Calculate overcurrent relay pickup setting, pickup current, or feeder load current from the other two values in A or kA units.
Overcurrent Relay Pickup Setting Formula
The pickup-to-load ratio expresses pickup current as a percentage of feeder load current. It is not necessarily the relay’s setting as a percentage of nominal CT secondary current. Pickup starts an element; a trip also depends on timing, logic and coordination. The calculator uses amperes as the base unit, so any value entered in kA is converted to A before the formula is applied.
- Pickup Current is the relay pickup current, in A or kA.
- Feeder Load Current is the normal feeder current used as the reference value, in A or kA.
- Relay Setting is the pickup setting as a percentage of feeder load current.
If you enter the feeder load current and relay setting, the calculator finds the pickup current. If you enter the pickup current and relay setting, it finds the feeder load current. If you enter the pickup current and feeder load current, it finds the relay pickup setting percentage.
Current Unit Conversions and Setting Reference
Use the following tables to check units and interpret illustrative pickup-to-load percentages. These are not recommended protection settings.
| Current unit | Equivalent in amperes | Example |
|---|---|---|
| A | 1 A | 600 A = 600 A |
| kA | 1 kA = 1000 A | 1.5 kA = 1500 A |
| Relay pickup setting | Meaning | Pickup current for 500 A feeder load |
|---|---|---|
| 110% | Picks up slightly above normal load current | 550 A |
| 125% | Pickup at 1.25 times feeder load current; illustrative only | 625 A |
| 150% | Higher pickup threshold for larger load margin | 750 A |
| 200% | Pickup at twice the feeder load current | 1000 A |
Example Calculations
Example 1: Calculate pickup current
You have a feeder load current of 400 A and a relay pickup setting of 125%.
The pickup current is 500 A.
Example 2: Calculate relay pickup setting
You have a pickup current of 0.9 kA and a feeder load current of 600 A. First convert 0.9 kA to amperes.
The relay pickup setting is 150% of feeder load current.
FAQs
What does relay pickup setting mean?
Relay pickup setting is the current level at which the overcurrent relay starts to operate, expressed as a percentage of the feeder load current. For example, a 125% setting on a 400 A feeder means the relay pickup current is 500 A.
Should pickup current be higher than feeder load current?
In most applications, yes. The pickup current is usually set above normal feeder load current so the relay does not trip during expected loading. The correct setting depends on the equipment rating, expected load, inrush current, coordination study, and protection requirements.
How are A and kA handled in the calculation?
The calculator converts current values to amperes before calculating. A value in kA is multiplied by 1000. For example, 2 kA is treated as 2000 A. After calculation, the result is converted back to the unit selected for the missing current field.
