Load current, the 125% continuous-load factor and the next standard transfer switch rating — and why a load that looks like 400 A needs a 600 A switch.
| Standby load | 250 kW at 0.9 power factor |
| Supply | 480 V, three-phase |
| Continuous portion | 80% of the load runs 3 hours or more |
| System | Optional standby (NEC Article 702) |
Standard ratings run …, 300, 400, 600 A. 400.9 A is just above 400 A.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Minimum ampere rating | ≥ 400.9 A | 600 A selected | ✓ PASS |
| 400 A switch | ≥ 400.9 A | 400 A | ✗ FAIL |
Key insight: Results that land just above a standard rating are worth a second look at the load data: confirming the continuous fraction or the power factor can avoid a jump to the next frame size.
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Open Transfer Switch Sizing calculator →Its withstand and closing rating (WCR) must be at least the available fault current for the upstream protective device type and clearing time, per its UL 1008 listing.
Motor loads affect transfer switch type (in-phase monitor or delayed transition) and inrush; check the manufacturer's application guide.