC) are in each conduit. This might be interpretted to allow 6 conduits configured as ABCN, ABCN, ABCN, ABC, ABC. We really do not need 6 750 kcmil neutral conductors as we have a very low neutral current (all motor loads). 4 750 kcmil neutrals should do...any thoughts on this interpretation ...thanks