Install a subpanel with ground - electricity.straw.subpanel PEP BB

I ran power out to my yurt, here's the process of hooking up the subpanel.  It's rated at 125amps, 3 phase.  I've got 3x 4 guage aluminum conductors running to it, and a 100 amp double pole breaker back at the main panel.  There are two ground rods spaced 6 ft apart per my jurisdictions requirements.  Right now it's just got one 20 amp circuit powering some outlets.  Here's to no heater on extension chords!

Pretty sure that's not 3-phase, but rather split phase or single phase 240 - there are 2 hot wires, a neutral, and a ground. 3-phase will have 3 hot wires, maybe a neutral and a ground.

It looks great, job well done, just not 3-phase?

I returned to the scene of the crime to collect more evidence - now I have pics of the grounding!

This sub-panel was installed as part of a project to build/install a "DIY" powerwall equivalent (actually better & cheaper than the commercially available Tesla V3).

It's a 100 amp sub-panel, supplying power to both of the ganged inverters and the server room directly, when the inverters are bypassed.

After locating where the new conduit would be ran, I had to source the correct ground/neutral lug for this GE breaker box (code requirement to keep UL listing of breaker box, is to only use GE parts inside of it...). The red cable on the right side with the white tape is the neutral for a too-small backup generator that I didn't install - but there's an open lug I can use for the new sub-panel neutral...

The ground is ran in green 6 Ga THHN coming off of the grounding bar on the right side, while the neutral is coming from the new lug on the top left side. It's 3 GA THHN, and NEC says that I can put up to 3 of those inside a 1" pipe - I'll run 2 parallel 1" conduit runs to the other side of the basement & into a square box, and sort out the conduit on the far side, so I have room for the ground cable and don't have to fight the heavy cables so much for the majority of the pull.

Fighting short sections of larger conduit, trying to bend it was a bit of a learning curve and I'm no where near an expert at that part, nor do I expect to be doing it enough to gain expertise, but perseverance eventually pays off - I probably created unusable scrap conduit 3 times the amount that actually fit well enough to be used on this one, but it's done. Running cable thru these is comparatively easy after the conduit trials. My helper was very useful with the overhead work, installing the conduit, and also with running the cable thru it, but they're a bit scared of high voltage work, so I got to do all the wire/cable stripping and termination, even though none of this was live yet.

They're actually a computer tech, so I can't really hold it against them...

The mounting plywood was put up, and the boxes were located, then mounted on the plywood. The conduit runs were completed to the box, and we ran the wire. I had only black 3 Ga cable, so colored tape was used to mark hot 2 (red) and neutral (white), per code.  All cables were secured to their respective lugs and torqued down. Per our local code, all grounds must tie directly back to the main breaker box and in that box is the only legal place to tie ground and neutral together.

At the main breaker box, the black and red cables are connected to the 2 terminals of the 100 amp breaker, it's confirmed to be off, and installed in the box. Double-check the installation on both ends, and turn the breaker on. Measurements with the multimeter confirm correct voltage both between legs, and split phase to neutral.

The only thing left to do with the sub-panel is to install those breakers, remove the appropriate panel knockouts and close it up.