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Frequently Asked

Before Ordering FAQ

1 - Which CT can I use with the RPICT series?

CT stands for Current Transformer. These are small transformers going around the wire to measure.

All RPICT cards support 'current output' CT type. This means the output of the sensor is a current and not a voltage. For example the SCT-013-000 is marked as 100A/50mA. Delivering 50mA output when there is 100A going through the core. The SCT-013-030 on the other hand is marked as 30A/1V. Note the output part of 1V is a voltage and hence not compatible with the RPICT series. For your convenience we provide the RPICT card with some pre-defined setup for a range of CT from YHDC. You can use any other current output CT brand of your choice. Use rating option 'other' and send us an email with the details of the CT you intend to use. We will then setup the hardware for this.

Note there is a set of RPICT card able to support voltage output CT. These are RPICT8 RPICT7V1 and RPICT4V3 in version 4 only.

2 - Can I change the rating of the RPICT series myself?

No. The rating of the RPICT series is set in the hardware. Make sure you order the right one at purchase time.

3 - Do I need to use a Voltage sensor?

Yes if you need to do more accurate readings on power. 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 (Usually 240, 220 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.

For any other readings using an estimated voltage (RPICT8, RPICT3T1) is usually sufficient.

4 - Why the SCT-013-000 rated for 100A must be used with RPICT series rated at 30A or others?

Short answer

It is perfectly valid to use the SCT-013-000 specified at 100A on a RPICT card rated for 30A. The 100A is a specification for the valid linearity of the CT. The 30A is the range the RPICT card can read. These are two different specs.

30A being an example here. The same applied to any other ratings like 60A/50A/40A/20A etc.

More technical answer

The valid range of use for the SCT-013-000 is from 0 to 100A. This does not mean we must digitise the entire 100A. When choosing a RPICT card at say 30A the hardware is set to digitise from 0 to 30A using the SCT-013-000 although this CT is capable to read up to 100A.

The burden resistor value is chosen to achieve the desired range of the RPICT card.

5 - 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.

6 - Which rating is best for me?

You will be asked at purchase time to provide the 'rating' and ct sensor you intend to use. All our board are initially 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 smaller the range it is capable to read. 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 (30A/1V)? SCT-013-030 or any SCT-013-0xx are voltage output CT type. SCT-013-000 (100A/50mA) 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 is the solution to use voltage output CT? Have a look at RPICT7V1 Version 4 or RPICT8 Version 4. 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.

Can I use another CT sensor that you do not list? Yes. Choose rating option 'other'. Then email us the details of the CT (brand/model/turn ratio).It might take few days before we can ship the board with the right components.

Can I have a card with different ratings? Yes. Pick rating option 'other' then email us the setup that you need. For example below CT1 - 100A SCT-013-000 CT2 - 50A SCT-013-000 CT3 - 20A SCT-006 CT4 - 200A SCT-019 etc

7 - Version 3, Version 4 and Version 5

This section is for units RPICT7V1 RPICT8 and RPICT4V3 to which there are currently three versionsd.

Version 3 are no longer sold and produced. Version 5 replace version 3 and is our most recommended version.

Version 4 is a higher spec unit. We would recommend to opt for it if you need lower noise and higher accuracy or if you need to use voltage output CT.


8 - Why do I see a constant small value while no CT is connected?

This is commonly asked for RPICT8 and RPICT3T1 units.

Also answers Why is the CT not showing zero's when not connected?

This is called the background noise and is due to the RMS calculation. To have perfect zero would imply the waveform is a perfect straight line. The CT sensor and the signal chain on the RPICT is analog. This means that noise is also measured. Therefore it reports zero + noise when not connected or when no current is flowing across.

For a board setup with the SCT-013-000 and 100A rating one might find a background noise around 0.1A (25W for 240V). Do take into consideration the range of the board versus the level of noise. In this example the reading range is 100A with a background noise of 0.1A. This is a 0.1% ratio.

What if I need to read something below or around the noise level?

One way to have a lower background noise is to opt for a lower range at purchase time (see option 'rating'). If you are using a 100A rated unit you can get lower background noise by using a 30A rated for example. Note this is at the cost of lower measuring range.

Another possibility is to use a voltage sensor and measure Active Power (also called Real Power) instead of Estimated Power. This might require to acquire a new board that support voltage reading.

Note that using additional hardware on the Raspberrypi like video monitor / keyboard and additional hats will increase the risk to have higher noise. The power supply unit quality has an impact on the level of noise. Make sure you are using a 2A minimum PSU.

The noise level also depend on the RPICT board used. RPICT units with atmega328p mcu (RPICT8 RPICT7V1 etc) do have a more consistent noise across channels compare to RPICTs with a attiny84 mcu (RPICT3T1 RPICT3V1).

Can I reduce the background noise with a calibration? No.

9 - What if the reading is different from my multimeter or watt-meter?

RPICT series are sold configured with theoretical calibration values. These provide good results in most cases. If you do trust the instrumentation device you are comparing against and want both of them to match then you should carry out a calibration of V and I. See this article. Calibration_VCAL_ICAL_PHASECAL.

Please consider these points below.

  • Make sure you are comparing the same values. If you are comparing a RPICT3T1 or RPICT8 power against a watt-meter power then it is very likely you are not comparing the same values. RPICT3T1 or RPICT8 provide estimated power based on current only. The watt-meter will give you active power based on current and voltage. In such case only Irms can be compared and calibrated together.
  • If you are measuring the power of an apparatus that states its nominal power do not expect to read that number exactly. For example the label of an apparatus may indicate 2kW. Nevertheless do not expect to read 2000.00W but rather something between 1500 or 2500 approximately. Also note some devices like ovens will be on and off and consume nominal power only for a short period of time.
  • The device compared against could be inaccurate. Note that many multimeter do not use True-RMS as performed by the RPICT series.

Usually not necessary but if you feel that you need to contact our support we will be happy to help. Before doing so please prepare the points below.

  • Only use the lcl-run command to view the data that you are comparing. We will refuse to investigate data coming from scripts or any kind of remote platform.
  • Send us a copy paste of the lcl-run output.
  • Indicate the model and ideally the four letters code that came with it.
  • Indicate which channels are used. For example CT1 and V1.
  • Indicate the estimated load applied. For example 'I have a 2kw heater' or 'my other device reads 5A'.
  • Indicate if a voltage sensor is used or not.
  • If you have modified the config dump it using -a. Then send the /tmp/rpict.conf file to us.

10 - CT is clamped and load is applied but reading does not change

1. Make sure the SCT is clamped around a SINGLE PHASE.

Clamping the CT on the whole mains cable will never read anything as it measures two currents. One positive plus one negative which sums to 0 (or near 0).

The easiest quick test is to connect the CT at the meter level on a single phase. Otherwise if willing to measure the power used by a single application then make a special cable that split phases.

Do not clamp CT This way Use splitter to clamp CT

There is a phase splitter kit available in the shop here. See also the instructions to mount the kit. Phase Splitter Kit Usage.

2. If using RPICT7V1 or RPICT4V3 or RPICT3V1 make sure the voltage sensor is plugged in and voltage reading is correct. Although even if the voltage sensor is not connected it is still possible to read Irms.

3. Make sure the load applied is sufficient and adapted. If using a 100A range board then a load of minimum 100W should be applied to be clearly visible. Anything in the vicinity of 25 watt would not stand out much from noise (with a 100A units).

11 - Why is Real Power sometimes negative?

Real power will be either positive or negative depending on the current direction. If the negative number is not desired flip the CT clamp the other way.

Also when power is near zero you will then just read the noise which might be randomly positive or negative.

12 - Can I use a voltage output CT with RPICT series?

Short answer: NO. Read below if you really need to use these.

CT Model Type Compatibility with RPICT
SCT-013-000 100A/50mA Current output Yes
SCT-006 20A/25mA Current output Yes
SCT-019 200A/33mA Current output Yes
SCT-024 400A/100mA Current output Yes
SCT-031 600A/100mA Current output Yes
SCT-013-020 Volatge output No
SCT-013-030 Volatge output No
SCT-013-040 Volatge output No
SCT-013-050 Volatge output No
SCT-013-010 Volatge output No

The vast majority of RPICT series only support current output CT's. The only units able to support voltage output CT's are RPICT7V1 RPICT8 and RPICT4V3 in version 4.

For information the SCT-013-000 is current output. All other SCT-013-030 SCT-013-005 and other SCT-013-0xx are voltage output.

The simple rule is when the second part of the rating is in Volt the CT is a voltage output type. e.g. 100A/1V is a voltage output.

Example (30A rating)

If you wish to read a range of 30A we recommend to use the SCT-013-000 with a 30A rated RPICT board.

"...but the sct-013-000 is for 100A? Not 30A??" These are 2 different specifications. The rating given by the manufacturer for the SCT-013-000 tells where the sensor provide a reading in spec with the datasheet. In this case if going beyond 100A this sensor will not respond linearly. The rating indicated for the RPICT is the range on which we scale the signal level to match the full scale of the ADC. In other terms we have 1024 or 4096 points of resolution and we distribute these resolution points over the chosen rating. The 30A rated board will have 4096 resolution points from 0 to 30A.

In short. 100A is the valid range of the CT. 30A is the range we zoom in.

13 - RPICT Pinout

Raspberrypi GPIO RPICT

The Raspberrypi pins used by the RPICT board are shown in bold in the table below.

3.3V 1 2 5V
SDA 3 4 5V
SCL 5 6 Gnd
Gnd 9 10 RXD
... 11 12 ...

The pin used are GPIO4 TXD and RXD (beside Ground and 5V).

If GPIO4 is used to reset the RPICT card when performing configuration. If GPIO4 is needed for another use it can be disabled by removing a resistor on the RPICT card. Note there will be no option to configure the card remotely after that.

Product RPICT3V1_RLY2 and RPICT3T1_RLY2 additionally use GPIO5 and GPIO6.

For your convenience we provide Raspberry Pi expansion card in the shop.

14 - Python 'device reports readiness to read but...' error

If you encounter the following error when running a Python program.

Traceback (most recent call last):
  File "/usr/local/bin/", line 564, in <module>
    L = wait_and_read(options)
  File "/usr/local/bin/", line 59, in wait_and_read
    r =
  File "build/bdist.linux-armv6l/egg/serial/", line 501, in read
serial.serialutil.SerialException: device reports readiness to read but returned no data (device disconnected or multiple access on port?)

This is usually due to a duplicate access to the serial port.

To solve this find the other program using the serial port and stop it.

Common causes are: * Another python program (or cat) is already running. To find out which program uses the serial port use the command below

fuser -v /dev/ttyAMA0

To find out if the serial port is correctly configured run the command below.

grep serial /boot/cmdline.txt

If the command returns nothing then it is setup correctly. If it does return something then please visit the setup guide above once more.

  • Emonhub is already running. Stop it using sudo /etc/init.d/emonhub stop.

15 - What to do if I see garbage characters on the output?

This is caused by the RPICT board being reset every x seconds.


This is commonly due to something using GPIO4 on the Raspberrypi. This could be both a physical connection or a software action.

  • Make sure there is no hardware connected to GPIO4.
  • Make sure there is no software using GPIO4.

Some systems have 1-wire enable which is using GPIO4. You can disable it with the raspi-config command.

sudo raspi-config

Choose option 5 Interfacing options. Then P7 1-Wire. Then disable it.

If you do need to use 1-Wire along side RPICT then you might have to remove a resistor on the RPICT to allow that. Note that you will loose the auto-reset feature.

16 - The cat or lcl-run command outputs nothing

  • First of all make sure you have religiously followed all steps in the Raspberrypi first setup.
  • Did you connect the RPICT board to an emonpi image at any point? See here
  • Check there is good comms with the firmware using the command below.

If it returns the firmware version and some more binary data then communications are good.

  • If the above point worked well it might be a good idea to restore the default configuration with the command.

17 - Why do I get nan's?

The 'nan' in the output indicates that a channel has reached the over limit. This could be caused by

  • The current is higher than the rating of the RPICT board.
  • A voltage port has been left unconnected. There are three possible options here.
    • Connect the voltage sensor if you have it.
    • Or remove any channels involved with this voltage port from the configuration.
    • Or short circuit the voltage input to keep tight at zero.
  • The system is placed in a noisy environment and spikes are triggering the over limit.
    • Try to isolate the Raspberrypi/RPICT stack.
  • If measuring the output of an inverter you will encounter heavy harmonic content. This results in the waveform requiring a much larger range than without harmonics. Under these conditions it is possible to see a 80A current triggering nan's on a 100A rated board. Do take into consideration this fact when choosing the rating of the RPICT board.

Note that once a channel as triggered an over limit all other channels involved in the same computation node will also indicate nan. For example if you have configured an active power with CT1 and V1. If CT1 triggers an over limit then not only Irms will show nan but also Vrms RealPower ApparentPower ReactivePower and PowerFactor of the same node.

Issues with nan' appearing too often can be diagnosed by transforming the RPICT into a scope. See Transform_a_RPICT_into_a_web_scope.

18 - Can I change the rating of my RPICT unit?

No. Not really. The rating is a hardware setup and can not be changed from software configuration. Please make sure you order the correct rating at purchase time. Every board is sold fixed for a given sensor and given range of reading. For example SCT-013-000 in 100A. Maybe Yes. However it might be possible if you are able to solder/desolder smd resistors. Note that we will refuse returns for unit that have been modified using the following steps below. Resistors are 0805 type and located close to the jack socket port. The required value of the resistor can be obtained from these links below.

* For RPICT3T1 RPICT3V1 RPICT4T4v2.5 and also RPICT8/RPICT7V1/RPICT4V3 in version 3
Gen3 Passive Component Setup

* For RPICT7V1 RPICT8 and RPICT4V3 in version 4
Gen4 Passive Component Setup

* For RPICT7V1 RPICT8 RPICT4V3 RPICT4T4 in version 5
Gen5 Passive Component Setup

After changing the resistor the calibration value in kcal must be changed also.

19 - Minimum Output Rate

See this article below RPICT Minimum Output Rate