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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?

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  • At 2' to the side just run a nipple then use single thhn wires from the main to the sub this would be the way most pros would connect the panels, there is no minimum height so you could mount the sub below the existing panel but in my opinion this looks like crap but would be legal. Main lug panels are usually 75c and this would be fine since the main panel should have the breaker to protect the sub, no real need for a second breaker this close together in my opinion.
    – Ed Beal
    Apr 17, 2018 at 23:11
  • Thanks for the reply. I'll do it that way then. Home Depot / Lowes doesn't seem to carry 3 AWG, I'll check around at the local supply houses and see what they have. Thanks!
    – J.B.Lee
    Apr 18, 2018 at 2:29
  • Yeah you definitely want to be dealing with real electrical supply. Home Depot and those guys either leave you high and dry for the parts you need, or stiff you on anything that isn't super mainstream. They know they gotcha, their typical customer has no idea where a real electrical supply is. Also you may notice the lugs are aluminum, might go Al just to avoid dissimilar metals... then it would be #1al. Your ground does not need to be #3, metal conduit is the ground...
    – Harper - Reinstate Monica
    Apr 18, 2018 at 6:04
  • Good info on the ground. I found a local supply place close, I'll give them a visit when my other parts come in.
    – J.B.Lee
    Apr 19, 2018 at 11:43

2 Answers 2

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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)).

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"Metal Conduit is the ground" is definitely NOT applicable to sub-panels per code. Code requires that sub-panels be grounded by a ground wire from the main panel to the sub-panel (even when metal conduit is used), generally ONE size smaller than the conductors. It has been found the the lug rings and other components may loosen or not have been tightened leaving the ground not able to provide protection as designed. In the sub-panel you must REMOVE the bonding screw so that the neutral and ground are not bonded inside that sub-panel. This makes the ground continuous to the main panel and separate from the neutral. You want the ground circuit to ONLY be bonded inside the main panel. This can be very important for the newer codes which require AFCI/GFCI protection.

This was not true in the past in commercial, however it has been code for a very long time.

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    No, the only places where bonding around knockouts is required are a) services and b) work above 250VAC (i.e. 277/480 commercial stuff), and even then it's only required where non-listed eccentric/concentric/tangential KOs are encountered. See sections 4.7.6 and 4.7.7 of the Steel Tube Institute's Guidelines for Installing Steel Conduit/Tubing if you want positive language on this (the NEC only specifies bonding means for >250V and for services, staying silent on non-service applications <250V).
    – ThreePhaseEel
    Jun 17, 2019 at 22:55

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