Central Office Loop Current

Loop Current is the amount of electrical energy flowing through the telephone and line, as opposed to the voltage which is the force behind the energy. There is a definite correlation between the Loop Current and Line Voltage. The normal voltage of a CO line is about 48-50 VDC, and the Loop Current of the line should be between 23ma and 27ma range, but in some instances that may not be true. Most phone systems work fine with Loop Current in the 23ma to 35ma range, to avoid any problems it is recommended to be 23ma to 27ma range. The distance between the Central Office to the subscriber’s location is a factor in Loop Current. There is a definite loss of electrical energy per foot of copper wire. At the end of a long loop the current that left the central office at 35ma might be reduced to 20ma and possibly less because of the wire loss.

Here are some common symptoms of High Loop Current and Low Loop Current.


Symptoms of High Loop Current Include:

  • Burned Out Key, PBX or Data Equipment
  • Garbled Data and Modem Failures
  • Cut-offs & Squealing on Lines
  • Crosstalk, Echo & Hollow Sounding Lines
  • Numerous Intermittent Circuit Failures
  • Off Premise Equipment Problems

Symptoms of Low Loop Current Problems:

  • Poor Voice Transmission Quality & Low Volume
  • Ghost Rings
  • Wrong Numbers
  • Lost Calls During Transfer
  • Data Loss

Measuring the Loop Current

Here is the hookup to measure loop current with a regular or digital multimeter. If you have bridging clips on a 66 block, just open one of them, and connect one lead of the meter towards the CO, and the other towards the equipment. It doesn’t matter which color lead goes where since the meter will display a + or – that you don’t care about (except on an analog meter which will make the needle go backwards and get damaged).

With the line on-hook, you should see almost no current flowing through the meter (it will read maybe .01ma). When you go off-hook with either the phone equipment, modem or your butt set, you should get a reading of between 23 and 35ma if the line is OK. Sometimes you can only get an accurate reading using the actual system trunk to pick up the line, using the butt-set showed lower readings. Repeat and record the readings for each line, since not all of the lines have loop current problems in many cases.

You can reduce the High Loop Current by adding a Loop Current Regulator or Attenuator on the line. That usually brings it down to 25ma with no loss of audio level, or you can add half watt resistors across the line. You can use 50 or 100 ohms but be aware of the audio level loss.


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