Mapping Your Home Electrical Circuits in an Afternoon

Got a friend and a couple hours? Grab a non-contact voltage tester and your phone and have a map of your home electrical circuits in no time! Really, every DIYer needs one of these.

I use “map” a little loosely here. What I really mean is a spreadsheet… but that’s less exciting sounding. Our goals for this spreadsheet were twofold:

  1. Be able to find out what fixtures/outlets were on any given circuit. If you’re cutting the power to a circuit, it’s helpful to know if your husband’s entire home office is going to go dark… for example
  2. Conversely, be able to find out which circuit any given fixture is on. This is essential for cutting power to the right circuit to work on, say, a light fixture without zapping yourself.

Hands down, the most surprising thing about moving into this house was the amount of electrical knowledge required to do just about anything. It all started when we decided to take down the awful, intermittently-functional fluorescent light in our kitchen:

yikes!

Before we go any further, I guess I need to tell you that I’m no licensed electrician (shocking!), so don’t let me be your only source of truth in any of this! Right. So. The first thing you need to do to take down a light fixture is to turn off the power to it – and not just at the switch, at the main service panel, wherever that happens to be. Ours happens to be in our lovely unfinished basement!

Anyway, to disconnect this lovely overhead light, we needed to know what circuit it was on! This was a trial and error process which took awhile. We decided we needed a more efficient process for next time…

The Main Service Panel

Opening it up, you see two things: on the inside of the cover, a list of labels:

(Hilariously, even the really broad labels like “kitchen” are incorrect. No help at all.)

And of course, you also see the circuit breakers inside the panel:

circuit breakers!

The 20A breakers *should* be the all-purpose circuits, and the 15A breakers *should* be the lighting circuits/light duty outlets/all the knob and tube. I won’t go into too much detail here about the panel, but you never want to expose any wiring to more amps than it’s rated to handle. This gets tricky when you’ve got an old house with knob & tube wiring, and you’re not sure if/where it turns into Romex.

The Audit

This part was totally brute force. Husband stayed down in the basement, opened up the panel, and called me on speakerphone. I grabbed a pencil & pad of paper, a non-contact voltage tester, an outlet tester (which ended up being pretty useless because we only had like 5 3-prong outlets in the whole house at the time), and my phone. (A water bottle or snack would have been a smart idea, too.)

As husband flipped breakers on and off, I started out by an outlet in the attic and waited until I no longer detected current. Then I asked which breaker he’d just turned off, wrote the result in my notebook, and moved onto the next outlet. If an outlet you’d detected current in suddenly goes dead when a breaker is flipped, it’s pretty certain it’s on that circuit! Light fixtures/appliances are a little easier and don’t require the voltage tester. Pretty obvious when a light is on or off!

Here’s the first page (of two, eventually) after going though the house methodically:

I tried to be as descriptive as possible, because the next step was to transcribe all of this into a Google Spreadsheet. We wanted to be able to sort by room & by circuit number and also have it easily accessible. After 3ish hours, our phone batteries were dead, but we had a pretty complete picture of the home circuits! Hooray!

Next Steps

As I mentioned, I put all this into a Google Spreadsheet. We were fascinated by what we found when we were able to sort through everything! It was obvious which ones were the original circuits (these ones meandered through all the areas of the house) and the ones that had been added in later. We started to get a sense for where in the walls these wires had to be located and started devising plans to replace/remove old wires. Someday we’d love to get rid of all the knob and tube… but that would require ripping all our walls apart and we’re not really ready for that yet. Or ever?

(Above, a snippet of our spreadsheet, sorted by circuit number)

This helped us identify things like unused circuits, potentially overloaded circuits, and curiously underloaded circuits. Like… why does a single 2-prong outlet in the attic bedroom need its own circuit??! As of this writing, we’ve used this document to help us decommission several old outlets/fixtures on the first floor and in the basement. We’ve also grounded some previously (extremely lazily) ungrounded outlets. Very exciting, right?!

I can tell you for sure that this wasn’t wasted effort. We’ve referenced this document SO. MANY. TIMES. during our various projects since then. Go ahead and give it a try! Not the most Pinterest-worthy project but SO useful.

 

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