Power, Lighting and Compressed Air
Circuit load and breakers, cord voltage drop, lighting levels, compressor and tank sizing, and dust collection airflow.
Circuits and loads
Each circuit has a breaker that trips when it carries too much current. Current is power divided by voltage, so the same tool draws twice as much on 120 V as on 230 V. Loads that run continuously should stay at about 80 % of the breaker rating.
I = P ÷ V, continuous limit ≈ 0.8 × breaker rating
Worked example: saw and dust extractor
- A table saw of 1 500 W and a dust extractor of 1 000 W together draw 2 500 W.
- At 120 V: 2 500 ÷ 120 = 20.8 A, above a 15 A breaker and well above its 12 A continuous limit (1 440 W). It will trip: put them on separate circuits.
- At 230 V: 2 500 ÷ 230 = 10.9 A, within a 16 A breaker and its 12.8 A continuous limit (2 944 W).
- Start-up surge. The saw alone runs on 12.5 A at 120 V, but an induction motor draws about 6 times that for a fraction of a second: 75 A. A slow-blow (type D or similar) breaker or a soft starter avoids nuisance trips.
- Plan the circuits: a circuit for lighting and sockets, a circuit for each large machine, and one for the bench.
- Count only what runs together. The sum of everything is not the load; the sum of the tools that run at the same time is.
- Fixed wiring is the electrician’s job. A new circuit, a sub-panel or a 230 V outlet should be done by a qualified person to your local code.
Extension cords and voltage drop
A cord has resistance. The current flows out and back through it, so the drop is 2 × length × resistance per meter × current. Voltage lost in the cord is power lost as heat in the cord and a lower voltage at the tool, which starves a motor and overheats it.
Worked example: a 30 m cord to a saw
- 14 AWG (8.29 Ω/km) at 15 A: 15 m loses 3.1 %; 30 m loses 6.2 %, about 7.5 V, leaving 112.5 V at the saw.
- 12 AWG (5.21 Ω/km): 3.9 % at 30 m. 10 AWG (3.28 Ω/km): 2.5 %.
- For a limit of 3 % at 15 A the longest cord is 14 m in 14 AWG, 23 m in 12 AWG and 37 m in 10 AWG.
- Heat in the cord: the 14 AWG cord dissipates 15² × 2 × 30 × 0.00829 = 112 W. A coiled cord cannot lose that heat and can overheat; uncoil the full length when the tool is under load.
- Use a short cord of the right gauge, or move the tool nearer a socket.
- Check the cord’s rating against the tool’s current and inspect it for damage every time.
- Keep cords out of the cut path and off wet floors; use a GFCI/RCD at the start of the cord.
Wiring and protection rules
- RCD / GFCI protection on all shop sockets, so a fault to earth cuts the supply fast.
- Do not chain adapters or power strips; use one strip of the right rating.
- Isolate machines with a switch within reach, and a no-volt release (a restart after a power cut must be a deliberate act).
- Label the breakers and keep the panel clear.
- Fire detection: a smoke or heat alarm in the shop, and a rated extinguisher at the door.
Lighting
Illuminance is measured in lux (lumens per square meter). A shop wants about 300 to 500 lux generally and 750 to 1 000 lux at the bench for fine work. The lumens needed are the area times the lux, divided by the fraction of light that reaches the work (the utilization, 0.4 to 0.6 in a small room).
Worked example: lighting a 12 m² shop
- The work needs 12 × 500 = 6 000 lm. With utilization 0.5 the lamps must give 6 000 ÷ 0.5 = 12 000 lm.
- 1.2 m LED tubes give about 2 500 lm each, so 5 tubes. At 100 lm/W the power is 120 W (0.12 kWh per hour of use).
- Task light at the bench: a lamp of 300 candela at 0.6 m gives I ÷ d² = 833 lux; bring it to 0.3 m and it gives 3 333 lux: the inverse-square law makes a close lamp the cheapest way to light fine work.
- A bench of 0.90 m² at 1 000 lux needs 900 lm on the surface, about 1 500 lm from the lamps at a utilization of 0.6.
- Color quality: a color rendering index (CRI) of 80 or more, and 4 000 to 5 000 K (neutral to daylight white).
- Avoid glare and shadows: light from both sides and above, and diffuse it.
- Avoid flicker: bad drivers flicker, which makes a rotating tool appear to stand still (the stroboscopic effect), which is dangerous.
- Windows help but do not rely on them: daylight comes and goes.
Compressed air
Air tools and spray guns use a flow of air at a pressure. Compressors are rated by the flow they deliver at a stated pressure (CFM, or L/min, at 90 PSI or 6 bar), and a tank smooths the supply.
Worked example: compressor and tank size
- An air tool uses 4 CFM at 90 PSI and runs half the time: average 2.0 CFM (57 L/min). With 50 % margin for leaks and fittings: 3.0 CFM (85 L/min) delivered.
- The tank is a buffer. A 100 L tank between 8 and 5 bar stores 100 × (8 − 5) ÷ 1.01 = 296 L of free air.
- If the tool runs continuously at 113 L/min but the compressor delivers 85 L/min, the tank supplies the difference of 28 L/min and lasts 10.5 minutes before the compressor runs out of buffer.
- A 2 hp (1.49 kW) compressor draws about 16 A at 120 V (efficiency and power factor 0.8): it needs its own circuit.
- Use the delivered CFM at 90 PSI, not the “peak horsepower” or displacement figure.
- Drain the tank every day; fit a filter and a regulator at the point of use, and a dryer for spray work.
- Hearing and eye protection for compressors and air tools; never point an air line at skin: air can enter the bloodstream.
- Pipe the shop in 1/2 inch (13 mm) pipe or larger with a slope to a drain; a long thin hose loses pressure.
Dust collection airflow
Wood dust is carried along a duct by air; below a conveying velocity of about 20 m/s (4 000 ft/min) it settles. The flow is the duct area times the velocity, so the duct size sets the airflow a machine needs.
Worked example: duct size and airflow
- A 100 mm duct has an area of 78.54 cm². At 20 m/s the flow is 0.008 × 20 = 0.16 m³/s = 9.4 m³/min (333 CFM): about what a table saw or planer wants.
- A 50 mm hose carries only 2.4 m³/min at the same velocity (a quarter of the area). A shop vacuum delivering 3 m³/min through it moves air at 25 m/s, good for a sander or a small saw but not for a planer.
- Long runs, bends and filters take much of the fan’s pressure and cut the real flow a lot; choose a collector with margin and keep the hose short and smooth.
- Fine dust (below 10 µm) goes through cheap bags into your lungs: use a good filter or a cyclone, wear a mask and see Workshop Safety for the health and explosion risks.
Troubleshooting
| Problem | Likely cause | Try |
|---|---|---|
| Breaker trips when a machine starts | Start-up surge; too many tools on one circuit; undersized cord | Separate circuits; slow-blow breaker; check the cord gauge |
| Motor hot and slow on a long cord | Voltage drop in a thin or coiled cord | Shorter or thicker cord; uncoil it |
| Shop is too dim | Too few lamps or old fittings | Add LED tubes and a task light using the lumen estimate |
| Compressor runs all the time | Leaks, too small for the tool | Soapy-water leak test; larger compressor or tank |
| Water in the air line | Tank not drained, no dryer | Drain daily; add a filter and a dryer |
| Dust everywhere | Duct too small, collector too weak, a leak | Check the hose and joints; measure velocity; improve the filter |
Related
- Shop Tools: the overview of this guide.
- Planning the Workshop: Space, Layout and Workflow: the layout and ventilation.
- Power Tools and Shop Machines: the tools that draw this power.
- Electronics Fundamentals and Workshop Safety.
FAQ
Do I need a dedicated circuit for my workshop?
It is a good idea, especially for big machines: a shop on a shared household circuit trips more often. Fixed wiring should be done or checked by a qualified electrician, and all sockets should have RCD/GFCI protection.
How long can an extension cord be?
As long as the voltage drop stays under about 3 %: at 15 A that is about 14 m in 14 AWG, 23 m in 12 AWG and 37 m in 10 AWG.
How much light does a workshop need?
About 300 to 500 lux generally and 750 to 1 000 lux at the bench; multiply the floor area by the lux and divide by 0.5 to find the lumens.
What size compressor do I need?
Add up the CFM of the tools that run together at 90 PSI, apply the duty factor, then add about 50 % margin, and compare with the delivered CFM (not the peak rating).
References
- NFPA 70, National Electrical Code (or IEC 60364 and your local code): circuits, cords and GFCI/RCD.
- EN 12464-1 and the IES Lighting Handbook for workplace illuminance.
- ACGIH, Industrial Ventilation: A Manual of Recommended Practice (conveying velocities for dust).
- Compressor makers’ sizing guides (delivered air flow at 90 PSI).
Last reviewed: October 2026. Power, lighting and airflow figures are examples: check local codes and equipment ratings, and use a qualified electrician for fixed wiring.