Phone Wiring

This webpage describes the basics of installing or wiring a phone line in your house. These wiring conventions are standard in the U.S. but may not be standard for other parts of the world.

Residential phone wiring: whose responsible for what?

Back in the day when you ordered phone service to your home, the phone company would provide a phone line to your house and also perform the phone wiring within your house as well.  Things have changed in recent years and they no longer perform the inside work.  When you order phone service to your house, the local phone company installs a network interface device (NID), and routes a line from their pedestal to the NID.  The NID looks like a grey plastic box usually mounted either in your basement or on an outside wall.  From the NID, you must route the cabling to your telephone jacks scattered through your home.

 residential phone wiring

You can do your inside wiring yourself, it is actually rather easy.  There is really no point is spending your hard-earned money for an electrician or phone company (if they offer the service) to do this task for you. Even if you have to buy wire and modular jacks, you're going to come out way ahead if you do your own work.

Installing Additional Phone Lines

Installing additional phone lines is one of the most common phone wiring tasks in this age of modems and fax machines. What's described here are the color coding conventions for phone wiring, and how to make the connections for some of the most basic configurations.

Phone wires carry low-voltage electricity, but there is enough voltage in a live phone line to give you a little jolt.  You should disconnect your house at the Network Interface Device before working on wiring.

In most residential phone wiring, the cable contains four individual wires. Most phone wire installed in the U.S. during the second half of the 20th century is of the following kind:

Four-strand wire:

Diagram showing old-style wire.  T1= Green, R1= Red, T2= Black, R2= Yellow 

The phonewireeshown above has now become obsolete. For all new telephone wiring projects, you should use CAT5 cable. All CAT 5 wire I've seen in phone systems use the following color coding:

Diagram showing new-style phone wire 

In either case, the important point is this: one phone line only requires two of these strands. In the vast majority of cases, the other two wires go unused-- but if you choose, you can certainly use them for a second line (i.e., a totally separate line with its own phone number, which the local phone company will connect to a second terminal in your network interface device). This means that if you are installing a second line for a fax, modem, etc., you usually don't have to actually physically run new wires; you can connect the extra two wires to the second phone line at the network interface device.

If you're going to buy a two-line phone, there's nothing more you need to do, since a two-line phone expects "Line 2" to run on the yellow/black wires. For ordinary phone equipment such a modem, however, you have to convert a "Line 1" jack to a "Line 2" jack. One way you can do this is with a plug-in adapter, but the method described here involves swapping around a few wires in the jack.

Buying a second line from the phone company is becoming less and less common in today's home as more folks are turning to Voice Over IP (VOIP) for their phone lines.  This technology utilizes your internet connection to establish a phone line outside your home.  With VOIP you can add as many lines as your internet connection will support (you'll likely be charged additional fees for additional lines with your VOIP provider).  VOIP typically supports fax machines as well as a normal phone.  Cell phones have also reduced the number of homes needing second lines, in fact many households have done away with their hard wired phone lines altogether due to having cell phones.

Phone Wiring at the network interface device

Don't be scared to poke around inside the Network Interface Device. It may look forbidding and official, but you have every right to be there.

The following two diagrams show the color coding scheme for the old kind of wire. This probably applies to your house if you're not running any new cable, and are simply running a second line thru the existing unused yellow/black wires. If you're running Cat 5 cable, you'll need to make the appropriate color conversions.

Diagram showing phone connections at NID for second line 

On standard 4-wire solid color wiring (common in most households) you can wire up your jacks as follows. Just remember that red and green make a pair. Also remember that black and yellow make a pair. You must swap the tip and ring for the correct pair with the other pair when converting between extensions.  

Wiring Jack

Wiring Jack
Line 1 only w/ Extension

Wiring Jack
Convert Line 1 to Line 2

\  \
 R Line 2

R Line 1
/ /
\  \ 
 T  R Line 2
 T  R Line 1  T  R 
/ \/ \_______/__/
   /        \
\ /\ /-------\--\
 R Line 2  T  R
 T  R Line 1  T  R
/ \/ \_______/__/




Note that black is swapped for green, and yellow is swapped for red. Of course, it would also work if you consistently swapped the black and yellow wires the other way (black for red, yellow for green) but that is not the standard. Given that you have to be consistent between the two ends of the wire, you might as well follow the standard.

Converting a jack to Line 2 means that you will no longer be able to use it for Line 1. In practice, you'll probably want to install a second wiring block beside the first, and use a short piece of four-strand wire to extend the system from the existing block to the new one. This way, you can have a Line 1 jack right beside the Line 2 jack.

Caveat: It occasionally happens that the red and/or green wires become damaged and unusable, but that the black and yellow wires are intact. Repair persons have sometimes remedied this by running the one phone line across the black and yellow wires rather than replacing the cabling. If this has happened, you won't be able to run a second line thru the four-strand wire. (This is uncommon, but it is a gotcha to be aware of).

Four-strand wire supports up to two phone lines. If you are installing three or four lines, you might also consider buying eight-strand wire. The color coding conventions for this kind of wire are as follows:

Diagram showing color convention for eight-strand phone wire 

The conventions for eight-strand wire are as follows:

  • Colored pairs match; e.g. WHITE with blue mark goes with BLUE with white mark for one phone line, etc.
  • WHITE with whatever color mark corresponds to the green line of four strand wire.
  • The pairs are used in the order pictured: thus, for the first line, you use blue; for the second, you use orange, etc.

If you have very old existing wiring in your house, it may not follow the conventions described above, but new wiring should follow them.

Troubleshooting Phone Wiring

If you think you've got everything hooked up correctly, but one or more of your lines is "dead" (no dial tone), the problem might be the local phone company's problem, or it might be in your own wiring. Be sure that the problem isn't in your own wiring or in one of your own phones before you call the phone company to check on the problem. If they determine that the problem is on their side of the network interface device, they have to fix the problem at no charge to you; but if they determine that the problem is on your side of the network interface device, they'll charge you just for having determined this, and they'll charge you a second time if you have them make the fix in your wiring for you.

So how can you tell whose problem it is? This is easy: when you open your network interface box, notice that there is a modular jack for each phone line. You can unplug the jack for the line in question (note that doing this unplugs your whole house from the phone company's network) and plug a working phone into the jack instead. This phone is now hooked directly into the phone company's network. If the phone works properly when connected in this manner, then the problem is in your own wiring. If the phone doesn't work, either your phone is broken or there's a problem in the phone company's network. Try a second phone which you know to work, and if there still seems to be no service on the line, the problem is probably on the phone company's side of the network interface device.

If the problem is in your own wiring, the following things might be wrong:

  • You might have colors reversed at some point. Check your connections.
  • A wire may have come loose from a screw in the network interface device or in one of your jacks. Check your connections.
  • You may have a damaged wire. Depending on where the damage is, you might have service to some jacks in your house but not to others; you'll have to trace the wiring thru your house and figure out which stretch of wire contains the damage.

If you're getting static on the line, it's possible that there's a hole somewhere in the wire insulation which is letting in moisture and causing a short. Follow the wire from the network interface device to the jack and look for holes. For example, if you've used staples to fasten the wire to the wall, check for a staple puncturing the insulation.

Modular Jacks

Two thing to what out for when buying modular jacks. First, for ordinary residential wiring, you should buy the kind of modular jack with four contacts inside the jack; don't make the mistake of buying the wider modular jack with six contacts unless you're sure it's what you need (you've got to look closely to see the difference).  Here are the different types of phone jacks you might see:

 RJ-12, 6P6C (6 position, 6 conductor)

RJ12 6 wire

RJ-11, 6P4C (6 position, 4 conductor)


RJ-45, 8P8C (8 position, 8 conductor) - typically used for home network wiring


Second, you can buy modular jacks either with or without the wiring block (this is the heavy plastic piece which you mount to the wall, with screws to attach the wires to; see the picture higher up on the page). If you're installing a totally new jack, then you need the wiring block. If you're upgrading an existing, old-fashioned (pre-modular) connection to a modular jack, you might be able to use the existing wiring block, in which case you don't need to buy the kind of jack with the wiring block included; sometimes you can take the old cover off and just put a new modular cover over the old wiring block. In the store, it's hard to tell from outside the sealed package whether the block is included. Read the label carefully!

Phone Jack Pinout



Where's Pin #1?


Additional Views Showing Telephone Pinout



As mentioned above, you should buy CAT 5 wire for all new phone wiring projects. The older four-color type allows more crosstalk between wires; this might be only a minor annoyance for voice lines, but it's a bigger problem for modems or DSL lines. Even if you don't have immediate plans to transmit this kind of data across your line, it's better to plan for flexibility in the future. CAT 5 is now the national standard. CAT 6 also works and has even better signal integrity than CAT 5.

 Wiring Requirements to Support DSL

Digital Subscriber Line (DSL) refers to a set of technologies used to provide broadband services via the existing copper cable plant. One variation of DSL is Asymmetric Digital Subscriber Line or ADSL, which has become very popular for residential internet access. ADSL can provide a subscriber receive speed approaching 1.5 Mbps and transmit speeds between 160 and 640 Kbps, which is more than adequate for most internet access applications. While there are numerous other variations of DSL (CDSL, HDSL, IDSL, RADSL, SDSL, and VDSL), ADSL has become so well known to the general public that it is often referred to as simply DSL.

DSL service can use the same copper path as an existing telephone line. Because they operate at drastically different frequencies, the DSL service and telephone line can use the same path simultaneously, as long as the two signals are separated properly. However, if the two frequency bands are not separated properly, telephones and other equipment such as modems and fax machines can interfere with the DSL service. Theoretically, DSL service itself should never interfere with the phone service, but the wiring and equipment added to support use of the line for DSL may become the source of phone service problems.

DSL Splitter or Filters?

DSL Splitter DSL Filter

Separating the DSL signal from the telephone service requires either a splitter in or near the NID or filters at each telephone device. Ideally, the DSL service provider will install a DSL splitter module inside the NID or in a separate enclosure next to the NID and run a new cable to the DSL modem location. But many service providers will avoid the cost of sending a technician to install a splitter and instead will provide filters as part of a "self installation kit." In addition to the DSL modem or router and the cables to connect it to the line and to your computer or LAN, these kits include a few filters (also known as "microfilters") for your phones. The kit may also include a few duplex adapters to so that you can use one jack for both the DSL connection and a voice connection or one filter for several voice connections. Of course, there will probably be an additional charge for the filters and adapters.

Using filters isolates the DSL service at each individual telephone jack, but the DSL signal is present on all the jacks. That's an additional physical connection for each telephone device. Interference on any phone jack may impact DSL speeds or could cause DSL to stop functioning entirely. Because the filters are connected to the line cord for each device, their connections are subject to damage by someone just tugging on a phone line cord and a single damaged filter connection could result in problems with both your DSL and telephone services. If you're willing to replace all of your telephone jacks, there are filters available mounted in faceplates that replace standard telephone jacks. The cost of a few of these will pay for a splitter. Using a splitter completely isolates the DSL service from the telephone line, so the DSL signal is not present at each telephone jack. A splitter also limits how many connection changes are required to complete your DSL installation and is not normally subject to incidental physical damage. DSL signal from a splitter is much more isolated from sources of interference, resulting in optimal DSL speeds.

If filters are used instead of a splitter, a maximum of five or six telephone devices with filters can be used before they begin to excessively degrade the DSL signal. Remember that "telephone devices" include answering machines, fax machines, and satellite/cable receivers with phone line connections. Don't forget modems - you'll want to keep dialup as a backup capability and may need a modem to connect to the remote access at your office. If you're looking at this page, I'd wager you have more than six telephone devices. Even if you can limit your installation to five or six telephone devices, you'll probably have to buy more filters than will be included in a self installation kit. The cost of just the additional filters would probably pay for a splitter.

A splitter is really a better solution because it allows you to connect the voice side to multiple jacks, normally does a better job of separating the data and voice bandwidth, and helps prevent cross-talk problems. A splitter requires a different cable/pair be used for the voice and data. However, if you use a DSL splitter, your existing inside wiring can be used for voice with very little change and without filters at each phone.

If you have an alarm system, installing a DSL splitter has an additional advantage. An alarm system will require either a special filter for the connection to the RJ 31X jack for the alarm system or a splitter to separate the DSL service from the telephone line before it goes to the RJ 31X jack for the alarm system. A splitter is the preferred option by most alarm system companies.

Splitter - Where and How to Connect

If you are lucky enough to have a splitter installed in or near your NID by your DSL service provider, your only concern is to establish a separate cable path to a jack at the DSL modem/router location. Browse though the following and determine your best option of how to arrange the wiring for your home.

Old "JK" quad cable does not meet the minimum specification for DSL. It doesn't really make a lot of sense to pay $30 or more per month for DSL service and suffer low speeds due to using low quality cable for your inside wiring. If you already have quad cable installed in your home, you can try running DSL on it, but you should expect significantly lower speeds than would be achieved with CAT3 or CAT5. I would highly recommend running CAT5 or CAT6 cable for the DSL data and only using the quad cable for your telephone jacks. If you choose to use quad cable for DSL, expect your DSL provider to blame it for any complaints about low speeds.

DSL Splitter wiring

If you are installing new inside wiring or making significant changes to your existing wiring, I recommend using a star topology with cables terminating on a hub/junction device such as a 66 block. For any new cabling, I recommend using four-pair CAT5 or CAT6 cable. CAT3 would be acceptable for telephone and would actually meet the minimum specification for DSL, but the cost for CAT5 is not significantly higher and it is much better cable.

If you have a ring, bus, or hybrid topology, it is probably most practical to locate the splitter near the NID and simply run a new cable from the splitter to the DSL modem location. You can use a splitter in a separate exterior enclosure or you can use a less expensive splitter without an exterior enclosure located inside the house at a protected location where the cable coming from the NID can be accessed before it goes to any jacks.

If you have a star topology, you have several other options for the location and type of splitter to install. You could locate the splitter at the hub/junction point or you could locate the splitter near the modem/router. Either of these options allow you to use a less expensive splitter without an exterior enclosure.

Even if you have a star topology, if you have an alarm system, you may find it preferable to use a splitter located near the NID, so that you don't have to reconfigure the connections to the RJ 31X jack. It is probably a better option to run a new cable for DSL from the splitter to your hub/junction and cross-connect to the cable to your DSL modem/router location, or you could run a cable directly from the splitter to a jack at the DSL modem/router location. Trying to splice a cable from the DSL modem/router into a spare pair in the cable between the NID and the RJ 31X jack is probably not worth the effort since there is a good probability that the splice would reduce your DSL speed and could result in a fault prone connection to the alarm system and to your phones.

Electrical - The Basics

Residential Electrical Guidelines and Codes

Rough-In Electrical and Pulling Cable

Common Electrical Wiring Diagrams

Breakers and Fuses

Wire Gauge and Voltage Drop Calculator

NEC Reference Tables (2011, 2008, 2005, 2002, and 1999)

Sizing Your Electrical Service

Electrical - Main Service Line Installation

Fire/Smoke Alarm Installation

Doorbell Wiring

Phone Wiring

Low-Voltage Wiring

Cable Pinouts

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