Electrical installation – building a mental image

Lesson #1: Chicago is special …

… in so many ways. And so is the Chicago Electrical Code (Municipal Code of Chicago – Title 18 Building Infrastructure – Chapter 18-27 Chicago Electrical Code).

The code prescribes the use of electrical metallic tubing (EMT), commonly known as conduits, through which the conductors (wires) get routed. EMT are thin-walled metal pipes that come in 10 foot sticks. Typical diameter sizes used in residential construction are 1/2 and 3/4 inch.

Lesson #2: Count your conductors – or wires …

I don’t mean physically counting wires, but counting them in your mind. Because of my electrical illiteracy, I left this mindful exercise in the capable hands of my electrician, Percy.

Why do we need to count the wires? The Electrical Code sets a limit to the amount of conductors (or wires) allowed in an EMT. This is for two reasons:

  1. “… [to] permit dissipation of the heat [from the conductors] …” – In other words, prevent overheating of the wires.
  2. “…[to ensure] ready installation or withdrawal of the conductors without damage to the conductors or to their insulation.”

The rule is that for a 1/2 inch diameter EMT we cannot have more than:

  • 9 conductors (12 or 14 AWG)

For a 3/4 inch diameter EMT we must limit the number to:

  • 13 conductors (12 or 14 AWG)

Lesson #3: Wire terminology

Or should I say conductor terminology. Either way, what is AWG?

AWG stands for American Wire Gauge [LINK]. Number 12 is the a heavier gauge and is used for 20 ampere circuits, while number 14 is a lighter gauge and is used for 15 ampere circuits.

Another acronym you may run into while digging through the Electrical Code is conductor type THHN or THWN [LINK].

THHN stands for Thermoplastic High Heat-resistant Nylon-coated. This conductor type is suitable for dry and damp locations.

THWN stands for Thermoplastic Heat and Water-resistant Nylon-coated. This conductor type is suitable for wet locations.

I noticed that the wire we eventually used is rated both, type THHN and THWN.

Why do I need to know all this?

The electrical plans for the building are a schematic representation at best of what electrical fixtures and loads will be where. But all the details, like what goes onto which circuit and how to route the circuits, conduits and wires, are left to the electrical contractor.

Percy had to develop a mental image of how he could route the conductors from the breaker panel to each circuit and subsequently to each fixture. He needed to get a mental count of the number and type of wires. Where do we have a 20 ampere circuit that requires a 12 AWG conductor, and where will 15 ampere with a 14 AWG conductor suffice?

I was pretty much overwhelmed by that task and had no idea how Percy was able to form and hold on to that mental image and how he kept track of the number of wires needed. I guess it all comes down to aptitude … and experience.

All of this information was needed to determine where we should use 1/2 inch or 3/4 inch EMT, so that we can get started with the installation.


About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

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