I’m (still?) getting up to speed on how electricity flows through our house and how use is commonly measured. ..

The Goal

The goal of this post is to:

  • explain why I chose to buy two SCT-013-000 CTs.
  • write down my understanding of how 240V electricity comes into our house.

Our House

All the energy monitors I know about use Current Transformers (CTs) to read the voltage and current used by appliances within a house.

Our house is wired with a three-wire single phased system.  As the Wikipedia article notes: This three-wire single phase system is common in North America for residential and light commercial applications. In this case, the voltages are 120 V line to neutral (grounded center tap of transformer) and 240 V line to line. This system allows 120 volts to be supplied for lighting and convenience outlets, and 240 volts for higher-demand appliances such as space heating, air conditioning, kitchen stoves or water heaters.

I copied this image from Home Depot’s Basics of Your Home’s Electrical System video:

Pasted Image 9 20 17 3 50 AM

The Learn section of OpenEnergyMonitor has a great article on North American Power Supply characteristics.  It includes this diagram:

As is true with my house: Standard practice among US electricians is to connect the circuits to the breakers in numerical sequence, with odd numbered breakers connected to one leg and even numbered breakers connected to the other leg. Thus, half of them will be on one leg, half on the other leg, with the aim being to balance the load evenly.

The implications of North American Power Supply characteristics to the firmware are briefly covered on the Use In North America/OpenEnergyMonitor’s setup guide.

Ordering CTs

How Many

Given the split of the 240V into two 120V lines, I will need two CTs, one for each line.

What Size

I used my caliper and measured the diameter of each of the 120V cables to be .32” or 8mm.

IMG 6222

According to the AWG wire sizes, this makes the cables 1/0 AWG.  This surprised me since I most cable examples I had seen on the Internet that had similar wiring to our house was 4/0, which is significantly wider.  Perhaps this is because the panel is actually a sub-panel that was put in place when we had a generator installed.  I didn’t open the original (main) panel.  Perhaps the cables for the 2 120V lines are 4/0 in the original panel.

IMG 6214

YIPPEE! This means I can use the common SCT-013-000 (data sheet).  YHDC sells several different CTs.  The SCT-013-000 is the one sold by OpenEnergyMonitor.  I would have ordered from their store.  Unfortunately, I would have to pay to have it delivered from the UK.  Instead I ordered two from Newegg.com.  


…so much for that…