NEC wire size / breaker matching

[video321]

The only caveat to oversizing the wire is that you are supposed to stay oversized downstream.  For example, if you run 12 guage wire, you should use 20amp receptacles, even if you put a 15amp breaker in the box.  That is why a 15 amp receptacle doesn't accept 12 gauge wire.  In this example, you wouldn't want a future person to open the panel and think that the entire circuit is rated for 20 amps because there is 12g wire at the panel and put in a 20 amp breaker.

Lou-
Sorry, but that is straight against code. The breaker and receptacles cannot be rated higher than the wire. You can have a 20A (or 15A) breaker with #12 wire and a 15A receptacle. Once you put a 20A receptacle on a 15A circuit (or #14 wire) you are basically stating that circuit can provide 20A at the receptacle - it can't! You can, and should, increase wire gauge to reduce voltage drop on long runs.
 
Edit for another recent task I did....
I ran a new circuit in buried conduit outside and used parts that I had on hand. I wired up a 20A breaker with 12/2 into a dual gang box and the first termination point was a 15A GFCI. It then feed the next receptacle, a 20A, with the downstream connections. How can I do this? The GFCI although rated for 15A at the receptacle is also rated for 20A pass-through via the back-wired terminals.
 
One more edit...
Happy 4th Everyone!!! B) 

 

[apostolakisl]

The breaker and receptacles cannot be rated higher than the wire. 

 
 
I think you misread my post.  I suggested no such thing.

 

[apostolakisl]

That's a reasonable line of thinking (IMHO) but I don't know that it's actually a rule, or perhaps just a local one.  Here we aren't allowed to use anything smaller than 12 AWG wire but are allowed to use 15A breakers and 15A outlets.  But that size wire won't fit in the quick connect holes so the screws must be used - a much more reliable connection anyway. 
 
I have heard of people using a smaller wire pigtail at the source breaker to make the sizing obvious to future workers. 
 
If a breaker is used as a disconnect at the unit then there is no problem with a breaker sized for the larger wire run at the source panel.  The disconnect breaker could be the smaller rating and the short run to the unit can be the smaller size.  No confusion later on either...

 
No, you are correct, it is not code.  But it is what an inspector once suggested is best practice, not to downsize downstream.  And I have also heard of pigtailing at the breaker or labeling the wire at the breaker to make it clear that the higher guage wire is not indicative of the entire circuit.

 

[apostolakisl]

So would a 40A breaker be able to accept 6AWG wire?  I know my device won't take 6AWG, so I'll have to pigtail that one, but don't want to pigtail the breaker of course.  The plan is to install a 14-50R outlet, and get a 14-50P cord set at Lowes to act as the disconnect.  The manufacturer requiring me to use a 40A breaker is really screwing up my plans

 
 
Why don't you skip the receptacle and hardwire it in.  It's not like you are going to unplug your care charger and plug it in somewhere else.  You'll probably save yourself $100 worth of NEMA receptacle/plugs.

 

[electron]

By making it a 14-50 outlet, I have a disconnect (since my breaker box isn't within line of sight, hidden in basement, I believe NEC requires a disconnect in this scenario), but by having a 50A 14-50 circuit, other cars, such as the Tesla Model S can charge at 50A (well, 40A really), instead of using my 30A unit.
 
If something goes wrong with my charging station, I can also use my backup, which can use a 14-50 outlet as well.
 
The 14-50 hardware isn't that expensive, a they sell the cord sets at Home Depot/Lowes for ovens.
 
Hardwiring is definitely the easiest route, I just want the circuit to be as useful as possible.

 

[apostolakisl]

I have a gigantic UPS unit at my office and I saved about $200 hardwiring it in instead of using plugs.  It passed inspection (twice actually 10 years apart).  The first time the breaker box into it was not in the same room, but the subpanel servicing the load items was.  The second time was the opposite, the breaker to it is in the same room, and the subpanel breaker box servicing the load items was not.
 
And yes, the dryer or oven type plugs aren't that pricey.  The nicer twist lock ones can be quite expensive and that was what I was thinking you would use for something like this.
 
As far as the receptacle, it would appear that there is no such thing as a 40amp receptacle.  After a google search, it looks like they go from 30 to 50.  So using #6 wire and putting a 50amp receptacle on it would be copacetic. 

 

[video321]

The only caveat to oversizing the wire is that you are supposed to stay oversized downstream.  For example, if you run 12 guage wire, you should use 20amp receptacles, even if you put a 15amp breaker in the box.  That is why a 15 amp receptacle doesn't accept 12 gauge wire.  In this example, you wouldn't want a future person to open the panel and think that the entire circuit is rated for 20 amps because there is 12g wire at the panel and put in a 20 amp breaker.

 
Lou-
This is what I'm referring to. As you stated.... when #12 wire is used it is o.k. to install a 20A receptacle along with a 15A breaker, but not o.k. to install a 15A receptacle when a 20A breaker is used.
 
However, that is backwards. Think about it... as a simple example you can have (2) 15A receptacles wired up to a 20A branch circuit and between those 2 receptacles get the full 20A out of the circuit. But... if you installed a 20A receptacle with a 15A breaker and connected a device with a 20A plug pulling that amperage - what do you think is going to happen?
 
So, there is nothing wrong with that future person putting a 20A breaker in there. The only thing the wire has to do is not be overloaded before tripping the breaker. Since #12 is designed to work with a 20A breaker it won't be overloaded.

 

[apostolakisl]

But... if you installed a 20A receptacle with a 15A breaker and connected a device with a 20A plug pulling that amperage - what do you think is going to happen?

 
 
What is going to happen is that you will pop the breaker.  No harm done.  There are a million ways to draw more than 15 amps off of a duplex plug.  No different than plugging 2 15 amp draws into each of the two plugs of a duplex, or into completely separate plugs since generally several duplex receptacles are all wired together on the same breaker.  I never said put a 20 amp breaker on a 15 amp wire or plug.
 
 However, at least at one time (not sure if this is current code), you were allowed to use 15 amp duplex receptacles on a 20 amp circuit.  The logic being that more than one receptacle would be the reason for exceeding the 15 amps.  Like 2 10 amp devices each plugged into their own 15 amp receptacle but both being on the same circuit.  Personally, I wouldn't do that, but it was (and might still be) allowed by code.
 
If you have wire rated for 20 amps, and a plug rated for 20 amps, you can still put a 15 amp breaker on it.