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A am installing a couple new 220 lines and I need the same plug and receptacles as my dryer which happen to be NEMA 10-50P & 10-50R. While looking at my box I noticed that the dryer was on a 30 amp breaker. But as mentioned, it’s wired to a 50 amp receptacle? This house was originally built in 1962 but I have had the upgrade from 100 amp to 200 amp service done.

I guess I just don’t understand why they didn’t install a 10-30R receptacle if the circuit was only going to be 30 amps and not 50 amps. Or doesn’t this matter?

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  • What are these 220 circuits feeding? Sep 7 at 11:39
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No, you won't be. Nothing uses NEMA 10 type connectors.

The NEMA 10 socket does't have a ground wire, which makes it pretty dangerous especially in a 120/240V appliance! It hasn't been legal to install groundless receptacles since the 1960s. There was a special exception for dryers and ranges particularly, but that too has been outlawed for 25 years now.

NEMA 10 sockets are still sold, only for exact replacement of a broken socket. NEMA 10 cords are still sold only for adapting ranges and dryers to legacy sockets. Fools use this "loophole" to cheat and install more of these dangerous sockets.

Most of the time when people choose NEMA 10, what they really need is NEMA 6, which does have a ground (but not a neutral).

The "universal donor" socket is NEMA 14, which has both neutral and ground, and thus, can support anything.

The dryer socket and plug were illegal the day they were installed. As such, they are not grandfathered and must be corrected ASAP. Dryers use the NEMA 14-30 since they need both ground and neutral. If there is not presently an actual ground wire to the dryer receptacle,

  • You can retrofit ground, by running a #10 ground to anywhere with a #10 or larger ground back to the panel.
  • Or you can fit a GFCI breaker and mark the NEMA 14-30 "GFCI Protected - No Equipment Ground".

As for the new circuits, use your pick of NEMA 6 or NEMA 14.

Change the appliance plugs or cords to NEMA 6 or NEMA 14. They should have never been NEMA 10 in the first place.

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  • For your GFCI recommendation isn't the purpose of 250.140 Informational Note No. 2 to remind you that the 250.114 requirement for grounding specific appliances is not satisfied by using an ungrounded GFCI? Sep 7 at 14:16
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We can only guess why they used a 50A receptacle, confusion or convenience (using what you got) are the top candidates in my opinion.

Using a 50A plug on a 30A drier would be a clear violation of the NRTL Listing which includes the installation instructions that certainly include cord rating, but there is a misaligned interpretation of the NEC eguarding the receptacle:

210.21(B)(1) Single Receptacle on an Individual Branch Circuit. A single receptacle installed on an individual branch circuit shall have an ampere rating not less than the branch circuit rating.

The Code doesn't clearly prohibit a higher rated receptacle on Individual Branch Circuits in this section. 210.23(B)(3) applies a different rule for multi-outlet circuits.

That being said a NEMA 10 receptacle that provides an equipment ground via the "grounded conductor" (the neutral) is receptacle only on allowed on legacy circuits specific to one exception in the NEC:

NEC 250.140 Frames of Ranges and Clothes Dryers. Frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be connected to the equipment grounding conductor in the manner specified by 250.134 or 250.138.

Exception No. 1: For existing branch-circuit installations only where an equipment grounding conductor is not present in the outlet or junction box, the frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be permitted to be connected to the grounded circuit conductor if all the following conditions are met.

(1) The supply circuit is 120/240-volt, single-phase, 3-wire; or 208Y/120-volt derived from a 3-phase, 4-wire, wye-connected system.

(2) The grounded conductor is not smaller than 10 AWG copper or 8 AWG aluminum.

(3) The grounded conductor is insulated or the grounded conductor is uninsulated and part or a Type SE service-entrance cable and the branch circuit originates at the service;

(4) Grounding contacts of receptacles furnished as part of the equipment are bonded to the equipment.

Notice the exception applies to existing installations only. For new installations a NEMA 6 must can be used for a grounded 240v receptacle, or a NEMA 14 must be used for a 240/120v circuit that also requires a neutral.

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  • I am running ASIC Crypto miners on the circuits. These are built in China and need 220v circuits because they need 3100 Watts at the wall. It’s their power cord that causes all the issues. It is (I believe) a NEMA 10-50P). It has 3 flat prongs (none of them have the curved prong in the 10-30.). I guess I should install the receptacle as you suggest and get an adapter if needed.
    – Carl Voll
    Sep 7 at 14:24
  • Are the miners UL/CSA/ETL listed? (Really Listed, not Asian counterfeit.) Sep 7 at 16:57
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    3100w seems underprotected at 30A. (3100w x 1.25 continuous rated) / 240v = 16A. Unless the UL/CSA/ETL Listing specifies specific overcurrent protection your breaker shouldn't exceed 20A. Sep 7 at 17:04
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    @CarlVoll -- what stops you from replacing the inappropriate power cord (maybe Chinese?) with an appropriate cord and plug (NEMA 6-20)? Sep 8 at 0:55

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