What REN Means and Why It Matters for Your Phone Line
The Ringer Equivalence Number (REN) is a standardised measure of how much electrical power a telephone device draws from the line during ringing. It is not a measure of how loud the phone rings or how much power it uses during a call. REN specifically measures the load a device places on the ringing current supplied by the telephone exchange or NBN modem.
Understanding REN matters whenever you connect multiple telephone devices to a single line or to the phone port of an NBN modem or VoIP adapter. Exceeding the REN limit causes phones to ring weakly, ring briefly and cut out, or not ring at all.
Page Index
- How REN Works
- REN Limits for Australian Lines
- Calculating Your Total REN
- Finding the REN of Your Device
- What Happens When You Exceed the Limit
- Frequently Asked Questions
How REN Works
When an incoming call arrives, the telephone exchange or NBN modem sends an alternating ringing signal down the line (typically 75V AC at 17Hz in Australia). Each telephone device connected to the line draws a portion of this ringing current to operate its ringer. The REN value of each device represents its share of the total ringing current available.
A standard analogue telephone has a REN of 1.0. A fax machine typically has a higher REN of around 1.5 to 2.0 because its ringing circuit draws more current. Some modern cordless phone bases have a lower REN of 0.1 or less because they use electronic ringing circuits rather than traditional electromechanical ringers.
The total REN of all devices connected to a line must not exceed the line's ringing current capacity. If it does, there is not enough ringing current available to reliably activate all the ringers simultaneously.
REN Limits for Australian Lines
| Line Type | Typical Maximum REN | Notes |
|---|---|---|
| Traditional copper landline (PSTN) | 3 to 5 | Varies by exchange. Telstra specifies a maximum of 3 REN for most residential lines. |
| NBN modem phone port (VoIP) | 1 to 4 | Most NBN modems support between 1 and 4 REN. Check your modem's specifications. Many budget NBN modems only support 1 REN. |
| VoIP ATA (Analog Telephone Adapter) | 1 to 5 | Varies significantly by device. Check the ATA specifications. |
The critical point for NBN installations is that many NBN modems supplied by RSPs have a low REN limit on their phone port. If you are connecting multiple phones to an NBN modem and they are ringing weakly or failing to ring at all, the modem's REN limit is the most common cause.
Calculating Your Total REN
Add the REN values of every device connected to the same telephone line or phone port. The total must be equal to or less than the maximum REN supported by the line or modem.
Example 1: Works correctly
- Cordless phone base: REN 0.1
- Second telephone: REN 1.0
- Fax machine: REN 1.9
- Total REN: 3.0 — within the typical limit of 3
Example 2: Likely to cause ringing problems
- Telephone 1: REN 1.0
- Telephone 2: REN 1.0
- Fax machine: REN 1.9
- Alarm panel: REN 1.0
- Total REN: 4.9 — likely to exceed a 3 REN limit and will definitely exceed a 1 REN NBN modem port
Example 3: Common NBN modem issue
- NBN modem phone port REN limit: 1
- Standard telephone: REN 1.0
- Total REN: 1.0 — exactly at the limit. A second phone connected in parallel would exceed it.
Finding the REN of Your Device
The REN value is printed on the base or rear label of most telephone devices. It is also listed in the device specifications and on the compliance label alongside the RCM mark. Common REN values you will encounter:
| Device Type | Typical REN Range |
|---|---|
| Standard analogue telephone | 0.8 to 1.0 |
| Cordless phone base station | 0.1 to 0.3 |
| Fax machine | 1.5 to 2.0 |
| Monitored alarm panel | 0.5 to 1.0 |
| Modem with built-in phone port | 0.0 (does not contribute to REN load) |
If the REN is not printed on the device, check the manufacturer's specifications or the compliance documentation. If the device predates REN labelling requirements, assume a REN of 1.0 as a conservative estimate.
What Happens When You Exceed the Limit
Exceeding the REN limit does not damage devices or the telephone line. The symptoms are limited to ringing behaviour:
- Phones ring weakly or with a quieter than normal ring.
- Phones ring once or twice then stop before the call is answered.
- Phones do not ring at all even though the line is working correctly for calls.
- Only some phones ring on a shared line while others are silent.
If you are experiencing any of these symptoms, disconnect devices one at a time until ringing returns to normal. The last device disconnected is the one that pushed the total REN over the limit. Either remove that device from the line or replace it with a lower REN alternative.
For NBN installations where the modem's phone port has a low REN limit, the practical solution is to use a single cordless phone system with a base station (low REN) and multiple handsets registered to that base, rather than connecting multiple independent phones to the modem port.
Frequently Asked Questions
Does REN apply to NBN VoIP services?
Yes. The phone port on an NBN modem or VoIP ATA generates a ringing signal just like a traditional telephone exchange, and is subject to the same REN limits. The key difference is that many NBN modems have a lower REN limit than traditional copper lines. Check your modem's specifications before connecting multiple phones.
Does my alarm panel count towards the REN total?
Yes, if it is connected to the telephone line. Most alarm panels with telephone line seizure capability draw ringing current and should be included in the REN calculation. The alarm panel's documentation should list its REN value.
Can I use a telephone line splitter to connect more devices?
A splitter allows multiple devices to be physically connected to the same line but does not increase the REN limit. All devices connected via a splitter still count towards the total REN of the line.
What is the difference between REN and line load?
REN only measures the ringing current load. Line load (or loop resistance) is a separate concept relating to the DC resistance of the cable run and affects whether the line can reliably carry a conversation over distance. The two are independent: a device can have a low REN but add significant line load, or vice versa.
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