If you have a piece of equipment that is 220 volt single phase,and nameplate of 70 amps, and has a 100 amp breaker installed in the machine,can you hook this up to a 20 amp breaker with # 12 wire,and be legal ? The reason I ask is that is how the machine is wired and it is only used at about 30 second intervals and it stays on,I'm thinking that regardless of if it runs or not you still have to size the breaker and wiring to the machine and not just what works

doc,
It may operate OK but it would not be a legal installation. You must be within the nameplate min. or max. ratings. Of course if it is a single motor with a H.P. rating on nameplate you would have to use NEC Table 430.148 to size circuit breaker and conductors. I hope I was of some help.

cinker (Frank)

The number 12 is protected at 20 amps per art.240-3(d) but you might want to look at art.670-4(a).I dont know you're circumstance but I dont think you will get to change it if it works as is (been there done that).

[QUOTE]Originally posted by doc:
[B]If you have a piece of equipment that is 220 volt single phase,and nameplate of 70 amps, and has a 100 amp breaker installed in the machine,can you hook this up to a 20 amp breaker with # 12 wire,and be legal ?

I realize that this is an old post, but I want to address the issue as best that I know how for others who may run into the same situation. If the load is a straight motor load on a branch circuit, the conductors feeding the load must be sized in accordance with 430-22 (which is 125% of the amp rating given in chart 430-148). The OCP for the branch circuit shall be sized in accordance with 430-152. If the equipment has a short time, intermittent, periodic, or varying duty cycle then you may use the chart 430-22(b) for the conductor ampacity. You'll notice that under certain conditions this chart requires that you raise the ampacity of the wire. So bear in mind that just because the equipment is not used for more than 30 seconds that the ampacity of the wire will go up (it may go down). Also remember that a "motor is to be considered to be for continuos duty unless the nature of the apparatus it drives is such that the motor cannot operate continuously with load under any condition of use (1999 NEC Handbook page 468 left side of the page)." Therefore, if there are no control devices that insure that the equipment will not run at full load for 3 hours or more, you must treat the equipment as if it were continuous. There are other issues involved here (110-14 (c), temperature, and others) from a stand point of design, that would be better addressed by an engineer. I won't bore you with the details.