A conventional tee is a fitting the pipeline is built around; a mechanical tee is added to a pipeline that already exists.

To install a conventional tee you cut out a section of the run, remove it, and connect the tee with a joint at each of its three ends. The line must be drained and out of service, and you have created three new joints.

A mechanical tee needs one hole cut in the wall of the intact run. The housing straddles the pipe, a collar seats into the hole to locate and seal it, and two bolts clamp the assembly. The run is never severed and its alignment is never disturbed.

The consequences are speed, far less disruption, and three fewer joints in the system. The limitation is branch size - a mechanical tee can only take a branch well below the run diameter, where a conventional tee can be full bore.

Substantially smaller than the run, with the exact ratio set by the manufacturer for each combination of run size and branch size.

Two things limit it. The hole must not remove so much material that the run's pressure capacity is compromised, and it must be within what the locating collar and gasket can seal around. Both scale with the run diameter, so a large main can accept a proportionally larger branch than a small one.

Manufacturers publish tables of permitted run and branch combinations, and these should be treated as absolute rather than as guidance. They also specify the hole diameter precisely - not the branch size, but the size of the hole to cut - and the tolerance on it.

Where the branch approaches half the run diameter or more, use a conventional grooved tee. The mechanical tee is the wrong tool at that point.

With a hole saw or a purpose-made pipe cutting tool, to the diameter the coupling manufacturer specifies, and the precision genuinely matters.

The locating collar on the tee seats into the hole. If the hole is undersized the collar will not enter and the tee cannot seat; if it is oversized the collar has nothing to locate against and the gasket may not seal. The hole must also be reasonably round and centred on the pipe crown.

Deburr the hole and remove all swarf from inside the pipe. Metal left in the line ends up in a valve seat, a pump or a sprinkler head, and on a fire system that is a serious defect rather than an untidiness.

On a wet system, drain it first unless using equipment specifically designed for hot tapping. Purpose-made cutters that retain the coupon are worth using, because a loose coupon dropped into a main is difficult to retrieve.

Extensively - branch lines and sprinkler drops off a main are one of their largest applications.

The attraction is that a sprinkler main can be installed as a continuous run and the branch outlets added where they are needed, which suits a trade where final head positions frequently change as the ceiling layout is settled. Threaded-outlet patterns take a sprinkler drop directly.

What matters on fire systems is approval. Components must carry the listings and approvals required by the governing standard for the installation, and those approvals are specific to the product, the size combination and the application. A coupling suitable for a heating main is not automatically acceptable on a sprinkler system.

Check the approval against the standard the system is being designed to, and keep the documentation - it will be asked for at commissioning.

Slightly, and the effect is on the branch rather than the run.

The run itself is essentially unaffected. The pipe is intact apart from the hole, and there is no fitting in the bore obstructing the main flow - which compares favourably with a conventional tee, where the fitting's internal geometry is in the flow path.

The branch is where the restriction sits. Flow has to turn sharply through a cut hole in a curved pipe wall and into the outlet, which is hydraulically less efficient than the smooth swept junction of a fabricated or cast tee. Manufacturers publish equivalent length or flow coefficient figures for this.

On most branch duties the effect is negligible. It becomes relevant when the branch is carrying a significant proportion of the flow, or on a hydraulically calculated system such as a sprinkler design, where the figure should be entered into the calculation rather than ignored.

Only with equipment and fittings designed for it. A standard mechanical tee requires the line to be drained.

The reason is simple: cutting the hole opens the pipe. On a pressurised line that means the contents escape, and on anything other than low-pressure cold water that ranges from unpleasant to dangerous.

Hot tapping - making a connection to a live main - is a specific technique using a purpose-built machine that seals to the pipe, isolates itself with a valve, cuts through the wall and withdraws while retaining the coupon. It is a specialist operation with its own equipment, procedures and competence requirements.

For ordinary work, plan the isolation. The advantage of a mechanical tee is that the outage is short - a hole and two bolts - rather than the extended shutdown a conventional tee needs, and that is usually enough to make the job practical.

Grooved and threaded outlets are both standard, and the choice follows what connects to the branch.

A grooved outlet continues the grooved system, taking a coupling and grooved pipe or fitting. It suits branches that will themselves be run in grooved pipework and keeps one jointing method throughout the installation.

A threaded outlet takes a screwed fitting directly, which is convenient for small branches - instrument connections, drain points, gauge tappings and sprinkler drops - where converting to grooved for a short run would be pointless.

Flanged outlet patterns also exist for connecting directly to valves and equipment.

Select on what the branch actually connects to, and check the outlet size is available for the run size you have - not every outlet type is offered across the whole size range, and it is worth confirming before the design assumes it.