Sizing an ATS for a Hospital Backup Generator: A Practical Guide

A hospital ATS is sized against three independent constraints: the continuous current of the essential-services load, the prospective short-circuit current at the switch, and the time it must spend across one fault clearing event without damage. NFPA 110 and IEC 60364-7-710 define the load classes and the maximum transfer times. Get the sizing wrong and either the ATS fails to clear a downstream fault or transfer times stretch beyond what life-safety equipment can tolerate.

Load classification first

Hospitals separate loads into three categories before sizing anything:

• Life-safety (Class 0 / NFPA 110 Type 10): Must restore within 0.5 seconds. Theatre power, ICU monitoring, dispensary refrigeration, life-support equipment. Typically fed via UPS that bridges the ATS transfer.
• Critical / essential (Class 15 / NFPA 110 Type 60): Must restore within 15 seconds. General theatre lighting, ICU general loads, medical gas compressors, X-ray and CT (cold restart acceptable), elevator priority, sewerage pumps, fire pumps.
• General / comfort: May or may not be backed up. HVAC for non-critical areas, kitchen non-essential, general lighting in admin offices. Some hospitals don't back these up at all.

The ATS sizing is based on the sum of critical + life-safety loads, not the total hospital connected load.

Step 1 — Calculate the essential-load continuous current

Sum the connected load of every circuit fed downstream of the ATS, apply the diversity factor appropriate for each load type (typically 0.5–0.7 for general lighting, 0.8 for HVAC, 1.0 for fixed medical equipment), and convert to current at 415 V three-phase:

I_continuous = P_total × 1000 / (√3 × 415 × PF)

Use a power factor of 0.85 as the design assumption unless plant studies indicate otherwise.

Step 2 — Add the 25% NEC / IEC headroom

The ATS continuous current rating must be at least 125% of the calculated continuous load (per IEC 60364-1 design margins, mirroring NEC 700.10). This gives:

I_ATS_rating = I_continuous × 1.25

Round up to the next standard frame size (typically 100 A, 160 A, 250 A, 400 A, 630 A, 800 A, 1000 A, 1250 A, 1600 A, 2000 A, 2500 A, 3200 A, 4000 A).

Step 3 — Check withstand and closing rating (WCR)

The WCR must be at least equal to the prospective short-circuit current (PSCC) at the ATS location, for the time the upstream breaker takes to clear a fault. Practically:

• Calculate PSCC at the ATS bus (utility infeed + cable impedance + generator contribution; use the larger of grid or generator scenarios)
• Look up the ATS WCR table — typically published as kArms for 30 cycles (100% WCR) or 3 cycles (10% WCR)
• Confirm the upstream breaker clears the worst-case fault inside the WCR's rated duration

For typical 250-bed Nairobi hospitals (with 1500 kVA utility transformer + 800 kVA standby genset), PSCC at the ATS is in the 25–35 kArms range; choose ATS with at least 50 kA 30-cycle WCR for design margin.

Step 4 — Account for motor inrush

If motors are a significant fraction of the load (large HVAC, lifts), apply motor-rated derating: typically multiply the ATS continuous rating by 0.8 for motor-rich loads, or specify a motor-rated ATS that publishes a separate motor current rating.

Step 5 — Choose transfer mode

• Open transition for general critical loads — 0.1 to 4 seconds outage, fits NFPA 110 Type 60 limit comfortably
• Closed transition where transfer outage is not acceptable to medical equipment — synchronisation panel required between grid and genset, near-zero outage
• Programmed transition where motor-heavy critical loads can't tolerate out-of-phase reclosing — brief mid-position delay (typically 100–300 ms) lets motors spin down before re-energisation

Worked example — 250-bed urban hospital

Sizing:

1. Continuous current at 0.85 PF: 584 × 1000 / (1.732 × 415 × 0.85) = 956 A
2. 125% NEC headroom: 956 × 1.25 = 1195 A → next standard size 1250 A
3. Motor-rich derating: 1250 × 0.8 = 1000 A — still fits the 956 A continuous, OK
4. PSCC calculation: 28 kA worst-case → specify 50 kA WCR (30 cycle)
5. Transfer mode: closed transition for theatre + ICU feeds (separate 400 A ATS) + open transition for HVAC and general (1250 A ATS)

Result: 1 × 1250 A open-transition ATS for general critical loads + 1 × 400 A closed-transition ATS for life-safety theatres and ICU, both with 50 kA WCR.

Common mistakes

• Sizing against total hospital connected load instead of just essential — over-specs the ATS, wastes capital
• Forgetting the 25% NEC headroom and operating the ATS at 95% of rating — no margin for thermal cycling
• Ignoring motor derating — ATS contacts wear from inrush, causes premature mechanical failure
• Choosing one mega-ATS for all loads when separate ATS for life-safety vs critical makes more sense
• WCR sized for utility-only PSCC without considering generator contribution during transfer

Bottom line

A hospital ATS isn't simply "size by total kW" — it's an essential-load calculation, a code-compliant headroom factor, a fault-current verification, and a transfer-mode choice. Split critical and life-safety into separate ATS where the operating-mode requirements differ. The cost difference between a correctly sized installation and an oversized one is small; the cost of getting it wrong shows up the first time a theatre loses power for too long.

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