How to Plan a 20 A Workshop Circuit
A step-by-step sizing walkthrough for a dedicated 20 A garage or workshop circuit. Each step uses one of the electrical calculators below, so you can follow along with your own cable run length, voltage and material. Covers IEC 60364 (EU/UK) and NEC (US/CA) in parallel.
Last updated: May 2026
The scenario
You are installing a dedicated circuit from a consumer unit (distribution board) to a detached garage or workshop. The circuit will feed: one 16 A outlet for a table saw or router table (2.0 kW at 230 V), two 13 A general outlets for hand tools and battery chargers, and a 6 x 5 W LED strip light. The cable runs 15 m one-way through a conduit in the wall. System voltage is 230 V AC single-phase.
The five steps below take you from a blank sheet to a signed-off specification, with the exact numbers to enter in each calculator.
Step 1: Define the circuit load
Add up the loads that could run at the same time. For this workshop:
| Load | Rated current | Notes |
|---|---|---|
| Table saw (2.0 kW) | 8.7 A | Intermittent: picks the heaviest single tool |
| Hand tool outlet (worst case: grinder 1.2 kW) | 5.2 A | Intermittent |
| LED strip 30 W | 0.1 A | Continuous |
| Total simultaneous | 14.0 A | Fits inside a 16 A or 20 A breaker |
The total simultaneous load is 14 A. A 20 A circuit gives comfortable headroom for the inrush current of the table saw motor at startup (typically 6 to 8 x rated current for 1 to 3 cycles) and for adding a second heavy tool later.
Is any load continuous? A load is continuous if it runs at more than 80% of its rated current for 3 hours or more (IEC 60364) or 3 hours or more (NEC 210.19). The LED lighting qualifies as continuous; the power tools do not. Because the continuous fraction (0.1 A) is tiny, the circuit as a whole is treated as non-continuous for sizing purposes.
Step 2: Size the wire gauge
For a 20 A circuit with intermittent loads, the design current equals the breaker rating: 20 A. The cable must have an ampacity of at least 20 A in conduit at 30 °C ambient.
- IEC 60364: 2.5 mm² copper carries 25 A in conduit. This exceeds 20 A, so 2.5 mm² satisfies the ampacity constraint.
- NEC: AWG 12 (3.31 mm²) is the minimum for a 20 A branch circuit per NEC 210.19(A).
Now check the voltage drop for a 15 m run. The IEC formula for single-phase is:
V_drop = 2 × 0.0172 × 15 × 20 / 2.5 = 4.1 V (1.8% of 230 V)
1.8% is inside the IEC 3% limit, so 2.5 mm² (AWG 13) is the correct choice for this run. If your run were longer than about 28 m, voltage drop would push you to 4 mm².
Use the calculator: Enter 20 A, 15 m, 230 V single-phase, copper, 3% limit into the Wire Gauge Calculator. The result should confirm 2.5 mm² (AWG 13).
Step 3: Size the circuit breaker or fuse
For non-continuous loads, the breaker must be rated at or above the design current. The next IEC standard size at or above 20 A is 20 A exactly. For motor loads like the table saw, check that the breaker type handles the startup inrush:
- MCB Type B (trips at 3 to 5 x rated): may nuisance-trip at startup if the saw draws 6 x rated at turn-on. Acceptable if the saw starts reliably under no-load conditions.
- MCB Type C (trips at 5 to 10 x rated): better for workshop motor loads. Recommended where startup current is a concern.
- NEC equivalent: a 20 A circuit breaker (OCPD). Motor branch circuits may use a time-delay fuse up to 175% FLA per NEC 430.52.
Use the calculator: Enter 20 A, non-continuous into the Fuse / Breaker Sizing Calculator to confirm 20 A and see the derating table.
Step 4: Verify voltage drop for your actual run
The calculation above used 15 m. If your garage is farther, or the cable routes around obstructions, the actual one-way length will differ. Use the voltage drop calculator to check your specific run with the wire size chosen in Step 2.
| Run length (one-way) | V-drop at 20 A, 2.5 mm² | Drop % | Action |
|---|---|---|---|
| 10 m | 2.8 V | 1.2% | 2.5 mm² is fine |
| 15 m | 4.1 V | 1.8% | 2.5 mm² is fine |
| 20 m | 5.5 V | 2.4% | 2.5 mm² is fine (within 3%) |
| 28 m | 7.7 V | 3.3% | Upgrade to 4 mm² to stay within 3% |
| 40 m | 11.0 V | 4.8% | Use 6 mm² for 3% limit |
Use the calculator: Enter your actual one-way run length, 20 A, 2.5 mm², 230 V single-phase, copper into the Voltage Drop Calculator. If the drop exceeds 3%, go back to Step 2 and increase the wire size.
Step 5: Check panel capacity
Before ordering materials, confirm your consumer unit has room for the new 20 A breaker and that the total load does not exceed the incoming supply fuse rating. A typical EU home supply is 25 A to 63 A single-phase; a 200 A service is standard in North America.
The new workshop circuit adds a maximum of 20 A to the demand. If your consumer unit is already loaded close to the supply limit, sum all breakers that could be on simultaneously and compare against the incoming fuse rating. In practice most residential installations are well below the supply limit, but garages and outbuildings with electric heating, EV chargers or compressors can push the total.
Use the calculator: Enter your total connected load into the Power Supply Calculator to check headroom against your supply rating.
Quick reference: 20 A workshop circuit summary
| Parameter | IEC 60364 (EU/UK/AU) | NEC (US/CA) |
|---|---|---|
| Breaker rating | 20 A MCB Type C | 20 A circuit breaker |
| Minimum wire (0 to 20 m) | 2.5 mm² copper | AWG 12 (3.31 mm²) |
| Minimum wire (20 to 35 m) | 4 mm² copper | AWG 10 (5.26 mm²) |
| Voltage drop limit | 3% IEC 60364 / 5% NEC total | 3% branch / 5% total |
| RCD/GFCI required | 30 mA RCD for garage circuits (IEC) | GFCI required for garage outlets (NEC 210.8) |
| Earth/ground conductor | Same cross-section as phase up to 16 mm² | AWG 12 equipment grounding conductor |
| Conduit or armoured cable | Required where mechanical damage is possible | Required in unfinished spaces (NEC) |
Common mistakes
- Undersizing for run length: using the minimum ampacity wire (2.5 mm²) for a 30 m garage run and ignoring voltage drop. At 30 m and 20 A, drop is 8.3 V (3.6%), outside the IEC limit. Use 4 mm².
- Skipping the earth conductor: the protective earth must run with every circuit. Do not rely on conduit metal or building steel for earthing continuity.
- Wrong MCB type: fitting a Type B breaker in a workshop with motor loads and then wondering why it trips at startup. Use Type C (or a time-delay fuse for motor circuits in North America).
- Sharing the circuit: plugging a compressor and a welder on the same 20 A outlet circuit. Each high-draw tool deserves its own dedicated circuit.
- Forgetting the RCD: IEC 60364-7-706 requires a 30 mA residual current device for all circuits in garages and agricultural buildings. NEC 210.8 requires GFCI protection for all garage outlets.
Frequently Asked Questions
What wire size do I need for a 20A circuit at 230 V?
For a run up to about 28 m at 230 V single-phase: 2.5 mm² copper (IEC) or AWG 12 (NEC). 2.5 mm² has a conduit ampacity of 25 A, which covers the 20 A breaker with margin, and keeps voltage drop under 3% for most garage runs. For runs of 28 to 45 m, increase to 4 mm² to stay inside the 3% voltage drop limit. Use the Wire Gauge Calculator with your exact run length for a precise answer.
Can I use a 25A breaker on a 20A circuit?
No: the breaker protects the cable, not the load. If the cable is rated for 20 A (2.5 mm² in conduit), the breaker must not exceed that ampacity. Fitting a 25 A breaker on 2.5 mm² cable means a fault could overheat the cable before the breaker trips. If you need a 25 A circuit to run larger loads, use 4 mm² cable (ampacity 32 A in conduit) with a 25 A breaker.
Does the workshop circuit need a separate RCD?
In most EU and UK jurisdictions, yes. IEC 60364-7-706 and Part P of the UK Building Regulations require a 30 mA RCD protecting all circuits in a garage, outbuilding or workshop. In North America, NEC 210.8(A)(2) requires GFCI protection for all 120/240 V outlets in garages. The RCD/GFCI trips within 30 ms when it detects a 30 mA earth leakage current, providing shock protection in wet or damp environments.
Can I run the workshop circuit with aluminium cable?
For a 20 A branch circuit, copper is strongly preferred. Aluminium branch-circuit wiring is permitted by NEC (with aluminium-rated devices, anti-oxidant compound and careful torquing) but is generally restricted to conductors 35 mm² and larger for IEC fixed wiring. At 20 A, you would need 4 mm² aluminium instead of 2.5 mm² copper (aluminium ampacity is about 78% of copper). For a garage circuit, the cost difference between copper and aluminium at 2.5 to 4 mm² is negligible. Use copper.
How many outlets can I put on a 20A workshop circuit?
As many as you like physically, but the total simultaneous load must stay within 20 A. In practice, one high-draw outlet (for a table saw, compressor or welder) plus two or three general outlets is a common and workable arrangement. If you regularly run two large tools at the same time, install two separate 20 A circuits rather than overloading one. IEC and NEC both allow multiple outlets on one circuit, but only one heavy tool should run at a time per circuit.