Split Single-phase Energy Meter

Affordable remote energy monitoring for your entire home

May 16, 2019

The Current Transformers

Hey Everyone!

It’s been an exciting week! We’ve officially reached our goal and almost doubled it! Thank you so much to all of you who have supported this campaign!

Many of you have had questions about the transformers - current transformers. Specifically about the sizing and if they’re safe. First, the sizing.

You may have noticed that the CTs in the pictures and video are blue. They are the YHDC SCT-013s. The NEC standard for service mains says that aluminum wire must be 4/0 AWG and copper must be 2/0 AWG for 200 A. Most people have aluminum wire because copper is more expensive. If you then look up the diameter of 4/0 wire you’ll see that it is 11.684 mm. The blue CTs have a diameter of 13mm. Perfect, right?

When going over the details for determining what CTs to include for the kit and solar package, not having standard mains wire myself, I overlooked one important thing - the diameter of mains wire WITH insulation. 4/0 AWG aluminum wire with insulation is ~16 mm. So, for most people, the blue current transformers will not fit.

This is why in the description of the campaign we list the similar, but larger, SCT016. This one can measure up to 120 A, and can fit wires with a diameter up to 16 mm. A special order will be placed for these with 3.5 mm plugs. The data sheet is located here if you are interested in more details.

A note on safety of current transformers. The ones we’ve chosen have a built in diode to suppress transient voltage (TVS). The energy meter also has built in 12ohm burden resistors. If you choose a different CT to hook up to the meter that has a built in burden resistor instead, you can disable the on-board burden via a jumper on the back of the board. It’s still possible for the current transformer to give you a shock if they are hooked up to mains wire, you are using a lot of power, and you grab the end of the 3.5mm jack while not plugged into the meter. You should never leave them disconnected (open-circuited) while also hooked around mains wire. They should be plugged into the meter before clamping them around mains wire. CTs should always be shorted in some way.

More updates soon! Happy energy metering!
John


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