Motor Disconnect Size Calculator | Required Disconnect Current & kVA

Result

Formulas

\(\displaystyle I_{\text{required}} = I_{\text{FLA}} \times \frac{p_{\text{disconnect}}}{100}\)
\(\displaystyle M_{\text{disconnect}} = I_{\text{entered}} - I_{\text{required}}\)
\(\displaystyle S_{\text{motor}} = \frac{V I_{\text{FLA}} k_{\phi}}{1000},\quad k_{\phi}=1\text{ for single-phase and }\sqrt{3}\text{ for balanced three-phase}\)

A motor disconnect must have a current rating appropriate for the motor load and the disconnect-sizing basis being applied. This Motor Disconnect Size Calculator produces the Required disconnect current by applying the selected Disconnect multiplier to the entered Motor FLA.

The result is useful when reviewing a motor branch circuit or feeder layout alongside conductor ampacity, overcurrent protection, starter or drive equipment, raceway fill, voltage-drop calculations, and disconnect location. It gives a direct current-rating target for comparing a proposed disconnecting means before the final equipment and code review.

Enter Motor FLA as the motor full-load amps or the applicable nameplate-current basis. Enter the Disconnect multiplier used for the current-rating screen, then enter an Entered disconnect rating only when a specific disconnect is being checked. System voltage and Phase are used to calculate motor load in kVA; they do not change the required disconnect-current arithmetic.

Required Disconnect Current Formula

The calculator uses the motor full-load current and disconnect multiplier:

\(\displaystyle \text{Required disconnect current} = \text{Motor FLA} \times \frac{\text{Disconnect multiplier}}{100}\)

The result may be a value that does not match a standard catalog disconnect rating. The calculated amperage is the required current basis generated by the selected multiplier. Selection of the installed disconnect requires comparison with available equipment ratings and the complete installation conditions.

Input	Function in the calculation
Motor FLA	The motor current basis, in amperes
Disconnect multiplier	The percentage applied to Motor FLA
Entered disconnect rating	A proposed disconnect current rating used only for comparison
System voltage	Voltage used for the motor-load kVA calculation
Phase	Selects single-phase or balanced three-phase kVA math

Disconnect Rating Comparison

When an Entered disconnect rating is provided, the calculator reports the difference between that rating and the calculated required current:

\(\displaystyle \text{Entered disconnect margin} = \text{Entered disconnect rating} - \text{Required disconnect current}\)

A positive margin means the entered rating is at least as high as the calculated required disconnect current. The result comparison is reported as within entered rating when the entered disconnect rating meets or exceeds that calculated value.

This comparison is not a confirmation that the disconnect is acceptable for the installation. A field selection must also account for the disconnect’s listed use, voltage rating, phase configuration, enclosure type, horsepower suitability where applicable, available fault current and short-circuit current rating, and the requirements governing the motor circuit and disconnecting means.

Motor Load kVA

The calculator also reports Motor load in kVA from Motor FLA, System voltage, and Phase. This supports load review and helps place the motor’s current demand in the context of feeder capacity, transformer loading, generator capacity, service load review, and voltage-drop analysis.

For balanced three-phase power:

\(\displaystyle \text{Motor load (kVA)} = \frac{\sqrt{3} \times \text{System voltage} \times \text{Motor FLA}}{1000}\)

For single-phase power:

\(\displaystyle \text{Motor load (kVA)} = \frac{\text{System voltage} \times \text{Motor FLA}}{1000}\)

The kVA output represents apparent power from the supplied voltage and current inputs. It is not a replacement for motor input-power, power-factor, efficiency, or demand calculations when those values are required for equipment sizing or load analysis.

Calculation Example

Given the following inputs:

Field	Entered value
Motor FLA	34 A
Disconnect multiplier	115%
Entered disconnect rating	60 A
System voltage	480 V
Phase	Balanced three-phase

Required disconnect current:

\(\displaystyle 34\ \text{A} \times \frac{115}{100} = 39.1\ \text{A}\)

Entered disconnect margin:

\(\displaystyle 60\ \text{A} - 39.1\ \text{A} = 20.9\ \text{A}\)

Balanced three-phase motor load:

\(\displaystyle \frac{1.732 \times 480\ \text{V} \times 34\ \text{A}}{1000} = 28.2671\ \text{kVA}\)

The calculator result is:

Required disconnect current: 39.1 A
Entered disconnect margin: 20.9 A
Motor load: 28.2671 kVA
Rating comparison: within entered rating

The 60 A entered rating exceeds the calculated 39.1 A requirement by 20.9 A under the selected 115% multiplier.

Field Verification

Use the calculated disconnect current as one part of the motor-circuit workflow. The final disconnect selection must be verified against the actual motor nameplate, branch-circuit and feeder design, conductor ampacity, AWG or kcmil conductor selection, insulation temperature rating, terminal rating, correction factor, adjustment factor, number of current-carrying conductors, raceway fill, and voltage-drop conditions.

The installed disconnect must also match the electrical system: System voltage, phase arrangement, utilization equipment, and the fault-duty conditions at its location. Confirm the manufacturer’s equipment markings and listing information, then apply the requirements enforced by the local AHJ.

Does this choose a disconnect switch?

No. It only compares current values. Final disconnect selection needs horsepower rating, enclosure, SCCR, listing, and code review.

Where should the multiplier come from?

Use the project, manufacturer, or reviewer basis. The calculator does not choose a code multiplier.

Can this approve a motor disconnect?

No. It is an arithmetic screen, not equipment selection or approval.