The short answer
Circuit Breakers in Control Panels
A circuit breaker in a control panel is chosen by what it is listed to protect, not only by its ampere rating. Molded-case and miniature circuit breakers listed to UL 489 provide branch circuit protection; supplementary protectors listed to UL 1077 do not and may only be used downstream of branch protection. The trip curve sets how the breaker responds to overload and to fault current, and the interrupting rating, together with any series rating, sets the short-circuit current rating of the panel.
Key points
- UL 489 breakers protect branch circuits. UL 1077 supplementary protectors do not, whatever they look like.
- The ampere rating handles overload. The interrupting rating handles a fault. Both must be right.
- Trip curves B, C, and D on miniature breakers differ only in the instantaneous trip point.
- The panel SCCR is set by the weakest device, and breakers are usually where it is won or lost.
- A breaker feeding a drive or a transformer needs a curve that rides through inrush.
Three devices that look alike
| Device | Listing | What it may protect | Typical use |
|---|---|---|---|
| Molded-case circuit breaker (MCCB) | UL 489 | Branch circuits, feeders, and service equipment; may be the panel main | Panel main disconnect, motor branch circuits above the miniature range, feeders to sub-panels |
| Miniature circuit breaker (MCB), UL 489 type | UL 489 | Branch circuits, within its rating | Control transformer primaries, small motor branches, receptacles and lighting in the panel |
| Supplementary protector | UL 1077 | Only equipment already protected by an upstream branch circuit device; provides supplementary protection within a piece of equipment | 24 V DC distribution, individual instrument and PLC circuits, small control loads, all downstream of a UL 489 device or a fuse |
The supplementary protector is the one that causes trouble. It is a DIN-rail device that looks exactly like a miniature breaker, is often cheaper, and is sometimes sold on the same catalog page. It is not listed to protect a branch circuit, and UL 508A does not permit it where branch circuit protection is required. Used where it belongs, downstream of proper protection to subdivide a control circuit, it is a good device. Used as the only protection on a transformer primary or a motor circuit, it is a listing violation and a hazard.
Ratings
- Ampere rating
- The continuous current the breaker carries without tripping. Chosen from the conductor ampacity and the load, with the NEC rules for motor circuits allowing a higher rating than the conductor ampacity so the motor branch rides through starting.
- Voltage rating
- The system voltage the breaker is listed for, including whether it is rated for the grounded or ungrounded system and for DC where used on DC.
- Interrupting rating
- The maximum fault current the breaker can interrupt safely at its rated voltage. Common miniature breakers are 10 kA; molded-case breakers range from 10 kA to 200 kA. A breaker applied above its interrupting rating can fail to open, or fail violently.
- Frame and trip unit
- On molded-case breakers, the frame sets the physical size and maximum rating; the trip unit sets the actual rating and, on electronic trip units, the adjustable settings.
- Poles
- One, two, or three, with common trip matching the circuit. A three-phase motor circuit requires common trip on all three poles.
Trip curves
A breaker has two trip mechanisms. The thermal element responds to sustained overload with an inverse time characteristic: a small overload takes minutes, a large one takes seconds. The magnetic or instantaneous element responds to fault current with no intentional delay. The trip curve plots both against current, and for miniature breakers the curve letter names the instantaneous trip range.
| Curve | Instantaneous trip | Use |
|---|---|---|
| B | 3 to 5 times rated current | Resistive loads, long circuits where fault current is low, lighting and receptacles |
| C | 5 to 10 times rated current | General purpose; control transformers with modest inrush, small motors, most control circuits |
| D | 10 to 20 times rated current | High inrush: transformers, drives with large DC bus capacitors, motors with high starting current |
A C-curve breaker on a control transformer primary that trips at every power-up is not defective; the transformer inrush is above its instantaneous trip point, and a D-curve breaker or a time-delay fuse is the answer. The reverse error, a D-curve breaker on a long circuit with low available fault current, may never see enough current to trip instantaneously and clears a fault on the slow thermal element instead.
Breakers and the panel SCCR
The short-circuit current rating of an industrial control panel under UL 508A Supplement SB is limited by the lowest-rated device in the power circuit, and the interrupting rating of a breaker is one of those numbers. A panel built with 10 kA miniature breakers has a 10 kA SCCR at best, whatever else is in it. Raising it means breakers with a higher interrupting rating, or a tested series combination in which an upstream breaker or fuse lets a downstream device be applied above its own rating, or current-limiting fuses ahead of the low-rated devices. The combinations must be published by the manufacturer; a series rating is never assumed. The SCCR page walks through the method.
Selection notes
- Drives: the drive manufacturer publishes the required upstream protection and the SCCR that results. Use the listed combination. Many drives require specific breakers or semiconductor fuses to achieve their published rating.
- Control transformers: primary protection per NEC 450.3 and the transformer inrush; a D-curve breaker or a time-delay fuse. Secondary protection sized for the secondary conductor.
- DC distribution: a 24 V DC circuit is protected by a device rated for DC. Many miniature breakers are rated for DC at reduced voltage; supplementary protectors are the common choice downstream of the power supply, and electronic circuit protectors that trip faster than a switch-mode supply current-limits are the better one.
- Coordination: a fault on one branch should trip that branch, not the main. Selective coordination is a curve overlay exercise, and on small panels it is usually achieved by a large enough ratio between the main and the branches.
- Auxiliary contacts: a breaker feeding a critical circuit should report its state to the controller. Alarming a tripped breaker is cheaper than finding it at a site visit.
- Lockout: the main breaker in a panel is the disconnect that is locked out for service. It needs a lockable handle, and the panel needs to be arranged so it is the only source, or every source is identified.
Frequently asked questions
- Can I use a supplementary protector on a transformer primary?
- Not as the branch circuit protection. A UL 1077 device can be used to subdivide a circuit that already has branch protection upstream, so a supplementary protector on a transformer primary is acceptable only if a UL 489 breaker or a fuse ahead of it provides the protection the code requires, and the supplementary device is then redundant.
- Why does the breaker on my drive trip at power-up?
- The drive charges its DC bus capacitors at power-up and the inrush exceeds the breaker instantaneous trip. Most drives have a precharge circuit that limits it; if the breaker still trips, it is the wrong curve, the wrong rating, or not the device the drive manufacturer specified.
- Fuse or breaker?
- Fuses interrupt higher fault currents, limit current more effectively, and are less expensive for a high SCCR. Breakers are resettable, report their state, and are easier to operate. Many panels use both: current-limiting fuses at the main for the SCCR, breakers on the branches for operation.
- How do I find the available fault current at my panel?
- From the utility at the service, then calculated through the transformers and conductors to the panel. The utility provides the value at the service; a short-circuit calculation, done by the engineer or with the transformer impedance and conductor data, gives the value at the panel. The NEC requires the value to be marked on service equipment and the SCCR on industrial control panels.
Related topics
- Short-Circuit Current Rating (SCCR)What a control panel’s short-circuit current rating means, how it is determined under UL 508A, why the NEC requires it on the label, and why the number is usually smaller than people expect.
- UL 508A Industrial Control PanelsWhat the listing actually covers, what has to appear on the label, and the requirements that most often cause a panel to fail inspection.
- Fuses in Control PanelsFuse classes, time-delay and fast-acting types, current limitation and what it does for the panel short-circuit rating, semiconductor fuses for drives, control circuit and DC fusing, and the habits that make a blown fuse a diagnosis instead of a mystery.
- Variable Frequency Drives in Pump PanelsWhat a drive adds to a pump panel and what it costs: speed control, soft start, reduced inrush, and diagnostics against heat, harmonics, cable and motor stress, bypass, and the minimum speed below which a pump does no work.
- How to Size a Control Panel Power SupplyAdd up the real load, account for inrush and simultaneity, add headroom, and check the heat you just added to the enclosure.
- Component LayoutArranging a panel so it is safe to work in, cool enough to run, quiet enough for its signals, and possible to modify: power and control separation, heat, wireways, terminals, and the working space rules.
Direct contact
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