How to Identify a Circuit Breaker Frame Size

Published: 2026-07-01 | 8 min read | Category: Technical Guide

A circuit breaker's frame size is the most important specification for determining compatibility with your electrical panel. The frame defines the physical dimensions of the breaker housing, the maximum continuous current it can carry, and the interrupting capacity at a given voltage. Getting the frame size wrong means the breaker literally won't fit in your panel — or worse, it fits but can't safely interrupt fault currents.

What Frame Size Means

Frame size refers to the largest ampere rating that can be installed in a particular breaker housing. A "250A frame" breaker has a housing designed to handle up to 250 amps of continuous current, but the actual trip unit inside might be rated at 100A, 150A, 175A, 200A, or 250A. The frame determines:

• **Physical dimensions** — height, width, depth, and mounting hole pattern
• **Maximum amperage** — the highest trip rating available for that frame
• **Interrupting capacity** — how much fault current the breaker can safely clear
• **Panel compatibility** — which panelboards accept that frame size

How to Read Frame Size from the Label

Every circuit breaker has a label on the front face or side. Look for these indicators:

1. **"Frame" or "FR" followed by a number** — e.g., "Frame: 250A" or "FR: 100A" 2. **The catalog number prefix** — the first 2-3 characters encode the frame 3. **Maximum continuous current** — listed as "Max Continuous" or the largest number on the rating label 4. **Physical size class** — some manufacturers list "F-Frame", "J-Frame", "K-Frame" etc.

Frame Size by Catalog Number

Square D / Schneider Electric

| Prefix | Frame Size | Max Amps | Typical Voltage | |--------|-----------|----------|----------------| | QO | 100A (Residential) | 100A | 120/240V | | QOB | 100A (Bolt-on) | 100A | 120/240V | | FA/FH | 100A (I-Line) | 100A | 480/600V | | KA/KH | 250A | 250A | 480/600V | | LA/LH | 400A | 400A | 480/600V | | MA | 800A | 800A | 480/600V | | PA/PH | 1200A | 1200A | 480/600V |

Eaton / Cutler-Hammer

| Prefix | Frame Size | Max Amps | Typical Voltage | |--------|-----------|----------|----------------| | BR | 100A (Residential) | 100A | 120/240V | | BAB | 100A (Bolt-on) | 100A | 120/240V | | FD | 100A (Industrial) | 100A | 480/600V | | HFD | 150A | 150A | 480/600V | | JD | 250A | 250A | 480/600V | | LD | 400A | 400A | 480/600V | | ND | 600A | 600A | 480/600V | | RD | 800A | 800A | 480/600V |

Siemens / ITE

| Prefix | Frame Size | Max Amps | Typical Voltage | |--------|-----------|----------|----------------| | QP | 100A (Residential) | 100A | 120/240V | | BQ | 100A (Bolt-on) | 100A | 120/240V | | ED | 100A (Industrial) | 100A | 480/600V | | FD/FXD | 250A | 250A | 480/600V | | JD/JXD | 400A | 400A | 480/600V | | LD | 600A | 600A | 480/600V | | MD | 800A | 800A | 480/600V |

GE

| Prefix | Frame Size | Max Amps | Typical Voltage | |--------|-----------|----------|----------------| | THQL | 100A (Residential) | 100A | 120/240V | | THQB | 100A (Bolt-on) | 100A | 120/240V | | TED | 100A (Industrial) | 100A | 480/600V | | TFK | 250A | 250A | 480/600V | | TFJ | 400A | 400A | 480/600V | | TKM | 800A | 800A | 480/600V |

Identifying Frame Size by Physical Measurement

If the label is worn or missing, measure the breaker:

| Frame Size | Typical Width (3-pole) | Typical Height | Typical Depth | |-----------|----------------------|---------------|---------------| | 100A | 3.0" | 5.5" | 3.0" | | 250A | 4.5" | 7.5" | 3.5" | | 400A | 6.0" | 10.0" | 4.0" | | 600A | 7.5" | 12.0" | 5.0" | | 800A | 9.0" | 14.0" | 6.0" |

Note: Dimensions vary by manufacturer and vintage. These are approximate ranges for modern molded-case breakers.

Why Frame Size Matters for Replacement

When replacing a circuit breaker, the new breaker must have:

1. **Same frame size** — it must physically fit the panel space 2. **Same or higher interrupting rating** — never downgrade KAIC 3. **Same mounting style** — bolt-on, plug-in, or I-Line must match 4. **Same number of poles** — 1P, 2P, or 3P 5. **Appropriate trip rating** — can be same or lower than original

Common Mistakes

• **Confusing trip rating with frame size** — a 100A trip breaker in a 250A frame is NOT a 100A frame breaker
• **Assuming same amperage = same frame** — a 100A breaker could be in a 100A, 250A, or 400A frame
• **Ignoring voltage class** — a 240V frame breaker won't fit a 480V panel even if the frame size matches

Bottom Line

Shop These Breakers

Popular models available for immediate shipping:

• Square D HJA36020 (H-frame)
• Square D JDA36200 (J-frame)
• Eaton HFD3100 (F-frame)

Call **(877) 611-0034** for pricing on any circuit breaker in our inventory.

Check the catalog number prefix first — it's the fastest way to identify frame size. If the label is gone, measure the breaker and compare to manufacturer dimension charts. AllBreakerSales.com stocks every frame size from every major brand. Call (877) 611-0034 with your catalog number and we'll confirm the frame size and find your exact replacement.

Frequently Asked Questions

What is a circuit breaker frame size?

A circuit breaker frame size is the physical housing that determines the maximum amperage the breaker can handle, its physical dimensions, and which panel it fits into. Common residential frames are 100A. Common commercial/industrial frames are 100A, 250A, 400A, 600A, 800A, 1200A, 1600A, and 2000A. The actual trip rating can be any value up to the frame size — for example, a 250A frame breaker might have a 150A trip unit installed.

How do I find the frame size from a catalog number?

The frame size is usually encoded in the first 2-3 letters of the catalog number. For Square D: FA/FH = 100A frame, KA/KH = 250A frame, LA/LH = 400A frame, MA = 800A frame. For Eaton: FD = 100A frame, HFD = 150A frame, JD = 250A frame, LD = 400A frame. For Siemens: ED = 100A frame, FD/FXD = 250A frame, JD/JXD = 400A frame, LD = 600A frame.

Does frame size determine what panel the breaker fits?

Yes. Each panel is designed to accept specific frame sizes. A 100A frame breaker won't physically fit in a slot designed for a 250A frame. Additionally, the mounting style (bolt-on vs plug-in) and bus bar configuration must match. Always verify both frame size and mounting type when replacing a breaker.