breakers and receptacles and cable, oh my! please help!

[Zach Duran]

The more I learn about electricity, the more i dig myself into a world of confusion. I recently had a family electrician install a 240 line in my basement to run my table saw and jointer. a 30 amp breaker was used at the service panel to send the two legs. But the table saw has a 20 amp turnlok plug on it, so we used a 20 amp receptacle as the 240 line outlet. I was told it would be fine, as the table saw doesn't draw more than 17 amps max.
Now, is this something I should not be doing, regardless of code? As it stands, the 30 amp breaker runs thru 10 gauge wire to the 20 amp receptacle. the table saw cord is 12 gauge wire, into the motor. Should i just change out receptacle to 30 amp, and rewire the motor cord with 10 gauge wire with an appropriate, 30 amp plug? If i did do that, would that harm the motor in any way? or should i leave all as is?

Enter confusion #2: The same outlet I am going to use interchangeable with my jointer. however, because the jointer has a 3 phase motor, I am running a VFD to take care of the power. The VFD instructions say to wire up the power in and send with 14 gauge wire, which, in my limited understanding is only used with 15 amp circuits. So could i even use the VFD wired with a 15 amp plug, on a 20 amp receptacle, going to a 30 amp breaker!?

Does all of this make sense, cause to me it is definitely greek! I would hate to destroy a brand new VFD, or worse, burn down my house. If anybody could shed some light on what is the best options, It would be much appreciated.

[Anthony Whitesell]

Both of your questions are answered by the same statement.

The breaker in the panel is to protect the wire in the wall first and foremost.

The break is sized to supply enough current to your load(s). The wire is sized according to the selected breaker. The plugs are selected based on the current requirement for the equipment.

Because you can run two 15A tools from one 30A circuit, means you would need a 30A breaker, which requires 10g wire, and 15 amp plug/receptacles on the tools. One could also use a 20, 25 or 30A plug/receptacle depending on what other outlets you are already using. Just don't go smaller on the plug than is required by the tools. Personally, I use a 20A 250V twistlock for all my tools on 220V less than 20A (I currently have no tools that are larger than 220V 20A).

If you want to protect the tool, you would need to add a properly sized breaker on the tool between the plug and switch.

[Dan Hintz]

Zach,

Look at it as two separate worlds: 1) The socket in the wall up to, and including, the electrical panel, and 2) the plug and everything up to, and including, the machine.

For world #1, the breaker protects all of the wiring up to the socket... and that's all. It is not there to protect your machine. Your minimum breaker size is determined by your wire size. You can use a thicker (larger gauge) wire than a breaker is rated for, but you cannot do the opposite. You size the socket for the machine you expect to plug in there, as long as the socket is not rated for more than the breaker (you won't hurt anything, but plugging in a bigger machine than the breaker is rated for will eventually pop the breaker).

For world #2, size the cable and the plug for the machine. These can be less than the breaker/wire will allow, but they should not be more. If you over-size the plug for the installed socket, they won't fit (different styles/sizes). As in world #1, oversizing the wire will do no harm, but it will also do you little good and is wasteful.



Sizing the socket for the 20A plug already on the table saw is perfectly fine. Adding a 20A plug/wire to the VFD is also acceptable, if you want to share the socket between them.

[Leo Graywacz]

If you have a 20 amp receptacle on the 30 amp circuit you should have a 20 amp breaker protecting it. That is the weak link in the system and what needs to be protected. You should either have an adapter to your saw or change the plug on the tablesaw. That's code. But if you are just using the saw on it you don't have anything to worry about.

[Anthony Whitesell]

If you have a 20 amp receptacle on the 30 amp circuit you should have a 20 amp breaker protecting it. That is the weak link in the system and what needs to be protected. You should either have an adapter to your saw or change the plug on the tablesaw. That's code. But if you are just using the saw on it you don't have anything to worry about.

Unfortunately, that is only true if it is the only receptacle on the circuit. You can remember this rule by looking around your house. Most of the receptacles in the house are 15A duplex outlets and most of the breakers are 20A.

[Leo Graywacz]

15amp duplex are allowed on a 20 amp circuit because each outlet is rated at 15amp. And most residential circuits are run with 14ga. Only certain circuits need 12ga. Garage, external outlets and appliance outlets. Good chance the countertop outlets are too depending where you live.

[Zach Duran]

thank you all for your input. it has cleared up a lot. The 30 amp breaker is only there for the one receptacle, which will be running my table saw and jointer/vfd, pending on which one i plug in.

[Rob Damon]

Don't have my current NEC, but from the 2006:

Table 210.21(B)(3) Receptacle Ratings for Various Size Circuits:

Current Rating Receptacle Rating
(Amperes) (Amperes)
------------------ -------------------
15 Not over 15
20 15 or 20
30 30
40 40 or 50
50 50

Yes the 30amp circuit breaker will protect the #10awg wire, but it will not protect the 20amp receptacle from being overloaded. If you die tomorrow and no one is aware you put the 20amp receptacle on the 30amp circuit breaker and runs a load higher than the 80% of the rating of the receptacle continuously, you run the potential of overheating the receptacle and causing a fire.

Rob