How to Size a Disconnect Switch for Your Project

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A disconnect switch may sound complicated, but it isn’t. It’s a simple on/off switch that cuts the power to your equipment or electrical enclosure. It’s also called the main switch in many applications.

Yet, though its use is straightforward, its installation can be tricky to beginners. The most important consideration during this process is determining the correct size your switch should be. Note that in this case, the size denotes the current rating of the switch rather than its physical measurements.

Today, we’re sharing a step-by-step guide to sizing a disconnect switch for your next project. Follow this tutorial and you’ll power up your projects in no time.

Ready to learn more? Let’s go!

Understanding IEC and NEC Requirements

Why is it so important to size your disconnect switch in the correct way? Industry guidelines require it.

IEC Standards

According to International Electrotechnical Commission (IEC) 60204-1:2016: Safety of Machinery – Electrical Equipment of Machines – Part 1: General Requirements, you have to provide a manually-operated disconnect switch for every main power supply you install.

This switch also has to follow certain design parameters.

For instance, it must include an indication of its on/off function, often provided through an O/I label, as this is the international symbol for on/off. In addition, the disconnect switch must include an accessible handle for easy, manual operation.

When you flip this switch to the “off” position, IEC standards mandate that it should disconnect all connected conductors. It should also integrate with an associated padlock that empowers operators to lock the power off.

NEC Standards

In addition to IEC standards, the National Electric Code (NEC) also dictates the presence of a disconnect switch.

This code states that any large and permanently-wired equipment must have a disconnecting means within sight distance. This applies to your HVAC unit, your attic exhaust fans, and other major household equipment.

In addition, the NEC states that this disconnecting means must be a device included on their list. It also should be capable of disconnecting the same horsepower as the equipment that it’s installed to support. For example, a 5-horsepower motor will require a switch rated at 5-horsepower or greater.

Keep in mind that most disconnect switches are fused, though some are non-fused. The NEC states that the one you select must have the right fuse size to fit the equipment.

With these requirements in mind, how do you approach the disconnect switch sizing process? Let’s take a look.

1. Determine Your Power Needs

Unsure which type of disconnect switch is best suited for your application? Consider your power requirements before you move onto any of the next steps.

If you’re using Alternating Current (AC) power, you’ll have a different kind of switch than someone using Direct Current (DC) power. While it may be tempting to interchange them, don’t do it. A switch designed for AC power will not accept DC power, and the opposite is also true.

The IEC code includes a breakdown of the different types of utilization categories and typical applications for both AC and DC power. Let’s review a few of the most common types.

AC Power: Frequent Operation Utilization Categories

The utilization categories designated for frequent AC power operation include the following:

  • AC-20A
  • AC-21A
  • AC-22A
  • AC-23A

Note that the application of AC-20A isn’t permissible in the U.S.

AC Power: Infrequent Operation Utilization Categories

The utilization categories designated for infrequent AC power operation include the following:

  • AC-20B
  • AC-21B
  • AC-22B
  • AC-23B

DC Power: Frequent Operation Utilization Categories

The utilization categories designated for frequent DC power operation include the following:

  • DC-20A
  • DC-21A
  • DC-22A
  • DC-23A

DC Power: Infrequent Operation Utilization Categories

The utilization categories designated for infrequent DC power operation include the following:

  • DC-20B
  • DC-21B
  • DC-22B
  • DC-23B

Typical Applications for AC-20A through AC-23B

What are the most common applications for each utilization category under AC-20A through AC-23B? Here’s a quick breakdown:

  • AC-20A and AC-20B: Connect and disconnect power under load conditions
  • AC-21A and AC-21B: Switch resistive loads and moderate overloads
  • AC22-A and AC22-B: Switch mixed resistive loads, inductive loads and moderate overloads
  • AC23-A and AC23-B: Switch motor loads and other highly inductive loads

Typical Applications for DC-20A through DC-23B

Next, let’s take a look at the most common applications for DC-20A through DC-23B. These include:

  • DC-20A and DC-20B: Connect and disconnect power under no-load conditions
  • DC-21A and DC-21B: Switch resistive loads and moderate overloads
  • DC-22A and DC-22-B: Switch mixed resistive loads, inductive loads and moderate overloads such as shunt motors
  • DC23-A and DC-23B: Switch highly inductive loads such as series motors

2. Calculate Voltage and Current Requirements

The equipment in question will have a nameplate that lists its manufacturer and other key details. Here, you’ll also find the equipment’s horsepower rating.

In some cases, you won’t find this metric listed as horsepower. If this is the case, look for numbers that measure volt-amps, watts or even kilowatts.

While it will take a little more time to convert these numbers into horsepower, it is possible. If you’re working with watts or volt-amps, divide those numbers by 0.7457. The resulting number is your horsepower.

Note that most disconnects max out at 600 volts AC power (VAC). Likewise, DC disconnects will rate for up to 1000 volts DC power (Vdc). These measurements are consistent with IEC standards.

Going higher than these voltages poses a significant risk of an arc flash, which can lead to serious operator injury or even death.

If your equipment requires a more powerful switch, you’ll need to research a specialty one designed to support the extra capacity.

Measuring the Current

When you install a disconnect switch, you must size its contacts to ensure they can accommodate the energy current traveling through them.

If your current is higher, you’ll need stronger contacts to support them. If you skip this step, your system could overheat in an instant. Most suppliers will provide AC disconnects ranging from 10A to 630A and DC disconnects rated for 32A at 1000Vdc or 45A for up to 600Vdc.

3. Consider the Application Type

Are you installing a disconnect for a single-motor application or a combined load application? The answer will help dictate the kind of switch you need.

The NEC outlines different requirements for each application type. Let’s take a high-level look.

Single-Motor Applications

When you install a disconnect switch for a single-motor application, it must meet the following two criteria:

  • Have an ampere rating at least 115% of the rated motor full load current
  • Have a horsepower rating the same or greater as the rated motor horsepower (at applied voltage)

The latter requirement is only applicable for installers who are working with a disconnect switch that is horsepower-rated.

Combined Load Applications

Although you’re working with multiple loads in this scenario, you’ll look at each one on its own (rather than the collective unit) to determine the size of the disconnect switch you need.

This means you’ll select the right one by adding together all of the simultaneous individual loads that are occurring within the circuit.

Begin by measuring one equivalent full-load current and one equivalent locked-rotor current. How do you obtain these metrics? You can look for the data in a few places, including:

  • Information on the equipment nameplate
  • Load data
  • Comparison tables via Section 430 of the NEC (Motors, Motor Circuits, and Controllers)

Once you’ve found the equivalent locked-rotor current, you can convert it to its equivalent horsepower rating using Table 430-151 of the NEC code. From there, look for a disconnect switch that meets the following requirements:

  • At least 115% of the equivalent current (full-load)
  • At least meets the equivalent HP rating

4. Choose Your Mounting Style

With most of the technical specifications out of the way, it’s time to consider aesthetics. Where should you install your disconnect switch and which mounting type is the most appropriate for your application?

Four-Hole Mounts

The most common mounting style is a four-hole mount, designed with a square-shaped mounting pattern that includes a screw hole in each of the four corners.

If you’re installing your switch onto a flat surface, such as the door of an electrical enclosure, this style is often your best bet, as it allows the switch to lie as flush as possible.

In this style, the base of the switch will go on one side of the door panel, while the front cover plate will go on the other side, sandwiching the panel in the middle.

You’ll hold the two pieces of the switch in place with four screws, placing one in each hole.

Other Mount Styles

If you’re working with a smaller switch, there is also a two-hole mount style that may be more appropriate. This model mounts in the same fashion as a four-hole design.

In addition, there are also central-mounted switches with a 22.5mm hole in the middle that meets IEC standards or a 30.5mm hole that meets standards set forth by the National Electrical Manufacturers Association (NEMA). If you go this route, you won’t affix the panels with screws. Rather, you’ll use a threaded plastic nut to secure the switch in place.

Larger switches can benefit from another mounting style: base mounts.

With this design, installers mount the switch body from the back, attaching it to a DIN rail or the back side of an electrical enclosure. In this case, the switch stands in the way of the power being opened when it’s live, which adds an extra layer of safety protection.

A shaft will connect the switch to a faceplate and the handle mounts on the enclosure door with four screw holes. When power is live and the door closes, the shaft fits in a hole located on the back of the switch handle, locking in place.

It will remain this way as long as the disconnect switch is in the “on” position. This way, the switch has to change to the “off” position (meaning the power isn’t engaged) before the lock will release and the door can open.

5. Browse Disconnect Switch Handles

Another aesthetic consideration is the handle associated with your disconnect switch. While your user preference will guide the style you select, some offer important technical functions in addition to mere looks.

For instance, some lever handles aren’t lockable while others are. If you need your switch to help control user access to electrical enclosures, this will be an important feature to look out for.

In addition, you can also select a lockable dial. You’ll also need to take into account the number of padlocks you can place on the handle to lock its position into place.

Most handles are black with a silver faceplate, though you may be able to specify a different color scheme instead. A red handle with a yellow faceplate is another common choice.

Find Switching and Control Solutions Today

In the market for a disconnect switch? It’s not quite as simple as picking the first one off the shelf. It’s worth taking the time to size yours the right way, ensuring that you stay safe and your project stays functional.

From voltage and current requirements to mounting styles, there’s plenty to consider. Take your time and do your research to make sure you’re making the right selection.

When you know what you need and you’re ready to make a selection, we’d love to help.

We offer a wide range of industrial electrical control products designed to fit every project need. From circuit breakers and relays to enclosures and terminal blocks, if you need it, you’ll find it here.

To get started, browse our extensive inventory, broken down by category.

Got questions? Feel free to contact us. We’ll walk you through this important step and give power to your best ideas.

The content provided is intended solely for general information purposes and is provided with the understanding that the authors and publishers are not herein engaged in rendering engineering or other professional advice or services. The practice of engineering is driven by site-specific circumstances unique to each project. Consequently, any use of this information should be done only in consultation with a qualified and licensed professional who can take into account all relevant factors and desired outcomes. The information was posted with reasonable care and attention. However, it is possible that some information is incomplete, incorrect, or inapplicable to particular circumstances or conditions. We do not accept liability for direct or indirect losses resulting from using, relying or acting upon information in this blog post.