I'm currently researching wiring a hot tub 240v 4 wire 50amp, but have an older Federal Pacific box as the main. Is there a relatively simple way to change a 3 wire system to a 4 wire where the hot tub connects. Eventually I will dump the FP box and rewire, but now is the time for tubbing :)

Here's the layout
Drop to Meter, 2 hots and one unshielded neutral. 200amp rated
Meter directly connects to the 150 amp 3-wire Service Panel. No Disconnects at the main. This is an older setup and there is no main breaker for the shut off.
Inside the main there are the 2 hot leads and one Neutral bar that both neutral and grounds connect.

Plan but incomplete due to questions on 3 and 4 wire setups.
Install a sub panel fed by a 50amp Federal Breaker
Red and Black feed the hots for the Sub Panel
White connects to the neutral bar in the Sub Panel

Hot Tub will connect to a 50amp GFCI breaker as follows via a spa disconnect.
Red and black leads connect to the 50amp GFCI (Cutler Hammer)
White from tub connects to Neutral on 50amp GFCI.
White GFCI connection from breaker connects to the neutral bar in the sub.

Questions are
What do with the grounds?
Hot tub feed and the sub panel both have grounds that would need to connect.
I can pigtail them together in the sub and connect to a ground bar (separated from the neutral) but, in the main, there is only the neutral bar and I would think this would fault the GFCI. My understanding is they must be isolated from the neutral bar in the sub but do they connect at the main neutral?

To add 4 wire service, is it possible to run a shielded ground wire from ground bar in the sub panel to a ground loop? For this I would drive a couple of ground bars @6ft apart outside. Then run the 2 shielded lines from the ground bar to the each ground stake.

Based on this my final grounding connections would be.
Ground from Hot tub and ground from main to sub panel supply would link together at the ground bar.
In the main the grounds would connect to the neutral.

Hope this is enough detail. I'm using the approved wire etc... just want to clarify the connections.

Thanks

As a follow-up to the above, I've viewed the diagrams at

Seimens GFCI wiring.
The Square D GFCI looks pretty much like my configuration.

Just a couple of clarifications needed. In the sub panel, does the ground bar need to connect to it's own ground source?
Or does it ground via the main breaker neutral bar? Where the main is grounded?

Thanks in advance.

The neutral/ground rod connection should be made at the first diconnect and the main panel treated as a sub-panel with independent grounds and neutrals.

Many times a sub-panel must be bought with an optional groud bar kit. There is also a green screw that needs to be to then isolate the neutral bar.

If your main panel was the normal way where it had a main breaker, the neutral and grounds and the ground rod would be joined.

The 4-wire feeder would be run to the sub-panel with neutral and ground tied to the same bar in the main panel. The sub would have an isolated neutral and a ground bar kit installed.

Is metallic conduit used to get power to the main box?

Care to post a pic of your main panel with the cover off? Use "go advanced"/manage attachments.

Here are a couple of photos.
The first is the full shot of the main breaker panel

Then there's one of the sub panel with it's GFCI installed

The Ground screw in the sub connects the neutral / ground from the main panel to the hot tub. The sub panel box itself is not grounded. Only the Main panel is grounded at the neutral / ground bar.

Many thanks for the help

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I think it looks OK. The only thing I would consider is putting a LUG at the ground screw connection to the box.

e.g.

Ilsco CC-SLU-125 1/0-#6 Copper Mech Lug