Circuit Breaker Frame Sizes Explained — A, B, C, D, E, F, H, J, K, L

What Is a Frame Size?

A frame size is a standardized physical housing that accommodates a range of current ratings. All breakers of the same frame size from the same manufacturer have the same physical dimensions and mounting footprint — only the internal trip mechanism and current rating differ.

This means a 100A breaker and a 250A breaker in the same frame size are physically interchangeable in the panel, even though they have different current ratings.

Common Frame Sizes and Current Ranges

Frame sizes vary by manufacturer, but the general industry convention:

A frame: 15–100A (residential and light commercial) B frame: 15–100A (some manufacturers) C frame: 15–100A D frame: 70–150A E frame: 70–250A F frame: 70–250A (Square D F-Frame: HFD, HFA, HFB series) H frame: 70–600A (Square D H-Frame: HJA, HJD, HKA series) J frame: 150–600A (Square D J-Frame: JDA, JJA series) K frame: 400–800A L frame: 600–1,200A M/P/R frames: 800–2,500A (large power circuit breakers)

Square D Frame Designations

Square D uses a letter-based naming system where the frame letter appears in the catalog number:

HFD3100: H-Frame, F-size, D-type, 3-pole, 100A HJA36150: H-Frame, J-size, A-type, 3-pole, 6=600V, 150A JDA36200: J-Frame, D-type, A-version, 3-pole, 6=600V, 200A

The 'H' prefix on many Square D industrial breakers stands for 'High performance' and indicates the PowerPact series.

Eaton Frame Designations

Eaton uses similar frame size conventions:

HFD3100: same as Square D (Eaton also makes HFD series) JD3250: J-Frame, D-type, 3-pole, 250A KD3400: K-Frame, D-type, 3-pole, 400A LD3600: L-Frame, D-type, 3-pole, 600A

Why Frame Size Matters for Replacement

When replacing a breaker, the replacement must be the same frame size as the original. A different frame size will not fit in the panel's mounting space, even if the current rating is the same. Always verify both the frame size and the current rating when ordering a replacement.