It's All Greek To Me!

Electrical Wires

I was recently at a client's home and while discussing how to solve a particular electrical problem they were having, I came across a bundle of wires. I looked that them in passing and just thought the wires were a bundle of Category 5 Enhanced (Cat5e) computer network cables; being that the clients work in the computer field and I never really gave it much thought after that. Upon digging a little further into the problem I was called there for, I grabbed the bundle pulled it down and was asked by the client "What do you suppose those are for?". I said that I originally thought that they were computer network cables and then I took a few minutes to look at the markings on the cables and explain what they meant and what they might be for. As you can imagine the response was, "it's all Greek to me".

The Canadian Safety Standards Association (CSA) dictates what additional markings are required on wire and cable how they can be deciphered. While there are quite literally 100's of different types of wire and cables with markings, let's focus on the ones typically found in the home.

The most common one you will find is NMD90. This is a fairly complex code as it stands for a number of things. The code NM refers to the construction of the wire meaning Non-Metallic; the D states where it can be used; in this case it means a Dry environment and the 90 refers to the maximum temperature rating of the wire which is 90*C.

Likely the next most common type you will find is NMWU. We already know what the NM, stands for, so what about the rest? The W stands for Wet. So this tells us that this cable can be used in a wet environment unlike NMD90 which must be dry. The U stands for Underground. This means that the cable can be buried directly in the ground, which normally is a wet environment. In most situations you would not run an NMWU cable in a conduit underground; you would use a single conductor and it would have to be W and U rated also. So you cannot use NMD90 in a conduit underground to feed your garage it must be something like an RWU, THWU or possibly an NMWU.

So other markings on the wire tell us what size it is such as 14 or 12 and the number of conductors in the cable. So if you look at a cable you might see something like NMD90 14/2 or NMWU 10/3 or see 10 AWG 3 Cond. Instead of 2 cond, you might see CU 2 or CU 3. That means that the there are two or three copper (CU) conductors; if you have aluminum wire you will see AL as the code.

Another marking that you may find useful are the voltage rating. While the maximum voltage in 90% of all residential application is 240 volts, the majority of common wire is rated at 300 volts. A good example of wire that is common that you may find has a lower rating is LVT Low Voltage Thermostat wire (used in thermostat, door bell, alarm systems, etc). This is usually brown or red and is either 30 volts or 300 volts, but at quick glance both types look the same.

One of the more cryptic markings is the fire rating of the wire. All wire has a fire rating (actually called a Flame Test (FT) and that rating determines where it can be used. While wire is installed according to the Canadian Electrical Code (CEC), the CSA determines what the rating of the wire and cable is and the National Building Code of Canada (NBC) determines where it can be installed. There are three common FT ratings found on Canadian made wire, FT1, FT4 and FT6. In most residential applications you will find FT1 rated wire.

There are a good number of online sources if you want to find out what a cable is that you have come across. Some of them are fairly obvious like ACWU90, Armoured Cable Wet Underground rated for 90*C while others like THHN Thermoplastic High Heat Nylon is a fairly common wire but the names are a bit obscure.

Now during your next project you can tell your buddy to "go get the SJO". . . as they look at you totally bewildered you can say "Oh that's Greek for extension cord".

About the Author

John Kuehnl-Cadwell is a Master Electrician and an owner of Datawise Solutions Inc., an electrical contracting company located in Ontario. He has been licenced as a Journeyman Electrician since 1992.

Posted by: John Kuehnl-Cadwell
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