I have a 30A 240V wall plug that has 4 pins. I am trying to connect this to a Mean Well PSU that takes 240V. The problem is the PSU only has 3 input terminals. It is rated only for 600W and it is just used to power a motherboard and some steppers. How should I make the connection? It should be noted that I have to connect 4x 700W silicon heaters to this circuit as well. And I would like to wire this as safely as possible.

Follow-up question what happens if you short the two hot wires of of the wall plug? I understand that 240V is very dangerous, I just am curious.


Based on the drawings below, of the 4 wires from the nema 14-30 I would connect the red, black, and green to my PSU.

wiring of nema 14-30 4 wire to 3 wire 240V

It should be noted that the PSU has a manual switch for its input power to be either 120V or 240V. 120V is simply not an option here though I would need a very long extension cord, and the 240V receptacle is right next to my project. Furthermore I have to connect my silicone bed heaters to 240V and this is a 2-wire connection. power supply unit

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    \$\begingroup\$ If everything is wired up properly, shorting the two hots will trip a breaker and/or blow a fuse. If everything is not wired up properly, it will start a fire. \$\endgroup\$
    – Hearth
    Aug 26, 2021 at 2:43
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    \$\begingroup\$ I guess the two hots are in a different phase, but what is the purpose of this if the neutral wire can handle 30A why split up the current between the two hots? \$\endgroup\$
    – Feynman137
    Aug 26, 2021 at 2:47
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    \$\begingroup\$ Both of the hots are only 120 V relative to ground, rather than 240 V. It's safer. \$\endgroup\$
    – Hearth
    Aug 26, 2021 at 2:51
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    \$\begingroup\$ Oh, to your question of "what happens", a 30A breaker is supposed to instant-trip at 300A. 240V@300A is scary enough. But many makes (FPE, Zinsco, Challenger) of breaker are known to jam (the mechanism trams and binds). Then you get this. youtube.com/watch?v=6hpE5LYj-CY Which I'll grant you is 277V and not 240V, but still. \$\endgroup\$ Aug 26, 2021 at 3:43
  • \$\begingroup\$ Why are you worried about the plug? Let us know which exact power supply model it is to find out if your plug selection even makes sense. Why would a 600W US power supply need 240V anyway. \$\endgroup\$
    – Justme
    Aug 26, 2021 at 4:22

1 Answer 1


That's a NEMA 14-30 plug.

You can't put that on a 600W power supply. Use a 6-20.

Take it back to the hardware store and get your money back. They should not have sold it to you.

Your 3400W power supply only draws 14 amps. Continuous loads need to be derated by 125%, so that becomes 4250W/17.7 amps.

So yes, that calls for a NEMA 6-20 plug. A 14-20 would also be acceptable, but that's a black swan. Stick with standard 6-20. Any hardware store has that as a field-installed plug, just install it on the cord the unit already has. If it didn't come with a cord, just buy any 14/3 cordage* by-the-foot from the wiring rack.

Note that cordage will be black-white-green color. You can leave it white because it's a plug-in device (not in-wall wiring)... but if it helps you mnemonically, feel free to mark it red.

NEC places no limits on the length of appliance cords.

Then get an adapter.

Once you have that, get a UL Listed NEMA 14-30P to 6-20R adapter.

While the Chinese will sell you anything on Banggood, AliExpress, Amazon, Wish.com and eBay... you need a UL listed one, because it has a distinguishing feature: Fuses.

That power supply needs to be externally fused at 20A.

A UL listed adapter block or cable will take care of that for you, and you are all set.

* Cordage means flexible cord intended for plug-in appliance cords. Unlike wall wiring, it has fine-stranded wires made for daily flexing. With flexible cordage, the ground wire counts, so /3 cordage is black-white-green.

14 AWG is UL-approved for <~15A appliances when it's cordage (i.e. not in walls packed in insulation.

enter image description here

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    \$\begingroup\$ I believe black and red are the hots and white is the neutral. I am just wondering what wire will be omitted in this connection? Based on your logic I would include the two hots and exclude the neutral and include the ground. \$\endgroup\$
    – Feynman137
    Aug 26, 2021 at 13:48
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    \$\begingroup\$ @Feynman137: the 6-15 and 6-20 outlets have the two hot wires to make 240 V, and Safety Ground. They do not have a Neutral connection. \$\endgroup\$ Aug 26, 2021 at 16:41
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    \$\begingroup\$ @Feynman137 Based on what you said, all your loads are 240V-only and there are no 120V loads. Due to the way 120V US electricity works, that means neutral is not needed. Hence the missing pin on the NEMA 6 family. \$\endgroup\$ Aug 26, 2021 at 20:42
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    \$\begingroup\$ By the way, 11:42 in that video directly addresses your neutral question. \$\endgroup\$ Aug 26, 2021 at 21:01
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    \$\begingroup\$ @Feynman137 Yes, however I am surprised UL would approve it with that marking. Check the UL listing. \$\endgroup\$ Aug 27, 2021 at 3:47

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