NEC 430.7(B) · NEMA MG 1 · Motor nameplates

Motor locked-rotor code letters, frames & service factor

What the CODE letter on a nameplate means, how to turn it into locked-rotor amps, why it is not the DESIGN letter, and how to read NEMA frame numbers and service factor.

Need the actual table? Look up Table 430.7(B) in the free NFPA viewer →

Nameplate markings that get confused

Marking	What it tells you	Where it comes from	Used for
CODE (a letter)	Locked-rotor kVA per horsepower range	NEC 430.7(B) / NEMA MG 1	Estimating starting (locked-rotor) current
DESIGN (A, B, C, D)	Torque and current characteristic class	NEMA MG 1	Load matching; also affects 75 °C terminal allowance and some Art. 430 rules
FRAME (e.g. a number + T)	Mounting and shaft dimensions	NEMA MG 1	Swapping motors, bases, couplings
S.F. (service factor)	Continuous overload the motor tolerates at rated voltage/frequency	NEMA MG 1	Overload sizing under 430.32
FLA	Nameplate running current	Manufacturer	Overloads only — not conductors or breaker

Our own summary of the 2023 NEC and NEMA MG 1, not the standards’ text. Look up Table 430.7(B) in the free NFPA viewer (nfpa.org/freeaccess), consult NEMA MG 1 for frame dimensions, and verify against the edition adopted by your AHJ.

How to turn a code letter into locked-rotor amps, step by step

Step	What you do	Watch out for
1	Read the CODE letter off the nameplate	Do not use the DESIGN letter — it is a different system
2	Find that letter in Table 430.7(B) and note the kVA/hp range	Use the upper end of the range for worst-case current
3	Multiply kVA/hp × nameplate horsepower = locked-rotor kVA	Multi-speed and dual-voltage motors have their own marking rules
4	Three-phase: LRA = kVA × 1000 ÷ (1.732 × volts). Single-phase: LRA = kVA × 1000 ÷ volts	Use the motor voltage, not the system nominal
5	Use LRA for generator sizing, voltage-dip checks, and disconnect/controller ratings where Art. 430 calls for it	For those Art. 430 disconnect rules, the NEC also publishes locked-rotor tables (430.251(A)/(B))

Worked example — 10 hp, 460 V, 3-phase, nameplate CODE G

Input / step	Value	Source
Code letter range (upper end)	6.29 kVA/hp	Table 430.7(B), letter G
Locked-rotor kVA	6.29 × 10 = 62.9 kVA	Step 3
Locked-rotor amps	62,900 ÷ (1.732 × 460) ≈ 79 A	Three-phase power formula
Compare to running current	≈ 5.6 × the 10 hp table FLC	Why motor breakers are sized high

Worked example values only (one letter). Look up the full table in the free NFPA viewer.

How to read a NEMA frame designation

Part of the number	Meaning	Example
First two digits (T-frames)	Divide by 4 = shaft centerline height (D dimension), inches	256T → 25 ÷ 4 = 6.25 in.
Third digit	Relates to the foot-bolt spacing (F dimension) — longer body, higher digit	254T vs 256T: same height, longer base
T suffix	Current (post-1964) rerate series	Replacing an older U-frame usually needs an adapter or new base
C / D	C-face or D-flange mounting	Pumps and gear reducers
S (e.g. TS)	Short shaft for direct coupling	Belted loads usually need the standard T shaft

Dimensions from NEMA MG 1 — confirm on the manufacturer dimension sheet before ordering a replacement.

Common field mistakes

Mistake	What to do instead
Reading the DESIGN letter as the code letter	CODE = kVA/hp at locked rotor; DESIGN = torque class. A Design B motor can carry almost any code letter
Using the low end of the kVA/hp range for generator sizing	Use the upper end for worst case
Treating service factor as extra nameplate horsepower	SF is a margin for voltage/temperature swings; running in it continuously shortens insulation life
Sizing overloads on a 1.0 SF motor like a 1.15 SF motor	430.32 allows less overload margin for motors without a 1.15+ SF or low marked temp rise
Assuming the same frame number means a drop-in replacement	Check C/D face, shaft suffix, and U vs T series

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Decoding the motor nameplate

Look up Table 430.7(B) in the free NFPA viewer. This page explains the code-letter system and the NEMA MG 1 markings; it does not reproduce the tables. Frame dimensions and design-letter definitions come from NEMA MG 1 — consult the standard or the motor manufacturer.

The CODE letter tells you how hard a motor hits the system when it starts. Each letter covers a range of kVA per horsepower with the rotor locked; letters earlier in the alphabet mean lower starting kVA. That matters when you size a generator, check voltage dip on a weak service, or explain to an owner why the lights blink when the compressor kicks on.

Worked example. A 10 hp, 460 V three-phase motor stamped CODE G. The top of the G range is 6.29 kVA/hp, so locked-rotor kVA is about 62.9 kVA. Dividing 62,900 by (1.732 × 460) gives roughly 79 A at the moment of start — several times its running current, which is exactly why Article 430 lets the branch breaker be sized well above the wire.

Frames and service factor. The NEMA frame number tells you shaft height and base footprint, which is what matters when you swap a motor. Service factor is the continuous margin above nameplate horsepower — useful headroom, not extra horsepower to plan on. It also changes how high you can set the overloads.

Next steps: size the circuit with the motor branch-circuit protection guide, pull running current from the motor full-load current chart guide, and pick the starter with NEMA starter sizes. Field PM keeps nameplate photos and motor data with the equipment record on the job.

FAQ

What does the code letter on a motor mean?+

It is the locked-rotor indicating code letter from NEC 430.7(B). It tells you the range of kilovolt-amperes per horsepower the motor draws with the rotor locked (the instant of starting). Multiply by horsepower to get locked-rotor kVA, then convert to amps. Look up Table 430.7(B) in the free NFPA viewer at nfpa.org/freeaccess.

How do I calculate locked-rotor amps from the code letter?+

Locked-rotor kVA = kVA/hp (upper end of the letter range) × horsepower. For three-phase, LRA = kVA × 1000 ÷ (1.732 × volts). Example: a 10 hp, 460 V, Code G motor at 6.29 kVA/hp gives 62.9 kVA, or about 79 A locked-rotor.

Is the code letter the same as the NEMA design letter?+

No. The code letter describes locked-rotor kVA per horsepower. The NEMA design letter (A, B, C, D) under NEMA MG 1 describes the torque and starting-current characteristic. They are separate markings, and mixing them up is one of the most common nameplate errors.

What is motor service factor?+

Service factor is the multiplier NEMA MG 1 assigns for the continuous load a motor can carry at rated voltage and frequency without damage. A 1.15 SF motor tolerates about 15% over nameplate horsepower, but running there routinely raises winding temperature. Under NEC 430.32, a service factor of 1.15 or more lets overloads be set a little higher.

How do I read a NEMA motor frame size?+

For NEMA T-frames, divide the first two digits by 4 to get the shaft centerline height in inches (a 256T is 6.25 in.). The third digit relates to the foot-bolt spacing. Letters after the number indicate series (T or older U) and mounting options like C-face or a short shaft. Confirm on the manufacturer dimension sheet.

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