Service Upgrade

[sgunsel]

Have an existing commercial building with 240VAC, 3 phase delta, 200 amp service (in addition to 120/240 service). Three phase motor loads are excessive now and they want to expand. Service comes in through meter to 200 amp disconnect, then to 200 amp MBP, then (via 3/0 Cu, THHN in EMT) from MBP to 600 amp MLO panel.

If 240 service is increased to 400 amps and a new fused disconnect replaces the existing 200 amp disconnect, can a parallel EMT with 3/0 Cu, THHN, run from the new disconnect to the existing 600 amp MLO panel?

Per 310.10(H)(2) and (3), it looks allowed, but how precisely must lengths match?

Any other issues with paralleling?

 

[infinity]

Yes, it can be done but you must follow all of the parameters of that section. The length of each phase conductor and neutral must the same as their paralleled conductor.

 

[LarryFine]

Per 310.10(H)(2) and (3), it looks allowed, but how precisely must lengths match?

Any other issues with paralleling?

There's no specified tolerance, but the closer you are to maxing the conductors' loads to their ratings, the closer everything has to match; pipe length and material, conductor size and material, insulation type, termination methods and quantities, etc.

In other words, you should ideally make the addition as close as possible to being as if it was installed at the same time as the original. Note that phases may differ, but each phase's paralleled conductors must match.