Motor Code Letter
A motor code letter is the NEMA locked-rotor kVA per horsepower code stamped on an AC motor nameplate, ranging from A to V, indicating the apparent power drawn at rated voltage with the rotor locked. This code is used for sizing conductors, protective devices, and ensuring compliance with NEC and OSHA electrical safety standards in industrial facilities.
On the shop floor, motor code letters are used by engineers and electricians for electrical design and protection. They determine feeder and branch-circuit sizing, overcurrent device selection, and transformer loading to prevent voltage dips and nuisance trips. High code letters (M-V) require staggered starts or soft-starters. In maintenance, code letters help diagnose abnormal starting behavior and guide motor replacements to avoid overloading existing feeders. Compliance with WorkSafeBC and OSHA requires correct use of code letters to ensure safe electrical installations and arc-flash studies.
How is locked-rotor kVA/HP calculated from the motor code letter?
The motor code letter gives a range of locked-rotor kVA per horsepower at rated voltage. For a motor rated P HP and line voltage V, select a value from the NEMA code table (e.g., code M: 10.0-11.19 kVA/HP). Compute total kVA at locked rotor as kVA/HP × P. Convert to line current for three-phase: I_LR = (kVA_LR × 1000) / (√3 × V). This current is used with NEC Article 430 for conductor and overcurrent device sizing.
How do NEC and NEMA use the motor code letter differently from the design letter in safety-critical calculations?
The code letter (A-V) is used in NEC 430.7(B) to determine locked-rotor kVA/HP and starting current for feeder sizing. The design letter (A, B, C, D) identifies speed-torque characteristics per NEMA MG-1, used in NEC 430.251 for current and torque considerations. Mixing them leads to incorrect NEC table application, risking unsafe electrical design.
Does WorkSafeBC explicitly regulate motor code letters?
WorkSafeBC Part 19 does not specify motor code letters directly but requires safe electrical systems designed per recognized standards like CEC and NEMA. Misapplication of code letters leading to overheating or arc-flash hazards can be evidence of non-compliance with general electrical safety provisions.