Questions & Answers > CT wire length

We have 600:5 CT's mounted inside substation transformer breaker. The metering equipment is 300ft away. What factors should I take into consideration in sizing the wire and how exactly will this affect the metering accuracy?
Thanks in advance for any reply!

December 26, 2013 | Unregistered CommenterNathan Xavier

Happy New Year Nathan!
I’m thinking that the 600:5 CTs you are talking about are primary CTs, which generally have a higher burden rating than secondary CTs. This higher burden rating should be, I believe, 1.8 ohms of burden and my suggestions are based upon this belief. This higher burden rating will allow for a longer run of CT secondary wires, but there are a couple of considerations that I would suggest:
1) 150 ft of #10 copper wire has .2 ohms of resistance, so at 300 ft the burden presented by the #10 wire would be the total of .8 ohms, using separate return leads. If you use #8-wire, there will be less burden presented to the CT secondary, which should allow for better meter accuracy.
2) A caution from the Meterman’s Handbook states: If the distance becomes significantly longer or >300 ft, don’t use a common return wire for the CT secondaries, use individual returns.
For this reference, I found in the Meterman’s Handbook (10th Edition – page 360) that: The resistance of a CT secondary lead adds to the burden, but unless this added resistance causes the total burden to exceed the burden rating of the transformer, it has a relatively small effect on the transformer accuracy. For very precise metering, separate return leads might be justified if the lead resistance is large.
It is standard practice to ground the non-polarity secondary lead of a CT. If separate return leads are used, each one should be grounded once. Interestingly, the Meterman’s Handbook recommends two different locations:
In the 10th edition it is at the meter cabinet or switchboard.
In the 9th edition the suggested ground connection is at the CTs.

I welcome further discussion by others who have other experiences with this type of set up. It would be interesting to see what else is happening for other metering professionals in this regard.
Be Safe All Ways,

December 30, 2013 | Unregistered CommenterBrad