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CT’s – installing and extending length of cables

by Darius Salgo
posted on Sunday, May 6th, 2018

CT’s used in the Wattwatchers Auditor series products – also commonly known simply as ‘Wattwatchers devices’ – are supplied with 1.7m cables.

Extending the length of the CT cables, using the following guidelines, will not generally affect accuracy.

The potential on the CT cables is the same as the Neutral connection to the Wattwatchers device.

WW CT cable is used for all WW CT’s, and has a white jacket. It

  • Conforms to UL2464, a classification for “data communication wire”
  • Has two conductors that are twisted together (to reduce induced currents from adjacent magnetic fields)
  • Has pink (+ve) and white (-ve) insulation over the two wires
  • Is quite flexible, with an outside diameter of 3.6 mm

The regulatory requirement is that the cable be

  • Non flammable .. if exposed to flame it should not continue to burn
  • Rated to 300 Vrms
  • Double insulated, if exposed to human touch

For convenience, the WW CT cables are provided stripped and tinned with solder, so that the multi-strand wires are easily inserted into the connector.

If the cable as supplied have excess length it is acceptable to coil and tie-wrap the excess length.

If the cable is not long enough it can be extended using cable that meets the above requirements. The best way to make the joint is with a soldering iron and solder. Shrink wrap each conductor, and shrink wrap the the outside of the entire joint. This maintains the “double insulated” requirement, and  provides a smooth surface that can be pulled through conduit. It is recommended to strip and tin the ends of the extended cable, as per the supplied termination.

The normal noise level reported by the Wattwatchers device current channel is around 34 mArms and less than 1W (with the current channel configured for 60A CT’s). This fact can be used to check that extended cables are not picking up more noise by closing the CT without a conductor and checking the reported current and power levels.

Note the 34 mArms is just noise .. it does not result in energy being reported. If a longer cable results in increased energy with no “actual” current this is caused by 50Hz pickup. Even so, a few watts is going to make very little difference in most situations.

Note that Wattwatchers devices report their noise floor accurately … regulated meters disguise their noise floor by reporting “zero” below a threshold.

WW CT’s are safe, even when not connected and clamped around current carrying conductors. The larger CT’s include clamps that prevent the open circuit voltage rising above 10V.