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I have a 40amp double pole breaker on my main with 30' of 8/3 to an L14 30R locking outlet, what is the max wattage?

Thank you.

  • What was this receptacle intended to power? – ThreePhaseEel May 27 '18 at 0:33
  • And what are you plugging into it? – ThreePhaseEel May 27 '18 at 16:05
  • Welder or a temporary distribution box. Which ever is needed in the shop at the moment. – Jeff F. May 27 '18 at 17:01
  • Can you give us the I1eff from the welder's nameplate, or barring that, its input current rating and maximum duty cycle? – ThreePhaseEel May 27 '18 at 17:33
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0 watts. A 30A receptacle on a 40A breaker is a code violation.

5760 watts continuous, 7200W intermittent, if you change the breaker to 30A.

7680 watts continuous, 9600W intermittent, if you change the receptacle to NEMA 14-50R.

Why are we using a NEMA 14-50 on a 40A circuit? They don't make 40A receptacles/plugs. You are expected to use 50A sockets on 40A circuits. This exception is coded into NEC table 210.21(b)(3).

  • Where’s the 50 amp breaker? – Tyson May 27 '18 at 0:37
  • Then why would you tell him to change the receptacle to a 14-50r? – Tyson May 27 '18 at 0:43
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    Interesting.. I also never realized L14-40 does seem to exist. – Tyson May 27 '18 at 1:25
  • I am going to change the breaker to a 30 amp and keep the 8/3 with the L14 30. If I change the receptacle to an L14 50, then the cable would have to be changed to a 6/3? – Jeff F. May 27 '18 at 9:10
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    @JeffF. No need. 40A breaker to 8/3 to 14-50 is correct for a 40A circuit. The reason for the 50A receptacle is they do not make 40A receptacles. – Harper May 27 '18 at 15:23

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