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A couple of wiring device questions

Preface: I am building a charging kit that contains a long, low gauge extension cord that can be used with various NEMA plugs. For example, I can connect it to a 14-50 to make a 14-50 extension cord, or a 6-20 to make it a 6-20 extension cord. I will be keeping all of these NEMA connectors with my car so that I can use it at any house with any connector I find.

1) I would like to protect myself and my charging kit by placing a GFCI at the front of the chain (closest to the outlet). Does anybody know where I can buy a 250V/40A rated inline GFCI device for less than $250? The closest I can find is a 250V/30A device for $263. Ouch! Why are these so pricy? At that price I can make my own with a small circuit breaker box and GFCI breaker. But I'd rather have something portable. DANIEL WOODHEAD Line Cord GFCI240VAC Voltage Rating, NEMA Plug Configuration30053-0M - Grainger Industrial Supply

2) I need to standardize on a male/female system so that I can quickly change adapters (without having to manually rewire them each time). I am hesitant to use a NEMA standard male/female system because when someone sees a plug like that, it implies a certain voltage/amperage capacity, which could be misleading depending on what connector is on the other end. I will label accordingly but I would rather not create the potential for someone to plug in and get something different than is expected/standardized. Is there any 250V/50A-rated male/female connector system (3-wire, or 2-wire + ground) that I can use to connect these things so there is no ambiguity about the NEMA implications?
You could use color-coded camlock connectors... those won't be mistaken.

As for GFCI, the best you're going to do is a small GFCI breaker in a mini 2-space box at the 50A rating. It's largely because there are so few things that require GFI protection at that current.
Your best bet is probably to use a standard breaker such as this:

New in The Box Square D QO240GFI QO 2 Pole 40 Amp GFCI Breaker | eBay

You'll need to put it in a small fuse box... this is the smallest I could find with a few minutes of googling...

Square D QO 70 Amp Main Lug Load Center Breaker Box Flush Mount | eBay

No need to go that big... Typical hot tub disconnects will work:
Eaton 50 Amp BR Type Spa Panel-BR50SPA - The Home Depot
50A GFCI included.

I looked through my usual supply houses' catalogs and found absolutely nothing for inline 50A GFI's... I think the best you'll do is a small spa disconnect like the one I linked.
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Thanks guys, that 50A spa panel is perfect!

Is there any cheaper option you know of than camlock connectors? They seem perfect but at $17 per 15 connector I'll be at $459 to do the whole rig that way.

Not for repeated use. If you didn't mind more set-up time, you might look at insulated Polaris connectors. Cap your wires with the giant wire nuts, then when you needed to use a new adapter, you'd uncap the wires, insert them into the connector, and torque them down with a tool you'd keep in the kit.

The CS (California) series of temp power connectors might work, there aren't too many of them around. They're kinda expensive as well.

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