However, they lead to standard 5-15P outlets. The electrician ran 10 gauge wire to the breaker from the outlets he installed. The electrician said its ok, just don't plug 110V stuff on there.
I think don't hire that electrician again. Putting 240v/30a service on Nema 5 is _crazy_. Forget the fact that it's a code violation: it's actually dangerous because some equipment will fail catastrophically (e.g. become electrified, burst into flames) when plugged into 240v.
The only reason I can think of that an electrician would do that is if he was just clueless.. and e.g. didn't know that you could easily pick up 240v outlets at the hardware store or that appropriate cables to hook IEC connectored equipment (like computer/miner PSUs) are easily available.
Sure _you_ know to only plug 240v stuff into it, but what happens when you have someone in to do some repairs in the room and they need to plug in some tools? What happens if you're out of town and some family member needs to plug in a shopvac to clean up? What happens if you're stumbling around in the dark and forget?
Also, something like this is bad enough that it might be used by an insurer to screw you over if there is a fire.
All my 240v stuff is either
NEMA 6 twistlocks or IEC 60320.
Non-twistlock NEMA6 is more common in US residential as well as NEMA14 'dryer plugs'... but I prefer twistlock because it locks so you don't accidentally yank it out or leave it half-pluged making a poor connection, plus it's commonly used on data-center equipment, so there is a lot of surplus data-center stuff that just works against it. (
See the picture here.)
I suggest you stop reading, right now, and go make a "danger 240v do not connect normal devices!" sign right now and stick it on it. That's the bare minimum. Then come back.
I would also be pretty dubious about having a 15a rated socket on a 30amp service especially with >15 amps of load. I've had NEMA 5 15amps outlets char up when just running with a near but under 15a load for an extended period.
Residential targeted electrical equipment doesn't seem to have a lot of safety margin: it seems to be mostly built expecting that you're going to have an intermittent high usage load (like a vacuum, blender, etc)... it'll survive a sustained load at the rated level but not with a lot of room to spare. Running a sustained 25a through a fixture specified for 15a seems actually likely to fail to me. Keep in mind, that damage from running it too hot can increase resistance by oxidizing the connectors... and the higher resistance means more current... which means more heat, and you get a run-away until it fails.
It's very easy to replace an outlet. You can get a NEMA 6 outlet that fits into the same wall box and will receive 10gauge wire. (I'm kinda surprised he got 10gauge to fit on a regular 5-15 outlet, they're really made to take 12gauge max normally). Turn off the breaker, remove a few screws, swap in the replacement, reinstall screws.. done. There are videos on youtube. Get a little IEC connector power strip rated for enough amps that plugs into your nema 6 outlet and can feed your miners using [ur=https://www.infinitecables.com/power-cables/iec-power-cords/c13-to-c14-power-cords/c13-to-c14-16awg-power-cords-black/iec-c13-to-iec-c14-power-cable-16awg-sjt/]IEC to IEC cables[/url]-- they're often inexpensive if you look some place that sells datacenter surplus (including ebay).
If the breaker is 30amps then I believe by the rules _all_ downstream wire must be at least 10ga (for copper, larger for aluminium). The purpose of the breaker is to prevent the wires from catching fire, and in theory any one wire downstream of the breaker could carry the entire 30a load. Though I know power cords are often sized way lower than their supporting breakers (like 18-gague wires connected to 20a circuits)... so I guess that rule doesn't apply to power cords.
That wouldn't wouldn't worry me too much in any case, a 14ga cable should be okay with a 15a load. Though if your cable runs aren't really short you might prefer to use a larger gage for the bulk of the run, because 15-amps over 14ga has a non-negligible loss (about 1.3% loss for 20feet).
If you look at most NEMA duplex outlets you will see each pole has 2 screws for wire and a brass link joining the contact pads together. For one, it makes it easy to daisy-chain circuits -- but -- when the link on the hot side is removed it also allows L1-N (110v) for one socket and L2-N (also 110v) to the other socket, in fact on many/most NEMA duplex outlets the link is scored to be easily broken off just for this purpose. The single neutral wire feeds both sockets via the link on its side. Considering that the device (duplex outlet) is only rated for a total of 15A due to thermal considerations, not sure why one would want to do that but it is allowed as the end result is 110v at each outlet.
Huh, I've only seen/used the splitting in order to make one outlet switched e.g. so that a floorlamp can be controlled by a lightswitch. I'd feel a little sketchy running both phases on one outlet with how close together the two hot nuts would be. A short there if it's just a lamp circuit would be mostly harmless, rather than fireworks.
