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Calcimator

Motor Circuit Calculator

Branch circuit protection, conductor, and starter sizing from motor HP per NEC Article 430.

Inputs

Results

Motor FLA (amps)

14

Max OCPD size (amps)35
Min conductor ampacity (A)17.5
Selected conductor ampacity (A)20
Overload relay trip (A)16.1
Conductor index0
StarterNEMA 1 (up to 18A)
Ground Wire10 AWG
How to Use This Calculator
  1. Enter the motor nameplate horsepower rating.
  2. Select single-phase (1) or three-phase (3) and the supply voltage (230 V single-phase or 460 V three-phase are common).
  3. Choose the protection type: inverse-time breaker (1) at 250% FLA or dual-element fuse (2) at 175% FLA per NEC 430.52.
  4. Read the full-load amperage (FLA) from NEC Table 430.248/250, the maximum overcurrent device size, and the minimum conductor ampacity at 125% of FLA.
  5. Use the overload relay trip amperage to set the thermal overload in the motor starter.

How the result changes with Voltage (V)

Voltage (V)Motor FLA (amps)
15042.9
27523.4
42515.2
55011.7

What each input means

Motor horsepower (HP)
Nameplate motor horsepower rating.
Phase (1 or 3)
Single-phase (1) or three-phase (3) motor.
Voltage (V)
Supply voltage. Common: 230V single-phase, 460V three-phase.
Protection type
1 = Inverse-time breaker (250% FLA), 2 = Dual-element fuse (175% FLA) per NEC 430.52.

What each result means

Motor FLA (amps)
Full-load amperage from NEC Table 430.248/250.
Max OCPD size (amps)
Maximum overcurrent protective device rating per NEC 430.52.
Min conductor ampacity (A)
125% of FLA per NEC 430.22.
Selected conductor ampacity (A)
Ampacity of selected conductor at 75°C (THWN-2).
Overload relay trip (A)
Overload relay set at 115% of FLA per NEC 430.32.
Conductor index
Internal index for conductor lookup.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Motor horsepower (HP) = 10, Phase (1 or 3) = 3, Voltage (V) = 460, Protection type = 1 = 4 input(s) provided
  2. Calculate Motor FLA
    14 = 14
  3. Calculate Max OCPD size
    Max OCPD size
    35 = 35
  4. Calculate Min conductor ampacity
    Min conductor ampacity = fla * 1.25
    17.5 = 17.5

Engine last updated . Checked against 1 independently-derived test — how we verify calculators.

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