A commercial building needs rewiring when its wiring can no longer carry the load safely or meet current standards. The usual causes are aged cable insulation, a switchboard that has run out of capacity, or a change of use the original wiring was never designed for. Most buildings do not need a full rewire. They need a staged partial one.
Most of the difficulty is scheduling rather than wiring. The building has to keep trading while the work happens.
What are the signs a commercial building needs rewiring?
The clearest sign is a building that trips on more than one circuit, in more than one part of the tenancy, with no single faulty appliance behind it. One dodgy fridge trips one circuit. Ageing wiring trips several, and it gets worse in wet weather as moisture finds the tired insulation.
The signs an electrician looks for on a walk-through:
- Rubber, fabric or VIR insulation. Cable from before the 1970s goes brittle and drops off at the terminations. If the switchboard tails look cracked, the rest of the run is the same age.
- Warm gear or discoloured terminals. Heat at a connection means a loose or corroded joint. Thermal imaging on the board picks this up before it fails.
- Circuits with no RCD protection. Older boards predate the requirement, and adding it often forces circuits to be split, which is rewiring work.
- Lights dimming when plant starts. Voltage drop under load points at undersized submains or a supply at its limit.
- Junctions taped up in the ceiling void. Layers of tenancy fitouts leave abandoned and joined cable above the tiles. It is a fire risk and it makes fault finding slow.
Is it a full rewire or a partial one?
Most commercial buildings get a partial rewire, because the problem is usually confined to the oldest cabling or to the circuits feeding new equipment. A full rewire is the right call when the insulation is failing building-wide, or when the fitout is being stripped back to shell anyway and the access will never be cheaper.
| Partial rewire | Full rewire | |
|---|---|---|
| What it covers | The oldest runs, a kitchen or plant room, one floor, or the circuits that keep tripping | Every submain, subcircuit and outlet from the main switchboard out, usually with a new board |
| When it makes sense | Testing shows the fault is localised, or budget has to be spread across financial years | Insulation is failing throughout, or the building is being stripped for a fitout |
| Disruption | Planned isolations circuit by circuit, business keeps trading | Areas handed over in stages, with one full outage for the board changeover |
| What it does not fix | The cabling left in place, which surfaces later | Anything upstream of the main switchboard, which is separate Level 2 work |
What usually triggers the decision?
Rewiring in a commercial building is usually forced by something else rather than chosen on its own. The common triggers:
- A change of use. Retail becoming food, an office becoming a gym, a warehouse taking on refrigeration. The circuits were never sized for the new load.
- New plant or equipment. Commercial kitchens, three-phase machinery, server rooms and EV chargers all land on a board that was full before they arrived.
- An insurer or a compliance report. A periodic inspection flags degraded cable, and cover or a lease renewal depends on fixing it.
- A fitout. Once ceilings are down and walls are open, rewiring costs a fraction of what it does in a trading building.
Not sure whether your building is due?
DM Electrical will walk the site, test the circuits that matter, and tell you plainly whether it is a partial rewire, a full one, or a switchboard problem in disguise. Fixed price against a defined scope, with the out of hours work priced in. Based in Brookvale, working commercial sites across Sydney.
How is a commercial rewire staged so the business keeps trading?
A commercial rewire is planned around the building's trading hours, not the electrician's. The work is broken into circuit groups, and each group is isolated for an agreed window while everything else stays live.
In practice that means:
- An isolation schedule agreed up front. Which circuits go off, on which night, and what the building manager tells tenants beforehand.
- Temporary supplies where trading depends on them. Refrigeration, tills, security and comms racks stay alive on temporary feeds while their permanent circuits are replaced.
- New cable pulled in live, then cut over. Most of the labour is running new cable alongside the old, which needs no outage. The outage is only the changeover at each end.
- One planned outage for the board. If the main switchboard is being replaced, there is a single hard outage to change it over, usually a few hours overnight.
In a strata or multi-tenanted building there is a second question underneath all of this: who is actually paying. That depends on whether the cable is common property or inside the lot, which we cover in who pays for electrical repairs in a strata building.
How long does a commercial rewire take?
Access drives the timeline far more than the size of the building does. A small tenancy with an accessible ceiling grid can be done in days. A whole floor with sheeted ceilings and after hours access only will run for weeks, because the crew gets a handful of usable hours per night rather than a full day.
What else gets upgraded at the same time?
Rewiring usually forces related work, and that work is cheaper done in the same visit. Expect the scope to also cover:
- The switchboard. New circuits need somewhere to land, and older boards rarely have the space or the RCD protection. See switchboard upgrades.
- The consumer mains. If the building's total demand has grown, the cable feeding the board may be the real bottleneck. Our guide to consumer mains upgrades covers how that is sized, and it is Level 2 ASP work.
- Three-phase supply. Commercial kitchens, plant and larger air conditioning often need it, and it is easier to bring in while the board is open.
- Exit and emergency lighting. The circuits feeding them get touched during a rewire, so testing and certification to AS/NZS 2293 belongs in the same scope.
Frequently asked questions
How do I know if my commercial building needs rewiring?
Look for faults that repeat across more than one circuit rather than one bad appliance. Breakers tripping in different parts of the tenancy, warm or discoloured switchboard gear, rubber or fabric insulation crumbling at the terminations, and circuits with no RCD protection are the common ones. An electrician confirms it with insulation resistance testing and a look inside the board and ceiling void.
Can a business stay open during a commercial rewire?
Usually yes. A rewire is broken into circuit groups and each group is isolated for a planned window while the rest of the building stays live. Retail and hospitality sites are normally done after close, offices over weekends, and warehouses around shift changes. A full outage is only needed for the main switchboard changeover, typically a few hours overnight.
Do I need a Level 2 electrician for a commercial rewire?
Only for the parts that touch the service connection. The wiring inside the building is standard licensed electrical work, but upgrading the consumer mains, moving the metering, or reconnecting the main switchboard to the network must be done by an accredited Level 2 ASP. DM Electrical holds Level 1 and Level 2 ASP accreditation, so one team can cover both sides.
How much does commercial rewiring cost in Sydney?
It is quoted per building rather than by a rate, because access and timing move the price more than the cable does. The main drivers are the number of circuits, whether ceilings and walls are open or sheeted, how much of the work has to happen outside trading hours, and whether the switchboard and consumer mains need upgrading at the same time.
The bottom line
If your building trips across several circuits, has cable old enough to crumble, or has quietly changed what it is used for, get the wiring tested before the next failure picks the timing for you. Most buildings need a staged partial rewire rather than a full one, and staging is what keeps the doors open while it happens. DM Electrical works commercial sites across the Northern Beaches, North Shore, Eastern Suburbs and greater Sydney as a commercial electrician. Get in touch and we will tell you what your building actually needs.