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Kevin, Licensed Electrical Contractor
Category: Electrical
Satisfied Customers: 3671
Experience:  30 years Licensed Electrical Contractor in Illinois, Adjunct College Electrical Instructor, Former Electrical Inspector, Diploma: Digital Electronics, FCC Technician License
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Hello, I have an old sears compressor that is rated 230v 15a,

Customer Question

Hello, I have an old sears compressor that is rated 230v 15a, it has a a 2-20r plug on it that does not have a ground prong on it(did not come with it), I was wondering if I could plug it into my unused dryer recepticle with a nema 10-30r if I just change the end on the compressor?
Submitted: 4 years ago.
Category: Electrical
Expert:  Kevin replied 4 years ago. name is Kevin and I will be happy to assist you with your electrical question. My goal is to exceed your expectations on Just Answer!


1) A 2-20R has only 2 hot conductors and no neutral comprising 240 volts between them. A 10-30R has 240 volts between the 2 hot's and 120 volts between each hot and the neutral. The 2-20R is only a 2 conductor receptacle while the 10-30R is a 3 conductor receptacle. A 2-20P will not fit into a 10-30R, physically impossible.


2) Since the compressor has a 20 amp plug, the circuit breaker will also need to be a double pole 20 amp. If attempting to re-use a 30 amp double pole breaker from a dryer receptacle, the dryer is also a 30 amp circuit breaker. This is "over-fusing" and would be a code violation. If the Full Load Current on the compressor is rated at 15 amps, codes requires the breaker to be 1.25 times the Full Load Current due to the "in-rush" current upon the motor start.. Therefore, 15 x 1.25 = 18.75 amps. Round it up to the next standard circuit breaker size = 20 amps. Therefore, you can re-use the existing dryer circuit conductors for the 2 hot's but the dryer receptacle will need to be swapped out to a 2-20R and the circuit breaker needs to be swapped out from a 30 amp double pole to a 20 amp double pole. The white neutral conductor from the existing dryer receptacle will not be required. Suggest removing the existing white dryer neutral from the main electrical panel neutral bus bar and cap it off and mark it as a "spare" to the location of the air compressor.




Hope this helps.........If you have any additional questions, let me know and I'll be glad to answer them for you.


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Expert:  Kevin replied 4 years ago.

1) Attached below is a NEMA configuration chart, notice how the blade configurations are different between the 2-20R and a 10-30R?


Attachment: 2013-08-30_024739_nema_configuration_chart.pdf


2) The cord connected plug for the compressor will remain as a 2-20P while the old dryer receptacle will need to be changed to a 2-20R using a 20 amp double pole breaker.

Customer: replied 4 years ago.

What if I wanted to run a new circut from my panel. I start with a 20a double pole breaker. How do I wire it. Ive done 110 but have yet to do 230.

Expert:  Kevin replied 4 years ago.

1) Hello..............If running a new circuit from the panel, what will be your wiring plan? In other words, do you plan on using Romex cable or EMT metal conduit or PVC conduit or Armored Cable, etc?


2) I would also need the approximate distance from your main electrical panel to the area where the air compressor will be installed since I need to calculate the voltage drop to ensure that the conductors are sized accordingly.


3) Does your main electrical panel have 2 consecutive spare circuit breaker spaces either on the left or the right hand side? The 2 spare slots need to be vertical and not horizontal. The 2 spare slots will need to accommodate a full size double pole breaker and not using a mini or tandem breaker.


Let me know the above answers and I can then provide you with the necessary information, parts, wire sizes, etc...........Thanks..................Kevin!