What would be the best way to apply an resistive load to a 20 amp circuit for the maxinum of the circuit ?

<Title change - KISS>

Your purpose?

It is a test requrirement for electrical regulation / circuit capacity test I have to load a circuit for the rated load and measure the voltage at 15 minute intervalls for the duration of the test I am not allowed to vary +/-3%

You asked for the "best way". The "best" way would be to use an "electronic load". One such company is this one: Programmable AC and DC Electronic Loads (http://www.electronicloads.com/). The loads have a "constant resistance" mode which is what you would use.

Programmable AC and DC Electronic Loads (http://www.electronicloads.com/)

You can purchase fixed resistors and a switch capable of switching the load. Circuit breakers, in general should not be used to switch a load on or off. When you use resistors, you will have to take into account the temperature coefficient of the resistor.

With a electronic load, you do not.

it is a test requrirement for electrical regulation / circuit capacity test i have to load a circuit for the rated load and measure the voltage at 15 minute intervalls for the duration of the test i am not allowed to vary +/-3&#37;If you could also write in sentences and use punctuation and capitalization it would help us comprehend you better.

KISS gave you as good an answer as you are going to get.

What is this for? Certification for yourself? Or some equipment?

You also need to understand Ohms Law.

R (Ohms) = Voltage (E) / Amperage (I)

Ohms = 120/20 = 6 Ohms. So you would have to load the circuit with 6 Ohms to force a 20 amp load. The resistor would also need to handle 2400 Watts.