1. Work out the full-load current
Start from the larger of the two sources the switch must carry, usually the generator or the transformer feeding the load. For a three-phase system the full-load current is:
I (A) = kVA × 1000 ÷ (√3 × V) where V is the line-to-line voltage
Most commercial and industrial supplies in the Kingdom are 380 V or 400 V at 60 Hz, depending on the age of the network and the transformer tap. Always use the voltage on the single-line diagram.
| Generator | Current at 380 V | Current at 400 V | Typical ATS frame |
|---|---|---|---|
| 250 kVA | 380 A | 361 A | 400 A |
| 500 kVA | 760 A | 722 A | 800 A |
| 800 kVA | 1,215 A | 1,155 A | 1,600 A at 380 V; 1,200 A at 400 V |
| 1,000 kVA | 1,519 A | 1,443 A | 1,600 A |
| 1,500 kVA | 2,279 A | 2,165 A | 2,600 A |
2. Round up to a standard frame
Transfer switches come in fixed frames such as 400, 600, 800, 1,000, 1,200, 1,600, 2,000, 2,600 and 3,000 A. Pick the next frame above the full-load current. Many designers simply match the ATS to the main breaker of the circuit it serves, which keeps coordination simple. Where the current lands just under a frame limit (800 kVA at 400 V is 1,155 A on a 1,200 A frame), go one size up if the load will grow or if the switch sits in a hot plant room.
3. Choose 3 poles or 4 poles
A 3-pole switch transfers the phases only and leaves the neutral solidly connected. A 4-pole switch also switches the neutral. Use 4 poles when the generator is a separately derived system with its own neutral-to-earth bond, when there is ground-fault protection on the utility side, or when the consultant's specification calls for a switched neutral. Getting this wrong causes nuisance earth-fault trips and neutral current flowing where it should not.
4. Pick the transition type
- Open transition (break-before-make): the standard choice for most buildings. A short interruption during transfer.
- Delayed transition: adds a timed centre-off position so motors, VFDs and UPS loads can decay before reconnection. Recommended for large motor loads.
- Closed transition (make-before-break): sources are paralleled for a fraction of a second, so planned tests and retransfer happen without a blackout. It needs synchronising and the utility's approval.
- Bypass-isolation: a second switch lets you maintain the ATS without dropping the load. Common in hospitals, data centres and critical facilities.
5. Check the withstand and closing rating (WCR)
The ATS must survive the fault current available at its terminals until the upstream breaker clears. Ask for the short-circuit level from the single-line diagram and check the switch's withstand rating with the specific upstream breaker or fuse. A switch that is correct on current but under-rated on WCR is a common reason for consultant rejections.
6. Fire pumps are a separate case
A fire pump fed from utility and generator needs a transfer switch listed for fire pump service, normally supplied as one assembly with the fire pump controller. A general-purpose ATS should not be used for this. See our page on fire pump controllers with built-in transfer switch.
What to send for a quote
- Current rating, or generator kVA and voltage
- Number of poles (3 or 4)
- Transition type and whether bypass-isolation is needed
- Short-circuit level at the switch, and the upstream breaker
- Enclosure type and IP rating, or open type for panel builders
- Approvals required by the consultant or Civil Defense
- Quantity and required delivery date
PEX supplies ASCO 300, 4000 and 7000 Series and Schneider Electric TransferPacT switches from Al Khobar to projects across the Kingdom. Send the details above on WhatsApp and we reply with price and lead time.