Start here: mains electricity kills people, and this article is an orientation, not a qualification. If you are building something that plugs into a wall and you are not confident about what follows, the correct move is to hire a qualified electrician or buy commercial equipment. That is not a disclaimer — it is the single most useful piece of advice in this tutorial.
What follows is the vocabulary and the reasoning, so that you can make competent decisions about what you build yourself, what you buy, and what you hand to someone else.
Work out your load first
Everything downstream depends on this number.
Add up the power draw in watts of everything in the piece. Then:
current (A) = power (W) / voltage (V)
A 600 W total on 230 V is about 2.6 A; on 120 V it’s about 5 A. Use the voltage of the country the piece is going to, which matters more than you’d think for touring work.
Two rules of thumb worth internalising:
- Derate. Design for about 80% of any circuit’s rating, continuous. A 13 A circuit should not be carrying 13 A all afternoon.
- Inrush is not steady state. Motors, big LED supplies and anything with a large capacitor draw far more at switch-on than they do running. A rig that trips the breaker only when you turn it on is telling you about inrush.
The fuse protects the cable, not your circuit
This is the most commonly misunderstood idea in the whole subject, and getting it backwards is how fires start.
A fuse or breaker exists to stop the wiring from carrying more current than it can survive. It is sized to the cable, not to your device. A fuse does not protect your electronics — by the time it blows, your electronics are already gone.
So: the fuse rating must be at or below what the cable can safely carry. Fitting a 13 A fuse to a device on thin flex because that’s the fuse that came in the plug is a genuine hazard, and it’s extremely common.
Earthing, and Class I versus Class II
Class I equipment has a protective earth. Exposed metal is bonded to earth so that if a live conductor touches the chassis, current flows to earth, the protective device trips, and the case never becomes live. If your piece has an exposed conductive enclosure and mains inside it, it must be earthed.
Class II (“double insulated”) has no earth and relies on two independent layers of insulation. Most plastic-cased consumer equipment is Class II.
You do not get to mix these casually. A metal enclosure with an unearthed mains supply inside is the classic dangerous homebuild.
The inlet: use an IEC module
Do not bring a captive mains cable into an enclosure through a hole.
Use a panel-mounted IEC C14 inlet — ideally a switched fused IEC module, which combines the inlet, a double-pole switch and a fuse holder in one part. Benefits:
- Detachable cable, so the piece can be transported and the cable replaced
- A fuse right at the entry point, protecting everything after it
- Double-pole switching, breaking both live and neutral
- A standard, tested, approved component rather than your own arrangement
Inside the enclosure: mains conductors terminated properly in an enclosed connector or terminal block, never twisted and taped; strain relief at the entry; adequate clearance between mains and low-voltage wiring, ideally physically separated; and insulated, shrouded terminals wherever a finger could reach.
RCD / GFCI protection
An RCD (Europe) or GFCI (North America) detects current leaking to earth — through a person, for instance — and disconnects in milliseconds. It protects people; fuses protect cable. They are not substitutes for each other.
For installation work: use an RCD-protected supply, always, and especially anywhere near water, outdoors, or where the public can reach the piece. A plug-in RCD adapter costs very little and there is no argument for not having one.
The venue will ask, and they should
Most professional venues in the UK, Europe and Australia will require PAT testing (portable appliance testing) on anything you plug into their supply, and many will refuse to power equipment that hasn’t been tested. North American venues often ask for listed equipment or an electrician’s sign-off instead.
Plan for this rather than discovering it on install day. Practical consequences:
- Homebuilt mains equipment is harder to get approved than a commercial power supply feeding your low-voltage electronics
- The easiest legal and practical path is to keep mains out of your work entirely — use an external commercial PSU or power brick, and build everything after it at 5 V, 12 V or 24 V
- Keep documentation: what draws what, what’s fused at what rating, what’s earthed
That last point is the real recommendation of this tutorial. The safest installation is one where you never wired mains at all. A certified external supply moves the dangerous, regulated, liability-bearing part of the system to a manufacturer who has tested it — and leaves you doing the interesting work at voltages that cannot kill you.