A Ringer Equivalency number is a value which indicates the amount of current a telecommunications device (modem, fax machine, telephone) uses. It is nominally set to a value of 1.0 for a conventional telephone. ndividual devices may have values greater than or less than 1.0, depending on their design and function.
Ringer Equivalency Numbers may have a letter after the numeric rating, either A or B. These numbers are documented in FCC Part 68 and EIA Document RS-470.
An "A" ringer usually indicates an old-fashioned telephone with an electromechanical bell. These ringers activate on an input frequency of between approximately 20 and 30 Hz (this frequency is based on an elevated, alternating-current voltage sent by the phone company).
A "B" is usually associated with more modern ringers, such as you might find on an electronic telephone. The frequencies for these phones are generally between 15.3 and 68.0 Hz.
An REN value of 1.0 is defined for an "A" class ringer.
The total REN value for your facility should not normally exceed 5.0. The exact maximum limit can be obtained from your phone company. If you have too many devices attached to your phone number, you may start to find that your fax machine or modem may not pick up, or that your phones won't ring at all. Reducing the load on the circuit by disconnecting extra devices is one quick way to test this.
(adapted from TELECOM Digests 10.011, 12.552, 12.524, and 12.536, and various Global Village manuals).