Ringer Equivalence Number (REN)

This system was introduced at the same time as the Plug and Socket system to reduce problems with too many telephones on one line circuit.  This is because customers could then connect any approved (and unapproved) apparatus into the new style sockets.

The REN value is based on the impedance of the ringer circuit and the current consumption, when idle, of the telephone.

Each telephone is given a REN value and the total REN of a BT line is assumed to be 4.  The standard BT telephone No. 746 (with a 4000 ohm ringer) has a REN of 1. It can therefore be assumed that 4 of these telephones can be fitted with no problems.

Nearly all old GPO will have a REN value of 1.

The REN value can be misleading. Sometimes if different telephones are mixed, this can cause ringing problems.  Mixing magneto bells and electronic ringers will no doubt cause lots of problems as the magneto ringers tend to hog the current.  Mixing GPO Trimphones with any other telephone is a no – no.  The answer is to try and see what happens and work from there.

Remember, that the REN is only generally relevant for incoming calls and telephones need not be counted if the ringer is disabled.

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Last revised: May 20, 2021