220 question

Reply

  #1  
Old 05-01-08, 03:14 PM
Member
Thread Starter
Join Date: Jul 2006
Posts: 15
Received 0 Votes on 0 Posts
220 question

I just went to wire a 220 receptacle for my table saw. I was surprised to remember that there is no neutral on a 220 receptacle. Why is that? If I'm wiring it using 12/3, should I use the bare ground or the neutral wire for the ground? Should I not use 12/3? What do "they" normally do? Is there special 12/2 that has two hots (appropriately colored)? This is kind of a curiosity thing for me.
 
Sponsored Links
  #2  
Old 05-01-08, 03:22 PM
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 10,944
Received 42 Votes on 40 Posts
If I'm wiring it using 12/3, should I use the bare ground or the neutral wire for the ground?
Use the bare as the ground; cap off the white and leave it unused.

Should I not use 12/3? What do "they" normally do? Is there special 12/2 that has two hots (appropriately colored)?
You can use 12/2 instead of 12/3 for a pure 240V circuit. You can special order 12/2 that comes in with red and black instead of white and black, but luckily the code allows you to simply color in a few inches of the white wire in normal 12/2 with a red sharpie marker to mark it as a 240V circuit.
 
  #3  
Old 05-02-08, 04:25 AM
Member
Join Date: Aug 2004
Location: Vancouver, Canada
Posts: 1,210
Received 0 Votes on 0 Posts
Originally Posted by sjacobso View Post
no neutral on a 220 receptacle. Why is that? This is kind of a curiosity thing for me.
It's because one live leg in your panel isn't simply "hot" all the time - it's oscillating, very rapidly. It peaks at +120v, then dips through +0v (safe, yeah right) on it's way to -120v, then again past +0v, and so on. Meanwhile, the 2nd leg in your panel is doing exactly the same thing, but perfectly out of phase with the 1st. Notice how the hot bars in your panel are interspersed so a ganged ("220") breaker draws off the two opposite phases? When you apply both to a motor, the peak difference is therefore 220 volts (i.e. the potential between +110 and -110, then -110 and +110, and so on). It's a kind of push/pull relationship, so a neutral (+0v) isn't required to complete the circuit.

You'd only need 12/3 cable for a table saw if that major appliance also featured a 110v (single phase) clock, light bulb, timer...
 
  #4  
Old 05-02-08, 08:44 AM
Member
Thread Starter
Join Date: Jul 2006
Posts: 15
Received 0 Votes on 0 Posts
What this reminds me is that there is much that I don't know about electricity. I don't understand how that works, but I'll take your word for it. It's wired anyway, so all is well.
 
  #5  
Old 05-02-08, 10:26 AM
Member
Join Date: Sep 2000
Location: United States
Posts: 18,497
Received 0 Votes on 0 Posts
With any single-phase electrical circuit, all you need is a complete circuit--a path for the electrons to flow out on and another path for them to flow back on. That only takes two wires, regardless of voltage. The only difference between a 120-volt circuit and a 240-volt circuit in North America is the voltage, and the fact that neither wire is grounded on a 240-volt circuit. Other than that, they're the same. And so they take the same number of wires.

What throws everyone off this simple concept is the fact that one of the most common 240-volt appliances in North America is the clothes dryer. But that appliance isn't just a 240-volt appliance. It's really a 120-volt appliance (the motor) and a 240-volt appliance (the heating element) contained in the same cabinet. That's the only reason it needs three wires.
 
  #6  
Old 05-02-08, 12:49 PM
Member
Join Date: Aug 2004
Location: Vancouver, Canada
Posts: 1,210
Received 0 Votes on 0 Posts
Originally Posted by sjacobso View Post
What this reminds me is that there is much that I don't know about electricity. I don't understand how that works, but I'll take your word for it. It's wired anyway, so all is well.
Well, the truth is, a little knowledge + ingenuity here is dangerous. There are so many shortcuts we could take, understanding how the system works. For example, you could [JN edit: remove example of bad practice for fear that somebody might actually do it]. But that's bad practice and forbidden for many reasons.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes
 
Ask a Question
Question Title:
Description:
Your question will be posted in: