RPICT Buying Guide

We aim to answer here most commonly asked questions at buying time.

Which RPICT board should I get?

The choice of the RPICT will depend on the following factors.

• Number of channels.
• 3 phase or single phase.
• Is temperature needed.
• Is a voltage sensor needed.
• RaspberryPi standard or RaspberryPi zero.

Do I need a voltage sensor?

If you are looking to estimate power consumption or detect if a device is on or off then you probably do not need a voltage sensor. In this case we use what we call 'Estimated Power'. RMS current is measured and multiplied by a fixed estimated voltage (say 240, 200 or 110V) to get an estimated power value. You can alternatively just read the RMS current in Ampere if you prefer. RPICT boards used for this are commonly RPICT3T1 RPIZ_CT3T1 and RPICT8.

If you are looking to measure Active Power, power factor and the like then clearly a voltage sensor is needed. This is also the case if you need to know the direction of power (import/export). Using a voltage sensor can also be used to allow measuring smaller power devices. For this we would use RPICT3V1 RPIZ_CT3V1 RPICT7V1 RPICT4V3 RPIZ_CT4V3T2.

Which rating is best for me?

To make things easier we ask that you indicate in 'rating' option which CT you plan to use and the max range that you intend to read. All our board are manufactured for SCT-013-000 100A. This is why there is a small fee to convert them for another CT or range value.

First of all you should estimate the highest current value your system is likely to have. Then pick a rating just above that. If you do not know just use the 100A to start with.

The lower the rating that you choose the better the resolution but the lesser this is capable to measure. For example a 20A rated board will measure small power devices (1kw to 5kw) with more resolution than the 100A. However if the current goes above 20A rms then the board will output a 'nan' meaning it has reached over limit.

Why are there several ratings for the same CT? For example SCT-013-000 is available in 100A 50A 30A and 20A. This is the ADC range*. If you look at the manufacturer datasheet for the SCT-013-000 it provides a maximum rating of 100A. This determines the range where the sensor itself is good to provide accurate reading. This is the working range of the sensor and not to be confused with the ADC range from above. The 'rating' value that we offer in the shop if the ADC range. (* the ADC is the Analog Digital Converter. The part that convert physical signal into numbers in the computer).

Can I use my SCT-013-030? SCT-013-030 or any SCT-013-0xx are voltage output CT type. SCT-013-000 is a current output CT type. Most of our RPICT board only support current output CT. Voltage output CT have the burden resistor fitted inside them. In order to have control on the most optimal scaling we chose to have the burden resistor on the RPICT board which require the use of current output CT's.

What solution is there for voltage output CT? Have a look at RPICT7V1 Version 4 or RPICT Version 4 or RPIDCV8. Voltage output CT require more expensive component due to the requirement to condition the signal. However one benefit is that only the calibration values need to be changed if you plan to use a different voltage output CT. Modifying the hardware won't be needed.