Low voltage switchboard Let’s start with some basic definitions from the United States National Electrical Code (NEC). An LV switchboard is generally defined as a large single panel, frame, or assembly of panels on which are mounted on the face, back, or both, switches, overcurrent and other protective devices, buses, and usually instruments.

Lessons I learned during installation and wiring of LV switchboard

These assemblies are generally accessible from the rear as well as from the front and are not intended to be installed in cabinets. Modern switchboards are totally enclosed to minimize the probability of spreading fire to adjacent combustible materials and to guard live parts.

Busbars are arranged to avoid inductive overheating of the enclosure or any nearby metal. Service busbars are isolated by barriers from the remainder of the switchboard to avoid inadvertent contact by personnel or tools during maintenance.

Here are some of the lessons I learned designing LV switchboards:

1. Switchboards and panelboards are different

Switchboards are typically used for service entry, and any distribution required there onwards. On the other side, panelboards are used primarily for branch circuits, and end of line loads. It follows therefore that switchboards feed panelboards.

One key distinction to remember is that panelboards typically have a maximum ampacity of 800A to 1200A and are governed by Underwriters Laboratories’ (UL) 67 – Standard for Panelboards.

The designs of switchboards are governed by UL 891 – Standard for Switchboards.

Left: Draw-out Molded Case Switchboard (photo credit: Eaton); Right: Low voltage panelboard (credit: Schneider Electric)

2. Access

This is always a big issue. Most codes and regulations will require two ways to enter and exit the switchboard area, likely at each end of the switchboard lineup.

Always plan for a dedicated room for the switchboard.

A room ensures safety due to the fact that access can be limited to only qualified personnel. If the switchboard is located within a room, which is the ideal case, the entry/exit doors should swing out in the direction of the exit path.

Over a certain amperage, many codes and regulations have this specific exit requirement but it is an engineering best practice to do it anyway. If you think about it, in an emergency, the easiest way to get out of an area would be to just have to push your way out.

To further ease your way out, another key item with doors is that they should have ‘panic hardware’ on them.

Doors equipped with panic hardware

3. Transport

Size your entry/exit doors to create a large enough opening that the switchgear can actually be brought into its dedicated room (or area) for installation. This seems obvious, but it is easily missed.

Pick the largest piece that needs to be transported! The manufacturer should be able to give you this information. Then work out an approximate path of how you believe that piece can get into the building and then into the room.

This is worked out by the installer; however, going through it as a designer helps prevent consternation later.

Transport of low voltage switchgear on site

Transport truck loaded with low voltage switchgear

4. Housekeeping

Equipment pads are necessary in order to ensure that the equipment is above the floor and safe during any accidental floods or just general cleaning/housekeeping.

A minimum height of 4” is a good idea and ensuring that the pad contours around the entire footprint of the switchboard. This will of course increase the overall height of the installation and therefore, the pad should be taken into consideration throughout the course of the design.

Low voltage switchgear pad

5. Insulating mats

In addition to housekeeping pads, another thing to keep in mind is maintenance. The safety of the individuals working on the switchboard can be ensured by installing high quality electrical insulating mats in front and/or back of the switchboard.

The mats are basically a way to ensure a zero hazard environment by preventing unprecedented shock to personnel.

The goal is to have the mats cover all places where live parts may be accessible during maintenance or repair by personnel.

Electrical insulating mats in front and/or back of the LV switchboards

6. Location

One of the key things that plays into this is equipment access requirements. Is your switchboard going to require front or rear access, or both? Rear access switchboards tend to be easier to install and maintain but they are also deeper.

The easiest way to mitigate this problem is to have front-access only switchboards. This may not always be possible but it is definitely a step in the right direction to simplify your design. It will make locating the switchboard design much easier.

Switchboard workspace (photo credit: mikeholt.com)

7. Architecture

This lesson relates to the location, but it is separated out due to the impact that an unrelated external factor can have on the design – Expansion Joints. It should go without saying that a switchboard should not be installed over an expansion joint.

However, unbelievably, I came across such an installation. As I remember seeing it, part of the switchboard was on one side of the expansion joint while part of it was on the other side. There were cables installed with enough slack going over the expansion joint connecting the two sides.

This meant the whole setup was unnecessarily long and used up quite a large footprint. The installer had to be careful on where and how the cable supports were installed because one wrong location, and it could all come down.

Basically, you should negotiate a location for your electrical equipment that does not result in such a complicated switchboard design and installation.

Switchboard expansion joints

8. Layouts and clearances

Clearances required vary based on overall layouts chosen. The idea always is to ensure the best possible use of the space available. Sometimes this will result in switchboard sections facing each other. While this may increase the clearance required resulting in a large isle between them, it often is the most economical use of the available space.

Other popular layouts are C or U-shaped, and L-shaped switchgear.

Switchboard layout ‘L’ Typical clearances start at 3 feet in front and up to 6/8 feet, if two switchboards face each other.

Electric switchboard clearances

9. Breakers, fuses or both

Prior to selection of whether you will have breakers or fused switches, ensure that a fault current study is done to know the expected fault current. For main service switchboards, the available fault at your service entry is provided by the utility company.

This will dictate what you can specify because if the fault currents are too high, fused switches may be your only option.

Sometimes if you have large meter or transformer loads downstream, a combination of breakers and switches can also be proposed in your switchboard to mitigate transformer inrush, motor inrush, etc.

[Position of circuit breakers and fuses in LV switchboard

10. Metering

Switchboards are versatile when it comes to metering and can combine circuit breakers, surge protection, metering CTs and the meters themselves in a single engineered assembly.

There are times when utility metering is done separately with bulb-type meters that are accessible to utility personnel, while electronic meters are also specified for the owner.

Metering panel

11. Preplanning

Because switchboard installations tend to be about getting it in place and forgetting about it, preplanning is essential. If you are going to have drawout breakers, it is important to include racking capabilities as part of the initial installation, instead of retrofitting it in later.

Switchboards can be expanded beyond their basic circuit distribution and protection functions to include various other equipment based on the requirements of the design. Low voltage distribution transformers, panelboards, even automatic transfer switches can be made part of the switchboard layout. This helps with preplanning space in the layout by including some spare room for future.

When mobile generators are being considered as a contingency backup, switchboards allow for quick connect setups for fast and easy mobile generator use.

Always assume that a pull section/structure will be required to pull cables into the switchboard prior to connection with the main breaker or disconnect switch.

12. Coordination

While this may be an obvious item, but a lack of coordination is a perennial problem. Mechanical, plumbing, fire protection, etc. all get in the way of or in dedicated electrical spaces with complete disregard. Therefore, ensure there is ample discussion with the other trades/disciplines.

The NEC has specific requirements on dedicated spaces about electrical equipment and what is explicitly allowed in those spaces. Fire protection sprinklers are allowed however it makes sense to be wary of including water in a place/area that has electricity.

Most country codes and regulations have ways around this problem by instead allowing electrical rooms to be two-hour rated construction. This is the case in the United States to preclude the sprinkler requirement.

Another way to prevent water and power mixing is talking with the fire protection engineer to consider a pre-action sprinkler system for your electrical switchboard room. This will allow for a time lag between detection and the eventual water spray.

If the location of a plumbing or mechanical pipe is in vicinity of your switchboard, consider drip pans or any other mitigation tactic.

Working space and dedicated electrical space according to NFPA 70 (NEC)