loganwest123
Member
- Location
- Central NC
- Occupation
- Electrical Engineering
Our team is currently working on an engineering analysis for a facility with a parallel service entrance using a Multi-Main Tie-Main switchboard configuration — two 3000A mains with a normally-open tie breaker, fed from separate utility transformers on the same secondary bus.
The question is around load analysis and overcurrent protection coordination between the mains and the tie.
During a planned transfer scenario where one main is opened and the tie is closed to allow one transformer to carry the full load — the combined load approaches the interrupting capacity of the tie breaker. We're looking at Eaton Magnum SB units (SBS630 series, 65kA @ 635V) for all three positions.
A few specific questions:
Any insight from those who've done the coordination study on similar configurations would be appreciated. Utility is 480V, 60Hz, facility is a data center — so reliability and selectivity are the primary drivers. Thank you.
The question is around load analysis and overcurrent protection coordination between the mains and the tie.
During a planned transfer scenario where one main is opened and the tie is closed to allow one transformer to carry the full load — the combined load approaches the interrupting capacity of the tie breaker. We're looking at Eaton Magnum SB units (SBS630 series, 65kA @ 635V) for all three positions.
A few specific questions:
- When sizing the tie breaker in an MTM arrangement, is it common practice to size it equal to the mains, or should it be derated to account for the worst-case single-transformer full-load scenario?
- For the trip unit coordination — if the mains are set with a long-delay pickup at 100% and a short-delay of 6 cycles, what's the recommended short-delay setting on the tie to maintain selectivity without nuisance tripping during transfer?
- NEC 230.71 limits service disconnecting means — in an MTM arrangement where the tie is internal to the switchboard, does it count toward the 6-disconnect maximum, or is it treated as a feeder breaker on the load side of the service?
Any insight from those who've done the coordination study on similar configurations would be appreciated. Utility is 480V, 60Hz, facility is a data center — so reliability and selectivity are the primary drivers. Thank you.
