I have a 30amp, 240v circuit running to a 240v water pump. Can I add a 20amp GFI to each leg?
In short, no. Each leg of a 240v circuit is is 180° out of phase and they share a common neutral. If both legs are hot, it will trip a 1 pole GFCI because the opposite phases cancel each other out on the neutral. You would need a 2 pole GFCI, which trips when there is a potential difference between the 2 hot legs instead of a single hot and neutral.
GFCI is a concept, not a socket
You have for years been dealing with one type of GFCI device: the GFCI+receptacle combo device. Your line of thinking is that all GFCIs are this.
GFCIs come in a variety of packages.
- A plain GFCI-only (actually you'd recognize that immediately)
- A GFCI+breaker
- A GFCI-that-is-a-switch (it uses TEST/RESET for ON/OFF)
- A GFCI+1 socket that also has a switch
- a GFCI+cord/plug (air conditioners and hair dryers have this)
- A GFCI+AFCI+breaker
- A GFCI+AFCI+receptacle (though this is a fairly stupid product)
All wires (not ground) must go through one single GFCI
GFCI works by making sure the current on all wires is accounted for (adds up and cancels out). So it needs to see all wires at once, or it cannot work.
Therefore it must be ONE device.
Since it needs to see 2 hots and a neutral (2-pole), there are only certain GFCI packages that will work. Generally it is rare to find 6 terminals on a thing the size of a receptacle, so a 2-pole GFCI-only would be unlikely. Your best bet is a GFCI breaker. These are readily available for modern panels, and even a few obsolete panels (Challenger takes Eaton BR, and some others take Eaton CL not to be confused with CH).
If your panel is too obsolete to support 2-pole GFCI breakers (Pushmatic), then have it feed a small subpanel and put it there. The subpanel's "trip amperage" can be larger than the load, e.g. A 30A plain breaker can feed a 60A "hot tub subpanel" which can then feed a 30A load; the plain breaker covers overcurrent and the hot tub panel covers GFCI.