Good Morning, while I saw this at a woodworking site, I thought I would post it for comments..
..the guy who wrote this claims he is an EI in 5 Western States.......
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...."Maybe it would help if I explained the situation in which multiple receptacles are typically installed on a 50A circuit, and why.
I'll use my own shop as an example. I have typical midrange/light industrial tools. For example, I have a Makita 2030 jointer/planer. It has a 2 HP motor and draws 18A when running. Typical startup current is double that, 36 A (double is a rule of thumb for motor startup).
Suppose I use a 20A circuit, with 10 guage wire, which appears to meet NEC for a motor circuit. I also use arc fault circuit interruptors and a GFCI. What happens when I turn on the motor? The breaker trips - the AFCI breakers respond quickly to overcurrent, unlike traditional breakers that respond slowly (if at all) to faults. If I use a traditional breaker, the motor will start, but the lights will dim and the motor voltage will drop, stressing the motor. That, and traditional breakers will only trip due to a sustained, significant overcurrent condition - so they often don't trip if a motor winding shorts, for example. (which is why AFCI's should be used for all circuits, even though that is not yet a code requirement - currently AFCIs are only required for bedrooms)
There are a couple of solutions to this problem. I could wire a dedicated 40A circuit with 6 guage wire to each machine. That would work, but is overly expensive. In addition, even with derating there are practical limits to how many such circuits I can legally put in my breaker panel. It's likely 200A service would not be sufficient if I am running my house off the same panel.
The better (IMHO) solution is to use a single 40A or 50A circuit with multiple 20A receptacles. I typically would wire this with 6 guage wire ( 4 is even better), which resolves the light dimming/motor drop issue. As noted previously, this meets NEC requirements. One significant benefit is that only one 40A or 50A circuit is added to the breaker panel, so I can still power my house with 200A sercvice. The only real disadvantage is that you can't start multiple machines simultaneously, but for most of us with small shops that's not a big deal. "
