Adding a Table Saw to a 16 A Workshop Radial
A technician is asked to check whether a new 2.2 kW table saw can be added to a workshop circuit. The workshop has a 16 A radial circuit feeding three double 13 A socket outlets in 2.5 mm squared twin and earth, clipped direct for most of its 18 m run, but passing through 3 m of wall insulation. Already in use at the same time are a 1.8 kW fan heater and a 1.5 kW dust extractor. The saw's rating plate reads 2.2 kW, 230 V, with a starting current around 2.5 times running current. The consumer unit has a 30 mA RCD, and the sockets are not shuttered to a particular IP rating. The room is 5 x 4 m with one window, and the extractor hose currently vents back into the room.
1. Calculate the running currents, decide whether the existing 16 A radial can carry the new load, and set out the options.
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Currents (I = P / V, single phase 230 V):
- Table saw: 2200 / 230 = 9.6 A
- Fan heater: 1800 / 230 = 7.8 A
- Dust extractor: 1500 / 230 = 6.5 A
- Total coincident running load: about 23.9 A
Cable check. 2.5 mm squared T&E clipped direct has a current-carrying capacity around 27 A, but the 3 m through thermal insulation derates it heavily - realistically to about 20-21 A. The 16 A breaker protects the cable, so the cable is not the immediate problem.
The problem is the load: 23.9 A exceeds the 16 A protective device. Diversity does not rescue this - a workshop heater, extractor and saw genuinely run together, and the saw is the load that will run longest on a cold morning.
Options, in order of preference:
- Give the saw its own dedicated radial (20 A in 2.5 mm squared, or 4 mm squared if the derated run is long) - separates motor load from heating load and removes the nuisance-trip risk.
- Interlock so the heater and saw cannot run simultaneously - cheap, but unpopular in winter.
- Upgrade the existing circuit to a 32 A ring or a 4 mm squared radial if the installation method and board allow, after a full check by a competent person.
Do not simply fit a larger breaker on the existing 2.5 mm squared derated cable.
2. The saw trips the breaker occasionally on start-up. Explain the likely causes and how the design should deal with motor starting, voltage drop and the derated cable.
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Starting current. 9.6 A running x 2.5 = about 24 A for a second or so. A 16 A Type B MCB trips magnetically between 3 and 5 times rated current (48-80 A), so a healthy saw should not trip it magnetically - but a 16 A device already carrying 14 A of heater and extractor is close to its thermal threshold, and repeated starts add heat. A Type C or motor-rated device may be appropriate, but only after the cable and load calculation support it.
Voltage drop. Roughly 18 mV/A/m for 2.5 mm squared: 18 x 24 A x 18 m / 1000 = about 7.8 V, near 3.4% while starting. On a long run this shows as slow starts, hot motor windings and dimmed lights. A 4 mm squared cable or a shorter dedicated run fixes it.
Derating. The 3 m of insulation means the cable cannot shed heat; any upgrade must recalculate the capacity for the installation method, not just swap the breaker.
Design responses: dedicated circuit; correct MCB curve for a motor; RCD protection retained; a local isolator within reach of the operator; motor-rated socket and plug (not a standard 13 A plug on a 2.2 kW motor if the manufacturer specifies otherwise); and a documented load schedule for the room so the next machine is not added to the same circuit.
3. The extractor discharges back into the room. Explain why that is a building services and health problem here, and what should change.
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Air quality. A dust extractor that returns air to the room is only acceptable with filtration fine enough to protect health - typically a fine filter with a rating well below the dust being produced. MDF and fine sanding dust are the worst case: the respirable fraction stays airborne for hours, and the workshop is 5 x 4 m with one window. Dust also settles on machine windings and light fittings, which is a fire and reliability issue.
Moisture. Warm, dusty air discharged back into a cool room raises local humidity and can cause condensation on cold surfaces; if the extractor is ducted outside instead, the make-up air must come from somewhere, or the room goes into negative pressure and pulls damp air through the building fabric.
What should change:
- Duct the extractor to outside with a short, smooth, large-radius run and a backdraught flap.
- Provide a designed make-up air path - a louvre or trickle vent - so the extractor can move the air it is rated for.
- If recirculation is unavoidable, specify fine filtration, keep the unit serviced, and add general ventilation to give several air changes per hour.
- Add a CO2 or dust monitor, or at minimum a rule that the extractor runs whenever a machine runs and for ten minutes after.
- Check that the RCD covers all socket outlets and that the extractor and saw are interlocked or clearly signed as a pair.