Protective Relay Pickup Calculator

A protective relay pickup setting is commonly entered in secondary amperes, while the protected feeder, transformer, bus, or equipment is evaluated in primary amperes. The Protective Relay Pickup Calculator converts the entered relay pickup setting through the current-transformer ratio and compares the resulting primary pickup current with the entered available fault current.

The main result is Primary pickup current: the primary-side current level at which the entered protection screen reaches pickup. This value is used during preliminary protection review to determine whether available fault current is above the intended pickup threshold and how much margin exists between a fault level and relay pickup.

This is not a conductor ampacity, feeder load, voltage-drop, raceway-fill, or breaker interrupting-rating calculation. It addresses the relay’s pickup threshold as represented by the entered CT ratio and secondary pickup setting.

Relay Pickup Inputs

Input	Electrical purpose
Relay pickup setting (A secondary)	The relay pickup value measured at the CT secondary. This is the entered protection-screen threshold.
CT primary rating (A)	The rated primary current associated with the CT ratio.
CT secondary rating (A)	The rated secondary current associated with the CT ratio. Common protection CT secondary ratings include 5 A, but the calculator uses the value entered.
Available fault current (A)	The prospective fault current used for a preliminary comparison against primary pickup current.

A CT ratio transfers primary current to a lower secondary current that can be measured by protective relays and associated metering or protection circuits. For example, a 600 A to 5 A CT ratio means 600 A primary corresponds to 5 A secondary at the CT’s rated ratio.

The relay pickup setting must be interpreted in the same secondary-current basis as the entered CT secondary rating (A). Changing either the relay secondary pickup or the CT ratio changes the calculated primary pickup threshold.

Primary Pickup Formula

The calculator converts the entered secondary relay pickup into primary amperes using the entered CT ratio:

\(\displaystyle \text{Primary pickup current} = \text{Relay pickup setting (A secondary)} \times \frac{\text{CT primary rating (A)}}{\text{CT secondary rating (A)}}\)

It then compares available fault current with that result:

\(\displaystyle \text{Fault current above pickup} = \text{Available fault current (A)} - \text{Primary pickup current}\)

\(\displaystyle \text{Fault-to-pickup ratio} = \frac{\text{Available fault current (A)}}{\text{Primary pickup current}}\)

A positive Fault current above pickup result indicates that the entered available fault current is higher than the calculated pickup screen. A fault-to-pickup ratio greater than 1 indicates the same condition.

Calculation Example

For the entered values:

Field	Entered value
Relay pickup setting (A secondary)	5 A
CT primary rating (A)	600 A
CT secondary rating (A)	5 A
Available fault current (A)	12,000 A

The CT ratio is:

\(\displaystyle \frac{600\text{ A}}{5\text{ A}} = 120:1\)

The primary pickup current is:

\(\displaystyle 5\text{ A secondary} \times \frac{600\text{ A}}{5\text{ A}} = 600\text{ A primary}\)

The available fault-current comparison is:

\(\displaystyle 12{,}000\text{ A} - 600\text{ A} = 11{,}400\text{ A}\)

\(\displaystyle \frac{12{,}000\text{ A}}{600\text{ A}} = 20\)

Result	Value
Primary pickup current	600 A
Fault current above pickup	11,400 A
Fault-to-pickup ratio	20 x
Pickup comparison	Available fault current is above the entered pickup screen.

The entered 12,000 A available fault current is 20 times the calculated 600 A primary pickup current. For this pickup comparison, the available fault level exceeds the entered relay screen by 11,400 A.

Protection Coordination Use

Primary pickup current is a starting value for protection-system review. It can be carried into a relay coordination study, time-current curve review, or equipment protection evaluation along with the actual relay function, relay curve, CT data, upstream protective-device characteristics, and downstream equipment limits.

The pickup value alone does not establish that a feeder, branch circuit, transformer, motor circuit, conductor, or bus is properly protected. Conductor sizing still depends on ampacity, insulation temperature rating, terminal rating, applicable correction and adjustment factors, current-carrying conductors, overcurrent protection, and installation conditions. Raceway fill, conduit layout, voltage drop, and equipment short-circuit current rating are separate electrical determinations.

For motor and transformer protection, the applicable relay element may also require review against normal operating current, starting or inrush current, locked-rotor current, transformer energization, load pickup, and expected through-fault duty. A pickup threshold that is low enough to detect faults may be unsuitable if normal or transient current causes unwanted operation.

Field Verification

Verify the protection basis before applying the result to a relay setting or coordination decision:

• Confirm that Relay pickup setting (A secondary) matches the relay element and setting convention being evaluated.
• Confirm the installed CT primary and secondary ratings, polarity, wiring, and the CT ratio actually assigned to the relay input.
• Use available fault current for the correct location, voltage system, configuration, and operating condition.
• Review relay curve selection, instantaneous or time-overcurrent settings, time dial or delay settings, and coordination with upstream and downstream protective devices separately.
• Evaluate CT performance, including burden and saturation behavior, where fault-current magnitude and relay operation depend on CT accuracy during fault conditions.
• Confirm equipment interrupting ratings, short-circuit current ratings, arc-flash study assumptions, and final protective settings through the responsible engineering and approval process.

The calculation converts the entered secondary pickup through the entered CT ratio and compares it with entered available fault current. It does not make a relay-setting decision, protection-coordination determination, CT-saturation evaluation, trip approval, or NEC/AHJ compliance determination.