Home-built machine: how to make the 110 V AC electrical installation part "by the book"? (safety, "code")

Question

I am building a machine to manufacture optical components. The source of mechanical power is a 1/4 HP motor that's plugged into 110 V AC. Various parts of the machine receive mechanical power via belts and pulleys. The machine is a large square-ish box made of plywood. Inside it, there's the powerhead module containing the motor, hooked up to the machine via belts.

Grinding / polishing the optical components takes place on top of the machine. The process can be occasionally messy, with water-based slurry droplets being sometimes splashed over the top side.

Here are some images of previous machines made by other folks. The front/right/outside:

The back/inside, with the powerhead module, and the motor almost completely hidden behind the central plate of the powerhead (see the AC cable going to the motor over that plate):

I want to do the 110 V AC part "nice", or "by the book". Near the front/right corner (closest to camera in the first image) I want to add a panel (a small box), jutting out of the big box, containing: the main AC switch, a 16x2 LCD and microcontroller, and perhaps a circuit breaker.

I want the main AC cord to go inside the machine, lay close to the bottom edge back-to-front, then up the front/right edge, come out of the box through a hole, and into the front panel. There, it should go into the main AC switch. From there, it should split in two:

One branch should go into the circuit breaker, then back into the big box through a hole, and then into a regular 110 V AC wall socket mounted inside the big box. The powerhead should plug into that internal 110 V socket, for easy removal (the pulleys on the powerhead need to be adjusted or changed periodically).

The other branch should go into a 5 V "wall wart" supply, and then into the microcontroller connected to the LCD display. All of this branch should stay inside the front panel.

I've done lots of low-voltage electronics design, analog and digital. But I have very limited experience with mains voltage AC, building "to code", etc.

I assume there are some rules to follow when designing the AC part for a machine like this, but what are those rules? In terms of parts and components - should the internal AC wall socket be contained in a box of sorts? Should I run the AC cables inside conduits? (and what kind?)

I assume the rules can't be too different from building a house "to code".

Answer 1

The things to worry about are 1) strain relief; 2) having an appropriately rated and sized fuse or circuit breaker; 3) having a large, easily-operated, within reach of the operator, power on-off switch or other control (http://dx.com/p/ac-220-380v-waterproof-on-off-push-button-switch-167441); 4) making sure everything is properly grounded; and 5) appropriate use of conduit for fire and abrasion protection. Use a C13-style connector mounted on the side of the unit and a standard power cord if the device is portable, or conduit if nonportable. An internal AC receptacle should be enclosed in a standard metal box.

If, as you say, there is water or moisture involved with the operation of the unit then the power source (wall socket) must be equipped with a Ground Fault Circuit Interrupter (GFCI).

Generally you won't have code-compliance issues if you're not manufacturing the units for sale and if your unit looks professional (no sloppy or frayed wiring). For legal advice consult a licensed attorney.

If your unit could injure someone if not shut off immediately (e.g. if a worker's hair or clothing were to become stuck in the mechanism) consider a pull-string or other emergency shutoff mechanism. The string would be connected to a shorting plug the socket for which is wired in series with the mains.

Answer 2

A different parts bin

First, anything involving mains power is not bought from DealExtreme, Banggood, Alibaba, AliExpress, eBay or Amazon.

Nor is it bought from Mouser, Digi-Key or RS Electronics. Most master electricians have never heard of those places.

The mains parts bin comes from home supply stores (Home Depot) and proper electrical supply houses. There is no dominant mail-order vendor for electrical equipment because the stuff is too heavy and low-value to ship. You can tire-kick at Platt, but don't pay their prices.

The gear

The general nuts and bolts you use for mains inside cabinets etc. are steel boxes, e.g. the $1.00 "Handy-Box", EMT thinwall conduits, FMC/Greenfield conduit, nipples and fittings, and common 50 cent snap switches and receptacles. More on conduit here. Metal conduit is a very safe plan for the inexperienced: just keep everything "mains" inside a full metal jacket, and it's easy to know when you've done that.

Handy-Box. Provides a standard form-factor that most switches, receps and GFCIs attach to. Knockouts knock ... out ... to allow fitting EMT conduit pipes or cable entry via a cable clamp/strain relief. Note raised nub for attaching #10-32 ground screw. If the boxes are connected with EMT metal conduit, that carries the ground everywhere else.

Simply site the Handy-Boxes where you want inlets, switches and outlets, and link them with EMT or MC conduit.

They make other boxes with extra flanges, holes or fittings for flush mounting e.g. on a machine.

You wouldn't want to put a GFCI inside the machine where one has to open it up to reset it.

Here's a nifty thing: A GFCI switch. This is heavy-duty-rated to use the GFCI trip as an ordinary on-off switch. You shut the tool off by tripping the GFCI, and turn the tool on by resetting. If this was the first stop your power cord made, it may satisfy any GFCI requirement while doubling as your on/off switch. Another option is this GFCI recep and switch, giving a convenience outlet e.g. for a work light.

For circuit protection, you generally don't need it especially if the motor has an overload breaker of its own. However if you really want it, you use a COTS fuse-switch intended for a furnace. I've also seen this critter in a variant offering a fuse and a single receptacle socket.