How to choose an enclosed disconnect switch for a control panel

Every control panel needs a way to cut power safely before someone opens it up or works on the machine it feeds. An enclosed disconnect switch does that job as a standalone unit: the switch mechanism comes already housed in its own enclosure, ready to mount on the panel, the machine frame or the wall next to a motor.

If you're building panels this quarter, the Altech KEM325UL is a UL-listed, 3-pole enclosed disconnect we keep in stock. But the right switch depends on your circuit, so here's how to work through the choice.

What a control panel disconnect switch does

A disconnect switch opens all the ungrounded conductors feeding a panel or load, so the equipment downstream is isolated from the supply. It's the point where maintenance staff turn power off, lock it off and tag it before they work.

That's why the disconnect shows up in the main standards panel builders work to:

• NFPA 79 (electrical standard for industrial machinery) calls for a supply circuit disconnecting means on each machine, with a handle that can be locked in the OFF position.
• NEC Article 430 covers disconnecting means for motors and controllers, including the requirement that the disconnect be within sight of the motor and controller in most cases.
• OSHA 29 CFR 1910.147 (lockout/tagout) requires that energy-isolating devices can be locked out during service.

Which requirements apply to your build depends on the machine, the installation and the local authority having jurisdiction, so treat this guide as a starting point and confirm against the current code editions.

Enclosed vs through-door disconnects

There are two common ways to put a disconnect on a control panel, and they solve slightly different problems.

Enclosed disconnects are popular with OEMs and maintenance teams because there's no door drilling or shaft alignment, and they can be placed exactly where the "within sight" requirement needs them.

Six things to check when choosing one

1. Number of poles. A 3-phase supply needs a 3-pole switch. Some installations also call for switching the neutral (a 4th pole), and single-phase loads may only need 2 poles. Check your supply and any local requirements before ordering.
2. Voltage rating. The switch has to be rated for the system voltage, for example 480 VAC on a typical North American industrial supply. Ratings often differ by voltage, so read the datasheet line for your voltage, not just the headline figure.
3. Current and horsepower rating. Size the switch for the load. For motor circuits, look at the horsepower rating at your voltage as well as the amp rating. Under NEC 430.110, a motor disconnect generally needs an ampere rating of at least 115% of the motor's full-load current.
4. UL listing and how it's marked. Not every UL-listed switch is approved for every use. A switch listed under UL 98 (enclosed and dead-front switches) and one listed under UL 508 as a manual motor controller marked "Suitable as Motor Disconnect" can sit in different places in the circuit. The NEC limits the second type to the load side of the branch-circuit protective device. Check the listing and markings against where the switch will go.
5. Enclosure rating. Match the housing to the environment: dry indoor, washdown, outdoor, oily or dusty. See the NEMA table below. Polycarbonate enclosures resist corrosion and are light; metal enclosures handle physical impact better.
6. Lockout and accessories. Confirm the handle can be padlocked in the OFF position. If your PLC needs to know the switch position, look for auxiliary contact options. Also check the short-circuit current rating (SCCR), since it feeds into the overall panel SCCR.

NEMA enclosure ratings at a glance

These are the ratings you'll see most often on enclosed disconnects in industrial plants.

IP ratings (like IP65) describe dust and water protection only. NEMA ratings also cover things like corrosion and ice, so a NEMA rating can be converted to an IP equivalent but not the other way around.

Pre-order checklist

Have these on hand before you request a quote and you'll get the right part the first time:

• Supply voltage and number of phases
• Load full-load current, and horsepower for motor loads
• Where the switch sits in the circuit (main disconnect or local motor disconnect)
• Mounting location and environment (indoor, washdown, outdoor, corrosive)
• Required panel SCCR
• Lockout and auxiliary contact needs
• The rest of the panel order: terminal blocks, jumpers, fuse blocks, DIN rail

Building a full panel? Our guide to control panel anatomy walks through the other components, and we stock Altech terminal blocks, jumpers, fuse blocks and TS35 DIN rail in bulk so you can order everything in one go.

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