How do you choose the right sized wire nut?

Question

Is there an exact rule and also a practical rule of thumb to tell if you have the right size wire nut for the wiring you want to connect?

Answer 1

It depends on the number of wires and what gauge they are. A general rule of thumb would be yellow with 14g and red with 12g, but as I said that will change with the number. The packaging they come in will have a chart for what to use when.
The only difference is size, so basically, if the nut won't fit on, you need a bigger one and if the nut slips on but won't tighten, you need a smaller one.

Answer 2

Those little black or white ones that come with a new light fixture are barely adequate but will work - Depending on the brand - some are soft some are hard - a soft (Scotch brand) yellow for 2 #12 wires and a soft red for 3 #12 wires. You use the size wire nut that will hold all the wires comfortably. For best results twist stranded wire individually before trying to join it with a solid wire. Twist solid wires together using pliers before inserting in the wire nut and twisting again.

Answer 3

Wirenuts are color coded for the maximum numbers and types of wires they can connect.
Yellow up to 3 #12, Red up to 5#12 IIRC.
Best look at the package.