I know this subject seems to be debated to no end but I have a few questions/comments.
My main service panel is in my attached garage and I want to add a 100 amp subpanel about a foot or 2 to the side of it to power a mini split, air compressor, welder, and few other future tools for my garage workshop (mill/lathe/CNC router, etc)
I have a Square D QO load center as my main breaker with lug attachments rated at 75C and it looks like Square D rates their breakers for 60/75C service. Looking at 310.15(B)(16), I can get away with 3 AWG CU or 1 AWG AL wire rated for 75C but I just need to make sure the subpanel terminal lugs are rated for 75C as well. Correct?
I'd rather work with copper conductors since they will be smaller and easier to manipulate but I'm not finding a whole lot of 3/3 cable available. I also figure that using SE cable would be easier instead of messing with conduit unless I come in right below the original service panel and run the wire straight down into the subpanel with a short, straight piece of conduit. Don't know if there's any code against doing that or not (height issues). But if I did it that way, I could use individual 3 AWG CU wires which might be easier to source.
Any thoughts/comments?
Best Answer
You have the right size wire, although the aluminum is no harder to work with than the copper
You are calling out the correct gauge wire for a 100A feeder, which is good, and you are also correct that distribution lugs are rated for 75°C wire, allowing you to use that column on the ampacity chart (110.14(C) point (a)(1) even permits it for <100A, never mind point (b) permitting it for 100A and up in any case). However, your assertion that copper will be smaller diameter/dramatically easier to work with isn't quite right. Fat aluminum wires are compact stranded by default, as opposed to the typical concentric stranding of a stranded copper wire, which means they do not take up much more space than a copper wire of equivalent ampacity.
There is also no minimum height for a panel
While you cannot have a breaker handle over 2m (6'7") from the floor (to make sure that folks can reach it!), there is no minimum height requirement, and large commercial panels sometimes are 6' tall or more, with breakers starting at 12-18" above the floor. This means that your plan to put the subpanel below the main panel is fine; do remember to pull the bonding screw out of the subpanel if it's shipped with one fitted, and torque all distribution lugs to their specified torque with a torque wrench or screwdriver that reads in inch-pounds. (This last requirement is a new one in the 2017 NEC, specified in 110.14(D)).