Understanding Electrical Prints for Devices and Circuits

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What We Cover in this Article:

  • Reading the Legend to Determine Symbols
  • How to Know What Circuits Go Where
  • What Type of Light/Device Do We Install in a Certain Location

As an electrician, we typically install lights, plugs, heaters, motors, exit signs, fire alarm, and if it’s in the contract, we will even run the pipe for security companies such as their cameras and doors.

With that said, our first step as electricians is looking at our electrical prints. It’s important to take the time out to understand the prints, otherwise, you risk the chance of doing the job more than once. Or worse, making a costly mistake.

When scanning our prints there are many things to take into consideration like what type of devices are in a certain area, their mounting heights, but also if other trades have any of their equipment in the way, too!

(This is even more important in a commercial atmosphere when running conduit because we have to determine how big to create our pipe racks and how big to size our pipes and boxes when pulling our wires!  For example, we may want to make our pipe racks double-sided if it’s a really busy area to save width space, or make the junction box extra big because it’s a busy area in case we need to add something later on.)

So in this article, I will cover how to read prints to know where our devices go, where those devices are powered from, and what type of box/enclosure to install in the rough-in stage to be prepared for finish time.


I will be basing this article off a multi-use building. The upstairs are condos for people to dwell in and the main level of the building will be a commercial space (coffee shop).

When being an apprentice, typically, you are just told what to do. You’re given your task without knowing the bigger picture.

So let me break down that bigger picture, okay?

Every job is different. This is because every general contractor (the one running the job site) has different ways of approaching construction.

A job’s timeline is typically determined by the general contractor which tells the site which tasks and areas to focus on. For example, they may want to focus on the condos first before the commercial space below in our case.

I say that because this determines where you’ll be looking on your prints.

The way you find this information is through weekly meetings your electrical foreman has with the general contractor and other trades on site.

After your electrical foreman finds out this info, he’ll know what tasks to assign to his crew. (And depending on how many guys you have on your site, and how close you are to your foreman, he may fill you in on the details of the meeting for you to learn, too!)

Now, I can tell you about a legend.

A legend is a chart/table that tells us what the symbols are.

Legends are typically always on the first page of your prints.

In a commercial setting, our electrical legend contains symbols for:

  • Power devices (plugs, switches, lights, motors, emergency lighting etc.)
  • Fire alarm (horns/buzzers/bells, pull stations/smoke detectors etc.)
  • Security (cameras, doors: electric strikes, request to exits, key cards)
  • Communications (internet, fiber, telephone)

You’d keep going back and referencing this legend for any devices you’re unsure about.

Here’s a typical legend I created to give you an idea:

Print Legend of Electrical Devices
Example of a Print Legend of Electrical Devices

Now, when we are looking at electrical prints, it’s important to mention about electrical engineers.

For the most part, electrical symbols are pretty standard from my experience, but sometimes different electrical engineers have their own symbols.  So don’t worry about memorizing these symbols because every job changes.

What’s most important is knowing how to read your prints.  Every set of prints will have a legend telling you what the symbols are.

Now that you understand about a legend, we can proceed with figuring what needs to be installed and where it gets powered from.


This is a main difference from residential to commercial electrical work.

In residential, we typically take a walk through and figure out how many devices are on a circuit, because by code, we’re only allowed a certain amount of devices per circuit breaker.

But in a commercial setting, the electrical engineer lays out all our circuits for us. From here, we devise a game plan on how to bring power to those destinations.

The planning is a crucial step because you have to take into account of other trades work, ceiling heights/types, and trying to find a path which will be easy to branch off throughout the job.

In most instances, being a commercial electrician is just a matter of following the prints. But sometimes it doesn’t work out that way because it’s easy to write on a piece of paper and say, “This circuit goes here” when in reality it would take way more work and material to achieve what the prints say.

So sometimes your electrical foreman will put in what is known as an RFI – Request for Information to the electrical engineer.

From here, the engineer may allow you to alter the prints, but it’s very important to get everything in writing so you have something to protect your decision if the change becomes a problem at a later date.

So with that said, typically residential circuits are made up on the job.

With commercial, we follow the prints as they are given to us.

Now I can show you what the circuit numbers mean and the type of device to be installed.

Electrical circuits on electrical prints
You can see the various electrical devices like from my example legend above, as well as the circuit numbers from different panels. Every job site is different, but in this picture example, 2H10 would be lighting circuits (2H is the panel, and 10 is the circuit number which is BLACK in color).

Here’s a real-world prints example of condos.

Typically how prints for condos work is there is the floor plan which is the ‘common area’. These are shared areas that all people in the building use like hallways, stairways, lobbies, elevators etc.

To see an individual condo’s layout, often times there are specific pages showing the general layout of where to install plugs, switches and lights.

Condos are kinda both residential and commercial in a sense that the actual condo themselves are residential, but once you enter the hallways, stairwells, and parkades, you get a sense of commercial.

There’s a lot going on in this picture, so let me break it down for you!

Even though condos are typically pretty cookie cutter (everything’s the same), it’s important to not get lazy and actually turn the page to the floor you’re actually on. Even though the hallway layout may be the same for every floor, the circuit numbers will probably be different.

With that said, you can see that this picture says we’re on the third floor. (Bottom left says ‘Third Floor Plan’)

Next, we’ll talk about how the individual suites get their main feeder cable.

Each suite in the picture has a little table. (A, 302, MC-3, 2P-100)

(In my experience, each condo lays this information out a little differently.)

The letter designation: A, A2, B, C3 etc. is simply the condo’s layout. Many times a condo has about 4-6 layouts that they repeat throughout the condo’s floors.

Next, the 301, 302, 303 etc. is the actual condo suite number. This is important for when you pull your feeder to the meter stack so each suite gets metered properly.

You don’t want to be paying for someone else’s power usage! (After pulling, it’s important to test, too!)

Now you’ll see MC-3… This just means ‘Meter Center 3’. You’ll see on the far right side of the prints it shows Meter Center 3 is in the closet.

So it’s just a matter of pulling a properly sized feeder cable from the MC-3 meter closet to each suite and label the wire for when it’s ready to cut in!

Pretty simple so far, right?!

Where we see 2P-100 – this means 2 pull 100 amp breaker.

In other words, we need two phases to achieve 208 volts in these condo suites because they typically have a range (stove/oven) and dryer. (In Canada, 208V is in condos because the building is 3 phase, but in single residential dwellings it’s actually 240V because of single phase.)

(On a side note, when buying a stove for a condo, which is 208V, it is important to buy a stove rated at 208V, too! Otherwise, you’d be underpowering your stove if you bought a 240V oven! And the other way around, if you bought a 208V stove and installed it in your home which is 240V, you’d overpower the oven and it’d probably burn out quicker than expected.)

Now, we’ll move onto the layout of the hallway devices which is the whole point of our legend introduction above!

And please note, my legend I created above does not relate to this prints picture.

This hallway contains lots of devices: lights, emergency lights, plugs, exit signs, and smoke detectors. (Many times security will have its own pages which you have to reference as well.)

We’ll start with lighting:

2H12 – This means it comes from Panel 2H and it is circuit 12.

Typically, electrical prints show how all of the switchgear (electrical panels) are connected. (Your main distribution powers another sub panel or goes to a transformer etc.)

So in other words, the page of the switchgear line-diagram will show where panel 2H gets power from.

And since it’s circuit 12, we know it’s the color blue when we pulling our circuits because it’s divisible by 6, right? 😉

For the plug, you’ll see it’s – 2H18. Again, fed from panel 2H and circuit 18.

And an important thing to mention about common areas is they typically use 20A plugs. So when looking at the symbols, make sure to reference your legend to see a 15A plug to a 20A plug.

A 20A plug usually has a little horizontal line, like you can see in the picture above!

Now on to emergency lighting.

If you look for the symbol containing EM7, this is your emergency lighting.

Emergency lighting is actually DC because they run off batteries when the power turns off.

This means we have to upsize our wires because of voltage drop, especially because the batteries are only putting out 12V, losing even 1V is quite a significant voltage drop!

In addition to voltage drop with emergency lighting, you have to take into account the number of EM heads you are running off the batteries!

So how emergency lighting works is there’s an EM pack (emergency pack) which only powers a certain amount of EM heads. (The EM7 battery pack powers EM7 heads. And EM5 battery pack powers EM5 heads, etc.)

In the condos I’ve worked at over my years, we tend to hide these emergency packs in the hallway closets and the lighting circuit in the area tends to be the same circuit that charges the EM pack’s battery.

This is because if the regular lights go off, the emergency lights will go on!

So if we look at the prints, we can see that EM7 is in the meter closet where MC-3 is, and the EM7 battery pack is powered by 2H12, which is the lighting circuit!

(Also, you can see that the EM7 battery pack is rated for 215 watts; that’s the number we have to keep an eye on for how many emergency heads we’re allowed to wire off of it.)

So here’s a real-world example of emergency battery packs.

If you look at the stairwell compared to the hallways emergency lighting, what do you notice?

I see the stairwell is EM5 but the hallway is EM7!

This means the stairwell must have its own battery pack and is separate from the hallway!

(Those are the important things to pay attention to, and can easily get missed!)

So that’s just a quick overview of how these prints work.

On a side note, I would like to mention that even though this hallway is a taste of commercial, it’s all pulled in NMD (loomex wiring).

This means you don’t have control over your wire colors.

In a real commercial setting, it’s important to look over the prints closely because we want to try and hit all the devices in that area that pertain to that panel.

This allows us to figure out pipe sizes and how big of a junction box to install in the area! (These main pipes that come from our panel are sometimes called main trunk lines.)

These trunk lines will typically pipe into a large junction box (12×12’s) which holds a lot of wires, and from there, we will pipe to other areas where only certain circuits are needed! So from that 12×12, we may pipe to another junction box which could be either a 4×4 or 6×6 depending on how many circuits are there.  (Always taking into account of pipe fill of our wires.. we may need to run a 3/4″ conduit, 1″, or 1 1/4″ for example!)

If you have a boss who knows what they’re doing, you will always have a gutter above your panel. This allows you to pull a circuit to multiple areas while still being able to make a splice in the gutter. You then send only the wires needed which gives your panel that super clean look!

In otherwords, the gutter is where you hide the mess and leaves the panel looking clean.


Roughing-in is the most crucial part of the job.

This is because once drywall is up, installing your light or electrical box becomes a much harder task.

In this section, I will show you how to know what type of light to install and also the right device box, too!

I’ll first start with lighting.

When we look at a light, there’s usually a little designation above the light. (For example, LD3)

For some reason, a lot of people think that’s the circuit number. But it’s not!

It’s actually the TYPE of light to install there.

This is especially important when it comes to pot lights because of their easy to miss differences in size (4″, 6″ etc.).

In a large commercial site, the order is the exact amount of lights and each light has their own designated spot. (If a light is damaged in shipping or through mistake, you’d just return it and get a new one shipped in. Sometimes if they’re specialty lights, they can take awhile to be shipped in!)

So for example, LD3 may be a 6″ pot light, whereas LD1 may be a 4″.

In this picture, there are two different types of lights.

You have the hallway lights which are the circled E, but then you also have a different type of light which is circled C, yet they have the same circuit number!

It all comes down to paying attention to the fine details.

Electrical prints showing plug and lighting circuits
Electrical print showing plugins, lights, smoke detectors, fire alarm bells and horns/strobes, and exit signs, and emergency lighting.

Now another really important point to mention about installing your boxes is plugs.

When you see a plug icon, usually it just means a standard duplex receptacle which is the most common in our homes.

However, if you see a quad symbol receptacle, you have to make sure to install a double gang, or in the case of metal studs, a 4×4 with a double gang mud ring! (This is not fun to fix.)

That’s a really common one that slips through the cracks.


That’s my overview of how to look over your prints to make sure you understand what you’re looking at and how the whole process works.

Keep in mind, this article does not take into account looking at your architectural prints which we use for exact placement of our lights and drop ceilings!

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