I did mine last year when I renovate and also did rewiring for the entire terrace house. In this case I used my electrical contractor. However, as my contractor is not under TNB panel, then he will arrange for a "TNB runner" to submit the application & liaise with TNB regarding this.

Indicative Cost:
1) 3-phase meter board with 3x 63A cut-out fuse & neutral connector : RM900

2) TNB runner fee : RM1,100

3) TNB connection fee : RM300 (for terrace house, tap directly from the supply line running across the row of houses, this should be what you need to pay. Expect to pay more if require relocation of cables, pulling new cables from pole, or even erecting poles)

4) 3-phase Distributiion Box (DB) with Main Switch MCB, RCCB & set of MCBs (the cost varies depending on brand, size, and whether you will have more than 1 DB) - in my case RM1,800 (rather huge 4-row DB)

Rewiring is not required as your electrician is supposed to do load-balancing by assigning circuits to the 3 phases (red, yellow, blue phases) - e.g. if you have 6 air-conds, ideally 2 air-conds on each phase, same goes for lighting points, water heater, sockets, etc. Get your electrician to quote you as there may be manpower charges for this.

However, if you plan to rewire the entire house, expect to spend an additional RM10,000 or more, depending on the number of points you wish to have.

In my case, the process took around a month.