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UL Molded Case Circuit Breaker Markings Explained — Part 2
Interrupting Ratings, Terminal Wiring, Adjustable Trips & Special Markings. This second installment covers interrupting ratings, terminal wiring limitations, adjustable trip controls, and special-purpose markings. Some of these items do not appear on IEC circuit breakers in the same way — they are specific to the UL system and reflect the realities of North American power distribution.
Visibility Position Codes
Code | Definition | In Practice |
B | Visible after removing trim or enclosure cover (no disassembly) | Trim = front face frame; Enclosure cover = panel door or cabinet lid |
D | Visible only after removing the frame cover or equivalent | Frame cover = breaker housing/body shell |
Fig.1 — UL MCCB with front trim removed showing position B markings
Section A — Interrupting Ratings
1) Interrupting Rating [Position: B]
UL Requirement: The circuit breaker shall be marked with its interrupting rating — the maximum fault current the breaker can safely interrupt at the rated voltage. In the UL system, this is the primary criterion for whether a breaker can be deployed in a given fault-current environment.
Example: RMS symmetrical interrupting capacity
240 V → 150 kA
480 V → 65 kA
Fig.2 — UL MCCB interrupting rating label showing 150 kA at 240 V and 65 kA at 480 V
Section B — Terminal Wiring Markings
Terminal wiring markings define the evaluated range of conductor types and sizes. Exceeding these limits — even if physically possible — is a code violation.
Fig.3 — UL MCCB terminal marked Cu/Al indicating compatibility with copper and aluminum conductors
# | Marking | UL Requirement | Position |
2 | Cu/Al Wire | Mark terminal conductor material (Cu, Al, or Cu/Al). | B |
3 | Small Wire Size | Mark small-conductor range if terminal accepts below standard minimum. | B |
4 | Tightening Torque | Mark required torque for each terminal screw. | B |
5 | Maximum Wire Size | Mark maximum allowable conductor size. | B |
6 | Multiple Conductor Connectors | If multiple conductors per terminal are allowed, mark explicitly. | B |
7 | 60/75°C Wire | Mark conductor temperature class (not ambient temperature). | B |
8 | Separately Shipped Connectors | Mark if connectors are supplied separately. | B |
9 | Cable Connection Only | Mark if approved for cable connection only (not busbar). | B |
10 | Bus Bar Sizes | Mark permissible busbar dimensions if evaluated for busbar connection. | B |
Fig.4 — UL MCCB terminal tightening torque markings: 35 lb-in, 70 lb-in, 150 lb-in
Fig.5 — Terminal marked 60/75°C showing conductor temperature class
Section C — Trip Unit and Control Markings
# | Marking | UL Requirement | Position |
11 | Instantaneous Trip Setting | Mark set value or range for adjustable instantaneous trip. | D |
12 | Type A / Type B | Mark classification if applicable. | D |
13 | Adjustable Controls | Mark all settings and their relationships for adjustable controls. | D |
Fig.6 — UL MCCB busbar connection size marking
Fig.7 — Adjustable trip unit with labeled setting dials for LT, ST, and instantaneous functions
Section D — Installation and Application Markings
# | Marking | Meaning | Position |
14 | Non-Conducting Enclosure Required | Breaker evaluated only in non-conducting enclosure. | B |
15 | Ventilated Enclosure Required | Breaker requires ventilated enclosure for safe operation. | B |
16 | 40°C Ambient Temperature | Rated ambient temperature condition (not conductor temperature). | B |
Section E — Special Capability Markings
# | Marking | Meaning | Position |
17 | Current Limiting | Breaker limits peak let-through current during a fault. | B |
18 | Class CTL | Circuit Total Limitation — limits total number of circuits in panelboard. | B |
19 | Delta System | Approved for specific ungrounded or corner-grounded delta system. | B |
Fig.8 — UL MCCB with Delta system marking
Sections F, G, H — Pole, Load Type & Restriction Markings
# | Marking | Meaning | Position |
20 | 2-Pole — 3-Phase Rated | 2-pole breaker evaluated for 3-phase systems. | B |
21 | 3-Pole — 1-Phase Rated | 3-pole breaker evaluated for single-phase systems. | B |
22 | 4-Pole — 3-Phase Rated | 4-pole breaker evaluated for 3-phase systems. | B |
23 | 2-Pole — 1-Phase Rated | Optional; standard combination, often unmarked. | B |
24 | Multi-Wire Circuit | Approved for multi-wire branch circuit (MWBC) use. | B |
25 | DC Rated 3-Pole | 3-pole breaker with a DC interrupting rating. | B |
26 | 100% Continuous Rated | May be loaded to 100% rated current continuously. | B |
27 | SWD | Rated for repetitive switching of lighting loads. | B |
28 | Independent Trip | Each pole trips independently. | B |
29 | Special Characteristics | Unique characteristics; refer to product documentation. | B |
30 | For Replacement Not CTL | Replacement use in old panels lacking CTL structure. | B |
31 | Special Purpose Not General | Restricted to specific applications only. | B |
32 | HID | Suitable for High Intensity Discharge lamp loads. | B |
33 | HACR Type | Suitable for Heating, Air Conditioning & Refrigeration circuits. | B |
Fig.9 — Multi-wire circuit and pole configuration markings
Fig.10 — Special purpose and replacement use markings
Summary
UL MCCB markings communicate far more than basic ampere and voltage ratings. The 33 required marking items reviewed across both parts of this series define: the fault level the breaker can safely handle; precise wiring constraints; how adjustable controls are set and validated; installation prerequisites; and special capabilities and restrictions.
Understanding these markings is essential for engineers and panel builders working in North American markets. They are not decorative — each one is tied to specific test conditions and application boundaries that determine safe and compliant use.
Reference: UL. Molded Case Circuit Breakers Marking and Application Guide. 2016.
New industry Technology regarding to Bussmann fuse, ABB breakers, Amphenol connectors, HPS transformers, etc.
TAG: UL MCCB Marking Molded Case Circuit Breaker Interrupting Rating Tightening Torque