Design Considerations for Generator Set Mounted Paralleling Circuit Breakers

Overview

Moving the breaker onto the set reopens questions of fault current, bus and cable bracing, load flow, grounding and maintenance isolation. The paper works through each with simple formulas. Every supply-side breaker needs an interrupting rating at or above Imafc, the sum of fault contributions from all gensets, motor loads and utility transformers that can be connected at the same time.

Genset fault current is Iafc = (kW x 1000) / (sqrt3 x V x pf x X"d), with X"d the per-unit subtransient reactance on the set rating. Motor contribution is approximated by locked-rotor current and utility contribution by transformer rating divided by impedance. For supply current, Igsc = kW / (sqrt3 x V x pf); the total must not exceed the paralleling bus continuous rating, shown with three 2,000 A sets on a 6,000 A bus.

On grounding, NEC 2014 230.95 requires ground-fault protection at the service disconnect, for example on 277/480 V or 347/600 V systems with an overcurrent device of 1000 A or more. NEC 700.6(D) requires only ground-fault indication at the emergency source. In four-wire systems neutral and ground are bonded at one place only, to avoid nuisance ground-fault tripping.

Key points

1. Supply-side breakers need interrupting rating at or above Imafc, the sum of all simultaneously connectable genset, motor and transformer contributions.
2. Genset fault current: Iafc = (kW x 1000) / (sqrt3 x V x pf x X"d).
3. Motor fault contribution uses locked-rotor current; utility contribution uses transformer rating over impedance.
4. Total generator supply current must not exceed the paralleling bus continuous rating (example: three 2,000 A sets on a 6,000 A bus).
5. NEC 2014 230.95 requires ground-fault protection at the service disconnect (e.g. 277/480 V with devices of 1000 A or more).
6. NEC 700.6(D) requires only ground-fault indication at the emergency source.
7. Four-wire systems bond neutral to ground at one place only.

Using it in design

• Sum fault current at the collector bus from catalog X"d, kW, V and pf, add motor and transformer terms, and check breaker kAIC and bus bracing.
• Verify bus ampacity against the sum of connected generator supply currents.
• Flag any design with more than one neutral-ground bond in a 4-wire generator system.

Who should read it

Electrical engineers and switchgear specifiers working on modular or distributed generator layouts.

Related standards

• NEC (NFPA 70) 230.95
• NEC 700.6
• UL 1558
• NFPA 110

Publication details

Authors

Hassan Obeid

Publisher

Cummins

Year

2025

Document number

Bulletin 5600618

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation