NEMA Code Letter Calculator

Purpose

A motor’s NEMA code letter — stamped on the nameplate between A and V — identifies the locked-rotor kVA per horsepower band the motor falls into at the instant it energizes across the line. This figure drives short-circuit protective device sizing, feeder and branch-circuit conductor selection under NEC Article 430, generator and UPS starting-capacity checks, and voltage-drop review during the start transient. The NEMA Code Letter Calculator works the problem in the direction most contractors actually need: given a measured or nameplate locked-rotor current, it back-calculates the locked-rotor kVA and the kVA/hp ratio, then reports which NEMA code letter band that ratio falls into.

This is the reverse of the more common lookup, where the code letter on the nameplate is used to predict locked-rotor current for breaker or starter sizing. Running it in reverse is useful when a nameplate is missing, damaged, or suspect, or when a field technician measures inrush current directly with a clamp meter and needs to verify it against the stamped code letter.

Inputs

The calculator uses four fields, matching the terminology on a standard motor nameplate:

• Motor horsepower — the rated output horsepower of the motor (10 hp in the worked example).
• System voltage — the voltage basis paired with the locked-rotor current reading. For three-phase motors, this is the line-to-line voltage (460 V), not line-to-neutral.
• Phase — either single-phase or balanced three-phase, since the kVA formula differs between the two.
• Locked-rotor current — the measured, nameplate, or estimated starting current in amperes at the instant the rotor is stationary and full voltage is applied (100 A in the example).

Formula

For a balanced three-phase motor, locked-rotor kVA is calculated as:

\(\text{Locked-rotor kVA} = \frac{V \times I \times \sqrt{3}}{1000}\)

For single-phase motors, the √3 term drops out:

\(\text{Locked-rotor kVA} = \frac{V \times I}{1000}\)

The kVA per horsepower ratio, which is the actual figure NEMA MG 1 and NEC Table 430.7(B) use to define each code letter band, is then:

\(\text{kVA/hp} = \frac{\text{Locked-rotor kVA}}{\text{Motor horsepower}}\)

That ratio is compared against the fixed NEMA bands (A through V, with I and Q unused) to identify the matching code letter and report the low and high edges of that band.

Calculation Example

With 10 hp, 460 V, three-phase, and 100 A locked-rotor current:

Step	Calculation	Result
Locked-rotor kVA	(460 × 100 × 1.732) / 1000	79.6743 kVA
kVA per hp	79.6743 / 10	7.9674 kVA/hp
Code letter band	7.9674 falls in 7.1–7.99	Code letter J

The calculator reports the band edges directly: Code range low = 7.1 kVA/hp, Code range high = 7.99 kVA/hp, matching NEC Table 430.7(B)’s published J-letter range. A 10 hp, 460 V motor drawing 100 A locked-rotor is consistent with a J-rated nameplate; if the nameplate showed a different letter (say, H at 6.3–7.1 kVA/hp), the discrepancy would flag either a measurement error, an incorrect voltage basis, or a motor that has been rewound or replaced without an updated nameplate.

Practical Use in Conductor and Protection Sizing

Once the code letter is confirmed, it feeds directly into NEC 430.52 for short-circuit and ground-fault protective device selection, since the maximum allowable breaker or fuse rating is expressed as a percentage of full-load current keyed to the motor’s locked-rotor characteristic, not the code letter itself directly, but the code letter is used with Table 430.7(B) to estimate locked-rotor current when it is not otherwise known. It also matters for generator sizing, where locked-rotor kVA sets the minimum instantaneous starting capacity the source must supply without excessive voltage drop, and for reduced-voltage starter or VFD bypass selection, where the starting current magnitude determines whether across-the-line starting is acceptable on a given service.

Field and Code Limitations

The result is arithmetic, not a code compliance determination. Three limitations apply directly:

• Voltage basis errors distort the result. Entering a line-to-neutral voltage instead of line-to-line for a three-phase motor produces a kVA figure roughly 1.732 times too low, which shifts the calculated code letter down by several bands.
• Measured locked-rotor current varies with supply voltage at the instant of test. Locked-rotor current scales roughly linearly with applied voltage, so a reading taken at reduced voltage will underreport the code letter unless corrected to rated voltage first.
• Code letter bands do not substitute for the nameplate value used in NEC calculations. Where NEC 430.7(B) and 430.52 require locked-rotor current for protective device sizing, use the nameplate-stamped code letter or manufacturer locked-rotor data as the basis; treat a calculated or back-derived letter as a field verification check, not a replacement value, and confirm any discrepancy with the motor manufacturer or the AHJ before resizing conductors or protective devices.