I am thinking about constructing a few ballast boxes, and I’m wondering what connector I should use for the output.
I have considered using the IEC C series connector, but I think it is too standard and somebody could plug something else that isn’t meant to be plugged into it by mistake.
The other connector I have considered using is the stage pin connector, which is commonly used in theatrical lighting (often on dimmed circuits) here in America, and comes in sizes from as little as 20A all the way up to 100A

What connector do you use?

I have never made a ballast box so I can't say what I use, but these are some recommendations:

- Stage Pin
A little bulky, kind of annoying to use sometimes because it is flat, but definitely does the trick.

- NEMA ML-2 or NEMA L5-15
Two different locking versions of a normal 120V 15A outlet. Connectors are easy to find everywhere, can use normal electrical boxes for receptacles.

- Neutrik PowerCON
Compact 120V 16A locking connector, looks very professional and is finger-safe, but they only come in blue for some reason.

Get ordinary plug and socket of another country -
Choose one that won't fit into any common socket or adapter in your country. (But there is risk of somebody using adapters, especially the Chinese ones with multiple holes that accept all plugs)

Swiss SN441011 won't fit into any other socket worldwide. The 16A variant is less common, and might have thick enough pins to prevent plugging into adapters

Italy CEI23-50 won't fit into any other socket worldwide. The 16A variant is less common, and have pins spaced far apart so it might prevent plugging into adapters
 - The socket is not recessed, and the sleeving on the pins does not make it "fool proof" at HID ignitor voltages. (Don't stick your fingers to the pins)
 - The plug is not polarised, but this generally is not an issue for any HID lamp

British BS546 won't fit into any other socket, but in itself is fairly common around the world. The 15A variant have thick pins sufficient to prevent plugging into adapters
 - The plug itself is huge, but this is not a bad thing at HID ignitor voltages

Israel SI32 won't fit into any other socket worldwide, but does fit into most adapters



Use IEC 60309 industrial connectors
 - Some types (typically Blue 2P+E 16/32A 6h and Red 4P+E 16/32A 6h) are fairly common in industrial use, so in factories etc there will be sockets where it can be plugged in and availability of adapters
 - There are more special (and harder to find) plugs in the 60309 series that are not compatible with anything else, like Green 2h for 400Hz



Modify an IEC C connector to make your own special version
 - https://www.lighting-gallery.net/gallery/displayimage.php?album=2096&pos=10&pid=58040
 - Keep clearance betwen any conductors you add and the L pin, especially considering HID ignitor voltages



If you use flex cable or moulded plugs with cable - Some flexes - especially the thin ones in the 0.5mm2 range (20AWG) or so - Have too thin insulation to withstand HID ignitor voltages at all

In all cases, any cable after the ignitor does pose some fire risk. If the cable gets damaged or its insulation fails, it will be subjected to ignitor voltage that will keep arcing going, and the ballast will prevent the tripping of any breakers, possibly including AFCIs

If this have to work unattended, either design it so the ignitor is in the luminaire and not in the ballast box, or use shielded cable and GFCI

@Multisubject Thanks for bringing the NEMA ML-2 to my attention! The L5-15 is reasonably popular but the ML-2 is a lot more uncommon, so I think that is a good option. The PowerCON is also an interesting item and I will consider it.

@Ash Also a good idea, but the consequence of said plugs and sockets being foreign is that they are really hard to obtain here, and shipping is often extremely expensive or most sellers won’t even ship overseas (I am all too familiar with this as the plugs and sockets of the world are one of my many interests)
I will definitely consider the idea of modifying an IEC or some other connector, but it is slightly more involved than using a premade plug and socket.

I agree with @Ash in suggesting the use of IEC 60309 connectors, those are hardly ever used here though they are a little bulky.

I just thought of the possibility of using an IEC C19/C20, those are pretty much only used in high power 240V computing applications, so I don't think anyone will end up misusing it. They are pretty compact, and of course finger safe as well. They can handle 16A at 250V.

Get ordinary plug and socket of another country -
Choose one that won't fit into any common socket or adapter in your country. (But there is risk of somebody using adapters, especially the Chinese ones with multiple holes that accept all plugs)

Quite interesting, here in Australia some gearboxes have a special round ground pin type I socket exactly for this purpose.