Summary: how to size a voltage sag protector
How to size a sag protector: 1) list the control circuit loads that reset on a sag; 2) measure their actual running current; 3) convert to kVA: single-phase is voltage × current ÷ 1000, three-phase is √3 × voltage × current ÷ 1000; 4) add margin and check inrush current; 5) choose the next standard size up and confirm the compensation time is enough. For a 100 kVA tool, control circuits usually need only a few kVA.
Why not buy for the whole tool's capacity?
When a voltage sag stops a tool, it's usually because the control system resets, not because high-power equipment can't hold on. Pumps, heaters, and motors ride through a 0.1 s sag on rotational inertia or thermal mass; what really needs protection is PLCs, industrial PCs, robot controllers, control power supplies, magnetic contactors, and EMO circuits. Together, these loads are usually a small fraction of the tool. Buying for the whole tool means paying a lot to protect equipment that would ride through anyway.
5 steps to size a voltage sag protector
List the loads
Use the electrical drawings to find the control circuits: PLCs, IPCs, touch panels, the "control power" of robots and servos (not motor power), control power supplies, contactor and relay coils, and EMO circuits. If you're unsure whether to include something, check whether it has tripped on past sags.
Measure current
The most accurate method is to measure actual current with a clamp meter on the secondary side of the control transformer or at the control circuit's main breaker, taking the maximum during normal operation. Adding up nameplate values usually overestimates, but if you can't measure, it's the more conservative approach.
Convert to kVA
Single-phase: kVA = voltage × current ÷ 1000. Three-phase: kVA = √3 × line voltage × current ÷ 1000. If the spec only gives watts (W), divide by the power factor to get VA; if you don't know the power factor, measure the current rather than treating watts as VA.
Margin and inrush
A common sizing practice for power equipment is to keep the load below 80% of rated capacity, with the actual margin set by on-site assessment. Also watch inrush current: contactor coils pulling in and power supplies starting up can briefly exceed the rating. For the sag protector referenced in this article, output shuts off automatically above 150% of rated current.
Pick a size, check hold time
Choose the next standard size at or above the calculated result. For the model referenced in this article, single-phase sizes are 850 VA and 1, 2, 3, 5, and 10 kVA, and three-phase sizes run from 5 to 200 kVA. At rated load, the 850 VA and 1 kVA models compensate for 2 s and the others for 3 s. If you need longer compensation, size up.
Worked examples
Example 1: 1-phase 220 V, 3.2 A
| Step | Formula | Result |
|---|---|---|
| Convert to kVA | 220 V × 3.2 A ÷ 1000 | 0.70 kVA |
| Add margin (load ≤ 80%) | 0.70 ÷ 0.8 | 0.88 kVA |
| Pick a size | Next size at or above 0.88 kVA | 1 kVA (2 s compensation) |
| Need 3 s? | One size up | 2 kVA (3 s compensation) |
Example 2: 3-phase 380 V, 20 A
| Step | Formula | Result |
|---|---|---|
| Convert to kVA | √3 × 380 V × 20 A ÷ 1000 | 13.2 kVA |
| Add margin (load ≤ 80%) | 13.2 ÷ 0.8 | 16.5 kVA |
| Pick a size | Next size at or above 16.5 kVA | 20 kVA (3 s compensation) |
Example figures only illustrate the method. Actual capacity must be assessed on site based on tool wiring, inrush current, and connection method. You can also enter your loads in the quick sizing tool on the product page to convert directly.
The 5 most common sizing mistakes
- Sizing for the whole tool→Only controls that reset
- Adding motors and heaters→They ride through on inertia
- Watts as VA→Divide by PF or measure current
- Ignoring inrush→Check overload limits
- Ignoring hold time→Small units hold less
FAQ
How large a sag protector does a 100 kVA tool need?
Usually just a few kVA. A sag protector protects only the control circuits that reset; pumps, heaters, and motors ride through on inertia and stay on the original supply. Actual capacity comes from measuring the control circuits' running current and converting it. For example, an air compressor with only its control unit connected needs just 850 VA.
How do you convert sag protector capacity to kVA?
Single-phase kVA equals voltage times current divided by 1000; three-phase kVA equals √3 times line voltage times current divided by 1000. For example, three-phase 380 V at 20 A is about 13.2 kVA.
How much margin should I allow?
A common sizing practice for power equipment is to keep the load below 80% of rated capacity. The actual margin depends on inrush current and future expansion plans, and should be set by on-site assessment.
What happens if the capacity is too small?
When the load exceeds the rating, compensation time may fall short; when inrush current exceeds the overload threshold, the compensator cuts its output, leaving the protected equipment without power. So size from measured values plus margin rather than cutting it close.