Lee Schierer
USNA '71
Go Navy!
My advice, comments and suggestions are free, but it costs money to run the site. If you found something of value here please give a little something back by becoming a contributor! Please Contribute
Sorry Tom, but this is correct. In full accordance with the NEC, youare permitted to install either 15 or 20 amp outlets onto a 20 amp circuit. You are also permitted to install 40 or 50 amp outlets on a 50 amp circuit. These are the only two exceptions, and all other circuit sizes and conditions must be equal.
Contrary to what I believe someone else may have stated, you cannot install a 30 amp outlet on a 20 amp circuit, nor can you install a 20 amp outlet on a 15 amp circuit.
I am talking about the plugs -- not the outlets!!!
Yes you can have 15A or 20A outlets -- as is the case in kitchen 120v 15A outlets on a 20A circuit and yes you can have the 40A/50A on 240v to cover motor startup etc... on the 40A line.
But we are talking about the PLUGS that we install on our larger 240v tools -- most items, that the average person buys come with the plug already installed. I think that all 240v tools should come with the correct plug installed -- if you want to wire it direct you should be forced to remove it.
This would help in having things wired correctly -- It would still be a problem in duel volt setups.
Read the manuals -- both of my newest (just delivered) tools state very clearly that they should be installed on a 15A not 20A 240V circuit.
This type of direction is the norm -- check your split AC units -- it is the same thing.
The internal wiring of the tool is built to a standard not to exceed. Why do you think that there are so many different plugs!
I am sorry to sound like I am ranting -- But I really have been shocked (joke!!) at the mistakes -- that I have noticed in some shops that I have looked at since I began to plan my new one.
The issue with tools or machinery and the maximum ampacity circuit you can connect them to, is due to the withstand rating of the tool or machine electrical component.
In a nutshell, a larger ampacity circuit will have lower impedance, and of course a larger ampacity breaker.
In the event of a fault in the machine or tool, the larger ampacity circuit can deliver more short circuit current than the tool or machine can withstand.
If the withstand ratings are exceeded, the tool or machine components can explode violently.
Anyone who has observed electric welding has seen a process that uses aproximately 25 to 30 volts at aproximately 100 amperes. Imagine the energy release and arc at 240 volts, 500 amperes.
A 30 ampere circuit with 3% impedance (usually the maximum allowed by code) is capable of delivering a maximum of aproximately 990 amperes of current into a fault.
The manufacturer of the tool or equipment, will design and test the tool to withstand the expected fault values. If you take a tool that is meant to operate from a 15 ampere circuit, and connect it to a 30 ampere circuit, you have doubled the available fault current.
If you aren't the electrical engineer responsible for the design of the tool, you have no way of determining whether that's safe. That is why machinery often specifies a maximum fuse or breaker size, and may even specify a certain time/current characteristic, or current limiting property for the fuse or breaker.
Regards, Rod.
Last edited by Rod Sheridan; 10-19-2007 at 3:20 PM.
A trouble light is a tool. Assume it contains a 100watt bulb which is rated for much less than 15 amps. Does that mean I cannot safely use a trouble light on a circuit that is protected by a 15 amp breaker?
The rating on a tool indicates the minimum rating on the supply circuit, not the maximum. My Dust Collector has a FLA of 20.7 amps. It is supplied by a circuit wired with 10ga copper and protected by a 30 amp breaker. Do I need to replace that 30 amp breaker with a 20 or 25 amp breaker. No, of course not.
The purpose of the breaker is to protect the circuit components and is sized for those components, not for whatever devices might happen to be plugged into the circuit. The breaker has absolutely no function or responsibility for protecting anything beyond the receptacle.
Tom Veatch
Wichita, KS
USA
Tom, there's something you're overlooking: a 20A receptacle (either 120V or 240V) is intentionally designed to accept either 15A or 20A plugs. So, the assertion that a 15A plug is meant to prevent a tool from being plugged into a 20A receptacle is incorrect.
You mentioned in another post that "both of my newest (just delivered) tools state very clearly that they should be installed on a 15A not 20A 240V circuit." I'm going to take a wild guess that your new tools are... Grizzly. Grizzly has chosen to put this restriction in many of their manuals, but the rest of the world (and the NEC, and the UL), has absolutely no problem with plugging a 15A (or less) tool into a 20A circuit.
I hope this helps clarify things. Enjoy the new tools and shop!
This is why fused plugs are required in some parts of the world (like Britain).
They are a good idea if you can find them.
Yes, I realized this distinction right after I made my posting, but I left my posting to stand as-is because this still overlooks the basic concept of circuit protection.
As another poster already pointed out, because you can have 15 amp outlets on a 20 amp circuit, there is nothing preventing you from plugging a tool with a NEMA 5-15 plug on it into a 20 amp circuit. Moreover, a NEMA 5-20 receptacle is designed to accept both a NEMA 5-15 or NEMA 5-20 plug.
When the NEC says that you should refer to the appliance's manual, they are referring to a properly and accurately created manual; not the type you find with woodworking machinery. (I write manuals, so I know exactly how this erroneous information makes its way into poorly written manuals.)