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I am in the process of starting a crypto mining farm. Eventually I will be getting upgraded outlets, but for now I am working with NEMA 10-30 dryer outlets that were installed back in the early 90s.

The outlet

I probed the outlet and got the following voltages: LINE1 to NEUTRAL: 125V LINE2 to NEUTRAL: 125V LINE1 to LINE2: 250V

The specification on the power supply I want to use on this outlet specifies 100-240V AC with no mention of split phase. https://shop.bitmain.com/productDetail.htm?pid=00020170920224132179WEIOvhXx06D6

My first question: I am assuming that the appliance will not care whether the power is supplied in single phase or split phase. I am also assuming that the appliance will not mind getting 250v instead of the rated 230v since it is within 10% of spec. However it seems a little on the high side, with only 3V to spare. Am I correct in these assumptions?

My second question is regarding the lack of a grounding conductor. It concerned me at first, however as far as I can tell the appliance is wired for single phase. Am I correct in my assertion that since it is wired this way, all current would be carried between LINE1 and LINE2 and that in this use case, the neutral could actually be considered a grounding conductor since it does not carry current? As opposed to when operating my dryer, it uses 125v from LINE1 to the NEUTRAL for the lights and timing mechanism, which would cause the neutral to cease being a grounding conductor. There is nothing else on the line, only one 10-30 outlet on a double pole breaker.

My last question is a more of a curiosity. In addition to the above measured voltages, I was reading about 4.5V AC between the neutral and LINE1 or LINE2 with the breaker OFF. Does this mean some other appliance is leaking current back into the breaker or something else?

All measurements taken with a Fluke 87.

My end goal is to have the highest level of safety for both myself and to protect the equipment. However it will be up to a month before I can get modern outlets installed so I would loose quite a bit of profit during that time span as my equipment is arriving before then.

I have an AS in EET so I feel pretty confident this is going to work but I came here for a second opinion as well as so future miners in my situation may find an answer to this.

Any help/tips/you're going to blow your miner up comments are welcome.

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  • Please edit your question to include the current (amperage) requirement for the power supply, or the power (in Watts) required for the miner hardware. Also, please clarify if the power supply has a GND connection. Neutral and GND serve different purposes. If the power supply is double-insulated and does not need GND, then you can use that outlet. Otherwise you absolutely need to replace it with one that is properly earth grounded. For example NEMA 6-20 or NEMA 6-30.
    – mkeith
    Nov 16, 2017 at 4:17
  • I wouldn't worry about the fact that your voltage is 125/250. That is pretty typical and within normal. It is still considered to nominally be 240.
    – mkeith
    Nov 16, 2017 at 4:20
  • Looks like the power input has a protective earth ground input. You need a grounded outlet. I would definitely not use the neutral.
    – mkeith
    Nov 16, 2017 at 4:26
  • @mkeith Amperage is no problem, the outlet is way overkill in that regard. It only requires 7A where as the outlet is 30A. The power supply does have a ground connection. As far as I can tell, it is built like a standard PC power supply. It does specify the leakage current to be <1.5mA, which IIRC means that no (close to zero) current would normally be flowing through the ground connection, especially with a 30A rated conductor. This is what lead me to believe that for all practical purposes the wire my dryer uses for neutral would act as a grounding conductor in this case.
    – Colby
    Nov 16, 2017 at 4:31
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    All the outlets I suggested are three prong. I agree with you that your supply does not need the neutral (assuming you go with 240V). But you do need a PE ground. I suggest you ask your question on the DIY stack exchange, because it is basically an electrician/electric code question. Neutral and protective earth are NOT the same. They are generally bonded together in exactly one place. But because neutral is a current carrying conductor, it is not allowed to be used as PE.
    – mkeith
    Nov 16, 2017 at 4:42

1 Answer 1

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There are only 2 ways to hook this power supply up.

  • 120V line (either line) to neutral. This would use the standard, common NEMA 5-15 connector, you know the one.
  • 240V line to line. (akin to using only +12 and -12 for a 24V load). Neutral is not used on this configuration, it doesn't even need to be wired to the receptacle. The proper connector is NEMA 6. This power supply will give 1600W of power on this config.

Equipment safety ground is not comparable to electronics GND. Code requires it today, and it does two things for you: helps fight ESD, and helps prevent electrocutions. A GFCI/RCD actually does a better job of the latter, but does nothing for ESD.

Getting a ground

This is such a super good idea I assume you'll find a way. Open up the box and look at the cable.

If the neutral is a bare wire or bunch of bare strands surrounding the conductors, permanently reassign it to be ground instead of neutral. Neutral and ground are tied in the main panel... but if it's a sub-panel, move the bare wire from the neutral bus to the ground bus. This is one-way; converting it back to neutral violates code.

If the neutral is a white wire, look and see if there's not already a ground wire in there. In 1993, maybe. Otherwise Code now permits you to retrofit a ground wire, minimum #10 size, green or bare, via any workable route back to the panel. Enter via a knockout and use a proper cable clamp. #10 to be compatible with future dryer use, so you don't fail inspection when you sell the home.

You can never retrofit neutral.

Wiring it up if only this plugs in here

Keeping in mind it will hurt your resale value if you can't roll it back to a dryer outlet.

Get a NEMA 6-15 dual receptacle, correct box lid and cover plate. Attach hot1, hot2 and ground to it. The dual sockets are for code compliance or a second server.

You must also change the 30A breaker to a 20A breaker. Normally socket amperage must match the breaker (hence not 30A) but there is a special exception for 15A sockets that makes multiples of them legal on a 20A breaker.

enter image description here

Yup, same form-factor. This a dual 6-15.

If the dryer needs to function too

You must have neutral and ground. Change the receptacle and dryer cord to NEMA 14-30, which will make your family safer too. Remember to remove the neutral-ground tie on the dryer.

Several options.

  • Ask the Code geeks on diy.se whether you can really put a 30A plug on a 1600W server. I don't think you can. If you can't...

  • Install a subpanel. Get at least an 8-space subpanel rated 30A or more, no "main" breaker required. One 30A 2-pole breaker goes to a NEMA 14-30 for the dryer, wired with #10 wire. A 20A 2-pole goes to the NEMA 6-15 dual as described above, wired with #12. The rest are for expansion, as you can put up to 24A continuous on this panel. (Continuous loads count as 125% of their actual measured draw.)

  • connect a UL-listed PDU that is rated for 30A or more. It must be installed according to its instructions, but fit a NEMA 6-30 or 14-30 plug if that is allowed.

If you want to use an L6-30, same deal as 6-30, the "L" means twist-lock. Ditto L14-30.

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  • Thank you for the precise reply! After looking into the facts I definitely will not be using a dryer on this outlet anymore. I wasn't previously aware that it was a non grounding appliance. The whole idea of having the metal case attached to a current carrying conductor is very sketchy to me and goes against everything they taught me in school regarding equipment safety. Not sure until I get into the wall, but if there is an existing grounding conductor i will use that. If not I will swap the neutral to be a grounding conductor and swap the outlet as well. Construction was ongoing during '93
    – Colby
    Nov 16, 2017 at 7:20
  • Also I spared a bit of detail in the OP. I was actually planning on wiring a 30A PDU to the plug itself, then plugging the server into the PDU with a C19 20A connector. So in the end the easiest solution for now would probably be to swap the neutral to a grounding conductor. Then swap the outlet to a L6-30P, instead of swapping the PDU's plug to a 10-30. This would also prevent accidental use of the dryer back feeding current to ground. With this setup I should be able to run at least 3 S9's and still have plenty headroom. Thank you again for the help.
    – Colby
    Nov 16, 2017 at 7:23
  • Oh well, if the PDU is what plugs in, and it requires 30A, then it is the utilization equipment and that's all we care about. Nov 16, 2017 at 7:54

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