Sizing a Voltage Sag Protector: It Starts with a Load Survey

The capacity of a voltage sag protector should be sized around the specific loads that need protection, not the facility’s total incoming power. The correct sequence is to first identify which loads stop during a voltage sag, then survey their rated current, inrush current, and simultaneous-operation conditions, and only then calculate the required kVA. Reversing this order leads to two failure modes: over-sizing that inflates cost, or under-sizing that leaves the exact equipment you meant to protect exposed to sags.

A common pattern on-site looks like this: “our line’s incoming capacity is 500 kVA, so let’s spec a 500 kVA TSP.” But within that 500 kVA, the loads actually sensitive to voltage sags — PLCs, servo drives, robot controllers — are often a small fraction of the total. Sizing around lighting, HVAC, and other insensitive loads only adds cost and installation footprint without adding protection.

Identify the loads that need protection first

Capacity sizing starts with singling out the loads that actually stop or misbehave during a sag. Common priority loads include:

• PLCs, sequencers, servo motor drives, robot controllers
• Control sections of semiconductor and display process equipment
• Inspection and control systems on automotive and battery production lines

Semiconductor and display tools are frequently referenced against the ride-through points defined in SEMI F47 (80% voltage retained for 1 second, 70% for 0.5 seconds, 50% for 0.2 seconds). Any load that could trip below those thresholds should be treated as a priority candidate for protection.

What to check during the load survey

Once loads are identified, the survey should confirm:

• Rated current / rated capacity (from the nameplate)
• Inrush current (servo motors, for example, can draw several times rated current at startup)
• Whether multiple loads operate simultaneously
• Planned expansion (future line additions should be factored into the margin)

A weak survey here puts every downstream sizing calculation on a bad foundation.

The basic logic of capacity sizing

With the survey complete, sum the kVA of the loads to be protected, then add margin for inrush current and simultaneous operation to arrive at the required capacity. WESCO TSP compensates voltage sags using EDLC ultra-capacitors instead of batteries, engaging within 2 milliseconds for small-to-mid capacity units and within 4 milliseconds for large-capacity units — fast enough that the load never registers the sag.

How much depth and duration of sag a given load can tolerate before tripping is documented in the published ITIC (CBEMA) curve, which is a useful reference for gauging a load’s voltage tolerance envelope.

The cost of over- and under-sizing

Getting capacity wrong causes problems in both directions:

• Over-sizing: unnecessary capital cost and installation space, protecting loads that never needed it
• Under-sizing: loads outside the protected scope remain exposed to sags, ongoing risk of line stoppage, and redesign work when the line is later expanded

IEEE studies report roughly 60-70 voltage sags per year at a typical industrial site. If capacity is under-sized, a meaningful share of that frequency can translate directly into line stoppages.

Deciding where to install

Once capacity is set, the next decision is placement. If the loads needing protection are concentrated on one line, installing near that load is typically the more cost-effective choice. If loads are distributed evenly across several lines, installing at the common bus or main incoming panel provides blanket coverage with fewer points to manage. Either way, the load distribution uncovered during the survey is what should drive the decision, not a default preference for one location over the other.

Key takeaways

• Size a voltage sag protector around the loads that need protection, not total incoming capacity
• The load survey — rated current, inrush current, simultaneous operation, expansion plans — has to come before sizing
• Over-sizing wastes capital; under-sizing leaves protection gaps
• Installation placement should follow whether loads are concentrated or distributed
• WESCO TSP’s EDLC-based, battery-free design responds within 2ms (small/mid capacity) or 4ms (large capacity)

Frequently Asked Questions

Q. Can I size a voltage sag protector based on total incoming facility capacity?

That approach is not recommended. Sizing off total incoming capacity pulls in loads that are not sensitive to voltage sags, such as lighting and HVAC, and results in over-sizing. The correct approach is to size around only the loads that actually need protection, such as PLCs, servo motor drives, and robot controllers.

Q. What should be checked first in a load survey?

First identify which loads actually stop or malfunction during a voltage sag or momentary outage. Then check each load's rated current, inrush current, and whether multiple loads run simultaneously. Loads like servo motors that draw several times their rated current at startup need to be accounted for separately.

Q. Should a TSP be installed near individual loads or at the main incoming panel?

It depends on how the loads are distributed. If protection-critical loads are concentrated on a specific line, installing near that load is more cost-effective. If loads are spread evenly across multiple lines, installing at the common bus or main panel provides blanket protection with fewer installation points to manage.