Locked-rotor amps calculator

Locked-rotor kVA and current of an induction motor from its horsepower, rated voltage and locked-rotor kVA per horsepower.

Supply
Nameplate
hp
V
kVA/hp

From the code letter's range in NEC 430.7(B) or NEMA MG 1: the upper end gives the largest current.

Result

Locked-rotor current
360.8A
Locked-rotor kVA
300kVA

At rated voltage. A lower voltage during the start gives proportionally less current.

Check the voltage dip for this start

What it calculates

The current an induction motor draws at the instant it is started across the line, before the rotor turns. Nameplates give it indirectly: the locked-rotor code letter stands for a range of locked-rotor kVA per horsepower. Multiply by the rated horsepower for the locked-rotor kVA:

SLR=kVAhp×hpS_\text{LR} = \dfrac{\text{kVA}}{\text{hp}} \times \text{hp}

Then convert the kVA to current at the rated voltage. For a three-phase motor, with line-to-line voltage:

ILR=1000 SLR3 VLLI_\text{LR} = \dfrac{1000\,S_\text{LR}}{\sqrt{3}\,V_\text{LL}}

For a single-phase motor:

ILR=1000 SLRVI_\text{LR} = \dfrac{1000\,S_\text{LR}}{V}

Where the kVA per horsepower comes from

NEC 430.7(B) covers the locked-rotor code letters, and NEMA MG 1 defines the same letters. Look up the range for your motor's letter in the edition you work to; this page does not reproduce the table. Each letter covers a range, so:

  • For the largest current the letter allows, for example to size for the worst-case dip, enter the upper end.
  • For the smallest, enter the lower end.
  • If the manufacturer publishes the locked-rotor current or kVA for the motor, that is better than either.

Worked example

A 50 hp, 480 V three-phase motor whose code letter range tops out at 6.0 kVA per horsepower. The calculator opens on this case:

SLR=6.0×50=300 kVAILR=300,0003×480=360.8 A\begin{aligned} S_\text{LR} &= 6.0 \times 50 = 300\ \text{kVA} \\[4pt] I_\text{LR} &= \dfrac{300{,}000}{\sqrt{3} \times 480} = 360.8\ \text{A} \end{aligned}

A 5 hp, 230 V single-phase motor at the same 6.0 kVA per horsepower:

SLR=6.0×5=30 kVAS_\text{LR} = 6.0 \times 5 = 30\ \text{kVA}ILR=30,000230=130.4 AI_\text{LR} = \dfrac{30{,}000}{230} = 130.4\ \text{A}

Next: the voltage dip

Locked-rotor kVA is what sets the bus voltage dip when the motor starts. The link under the result opens the motor-starting voltage dip calculator with this kVA filled in.

Assumptions

  • The kVA per horsepower is per rated output horsepower. Do not convert horsepower to kilowatts first: that understates the result by about a quarter.
  • The current is at rated voltage. If the voltage at the terminals dips during the start, the current falls roughly in proportion.
  • The result is the symmetrical RMS current at standstill. The first-cycle asymmetrical inrush can be higher, and the current falls as the motor accelerates.

Questions

Where do I find the kVA per horsepower?
From the locked-rotor code letter on the motor's nameplate. Each letter stands for a range of locked-rotor kVA per horsepower, listed in NEC 430.7(B) and in NEMA MG 1; this page does not reproduce that table. For the largest current, enter the upper end of the letter's range. If the manufacturer gives the locked-rotor current itself, use that instead.
Is the code letter per horsepower or per kilowatt?
Per rated horsepower. Multiply the kVA per horsepower by the horsepower directly. Converting the horsepower to kilowatts first understates the locked-rotor kVA and current by about a quarter.
Why is locked-rotor current so much higher than full-load current?
At standstill the slip is 1, so the rotor's resistance term in the equivalent circuit is at its smallest and only the winding impedances limit the current. For general-purpose motors, locked-rotor current is commonly five to eight times full-load current, which is why an across-the-line start dips the bus voltage.

Free practice problems that use this

Study areas that use this

For learning, not for design

PhasorPrep is an independent study resource. It is not affiliated with, endorsed by, or sponsored by NCEES. Questions are original and written for practice; they are not actual exam questions. Calculators are for learning, not for engineering design.

Put the methods to work

Original PE Power practice questions, each with a worked solution and an answer recomputed by a script. Find your weak areas with the free diagnostic, or try a sample question first.

Take the free diagnosticGet the 6-month pass — $79Try sample questions