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I want to add a sub panel in my basement. The main panel is in my garage, the next level up. The distance from the main panel is roughly 10 feet. The main power comes in through the basement and up to the garage panel. If I add a sub panel in the basement, there is no where on the underside of the main panel where I can feed the #2 cable. All the spots left on the lower part of the box are too small.

Obviously the main panel sits between 2 2x4’s. If I ran the cable between the next set of 2x4’s, is it okay by code to run the cable though the 2x4 above the main panel the have it come through the top of the main panel, is that okay.

In addition, does that #2 cable have to be secured to the 2x4’s along it’s route?

One last thing. I purchase a 125 amp panel and I was told the the #2 wire is sufficient by code. Is that correct? I definitely do not need 125 amps in the basement. I will only be using 3 or 4 breakers in that box for lighting and outlets.

I just want to make sure I run this correctly the first time because I will be tearing out drywall in the garage and I will be replacing it too and only want to do the job once. 😀

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    Did you already buy the cable, and how many wires does it have in it? – ThreePhaseEel May 6 '18 at 3:38
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    Is it copper or aluminum wire? – Harper May 6 '18 at 4:09
  • It is aluminum 2/2/2/4 – Bob May 6 '18 at 4:28
  • Yes! I already purchased the cable. – Bob May 6 '18 at 4:30
  • What size is the main breaker in your main panel? – Retired Master Electrician May 6 '18 at 14:04
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Take the subpanel back and get another one, with an eye toward two features:

  • Auxiliary ground bus included, not an extra-cost add-on
  • lots and lots of spaces. Right now in your construction, spaces are dirt cheap; regrets are expensive. Especially if drywall is involved.

Also look for

  • being bundled with useful "bonus" breakers
  • being a quality type (Siemens, QO, GE Qline, CH)
  • having sensibly priced AFCIs ($40ish) and GFCI/AFCI combos ($55ish). Can't use brand X breakers on brand Y panels; that's not brand loyalty but a matter of proper fit.
  • same type as your main panel (if you care)

It does not matter what the panel ampere rating is, as long as it is >80A because that is the breaker to use with your 75A 2/2/2/4 cable.* There is nothing wrong with getting a 200A subpanel for instance, as long as it has a plentiful supply of breaker spaces. Headroom increases safety and also allows for easy upgrades (just replace the cable/breaker).

The subpanel does not need a main breaker, but if it's cheaper (with bonus breakers) to accept a panel with one, that's fine.


* 75A -- yeah, the advice you got was that wrong. Never take advice from big-box again. In fact let's get you shopping at a proper electrical supply house. Most are locally owned businesses, and most will be happy to deal with you at sensible prices. Be honest with them that you're sick of dealing with big-box and their limited selection**, terrible advice and you're heard electrical supply prices are actually better on most things.

** I suspect that's why they put you into 2/2/2/4 Al.

  • Thank you! So you suggest 80 amp breaker to put in the main? The sub panel I purchased was a Square D 125 amp 12 space with ground bar kit to handle a 2/0 lug and it plug on neutral ready. – Bob May 6 '18 at 21:02
  • My basement already has a couple of lines that go to the basement for the Hvac for the first level, the lighting in all of the rooms and some outlets in a couple of the rooms. So I am just planning on using this panel for outlets for my shop and a soon to be rec room. Down the road I might put a small Hvac down there, but very unlikely. My other option is to put in two 20 amp tandem breakers for the basement and hold off on installing the sub panel. – Bob May 6 '18 at 21:19
  • Homeline panel... Standard big-box dreck and they stiff you for the extra ground bar. Tandems, so your main panel is full, then? Not good. With so many main-panel circuits passing near this panel, count on future moving some/all of them to this panel to free up space in the main. Plus 2 spaces for today's local loads, plus 2 for a future mini-split, plus 2 for an EV charger, we're at 12 already. Spaces go faster than you think, especially when you have them to spare. I would get a 30 at least. Trust TPE above. – Harper May 6 '18 at 21:31
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If I ran the cable between the next set of 2x4’s, is it okay by code to run the cable though the 2x4 above the main panel the have it come through the top of the main panel, is that okay.

Yes, that perfectly fine by the National Electrical Code.

In addition, does that #2 cable have to be secured to the 2x4’s along it’s route?

Yes if the cell between the 2x4s is open. If you are fishing the wire into an enclosed wall then you don't need to support it inside the wall.

One last thing. I purchase a 125 amp panel and I was told the the #2 wire is sufficient by code. Is that correct? I definitely do not need 125 amps in the basement. I will only be using 3 or 4 breakers in that box for lighting and outlets.

The conductors have to be sized for the load to be served. A 125 amp breaker is considerably more expensive than a 100 amp. Then you could use #3 copper.

So, you don't need #2 if you are not going to fully load the panel. However, if you ever plan to add more to the panel you Should consider wiring it for the full 125 amps and be able to fully load the panel at a later date.

Good luck!

  • His #2 is aluminum – Harper May 6 '18 at 15:20
  • @Harper well then that would be quite undersized for 125 amps. – ArchonOSX May 7 '18 at 16:48
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Your cable will do the job you ask of it, but it won't let you have the full 125A down the road

The NEC rated ampacity of a 2/2/2/4 aluminum SE cable when used as a feeder run within an insulated wall is 75A (as the surrounding insulation limits you to the 60°C column in the ampacity chart). However, 75A isn't a standard breaker size, so we round up to an 80A breaker as per 240.4(B). Either way, this is plenty of current available at the subpanel, so enlarging the cable would only be desired if you wished more upgrade room in the future (say -- if you had future plans to convert the basement into an accessory dwelling unit).

If you do want to take that cable back and get something bigger and better, you'll need to get a 2/0-2/0-2/0-1 SER cable to feed the subpanel fully -- this gives you 115A of usable ampacity, and the ability to use a 125A breaker as per 240.4(B). (It also means your minimum bend radius goes up to about 6" due to the cable being fatter.)

As to cable routing...

A 2-2-2-4 SER cable has a minimum bend radius of 5" (5 times the cable's 1" diameter), so getting it to make a U-turn into the top of the panel shouldn't be an issue. You'll need to support it at 4.5' intervals and within 12" of the loadcenter cabinets unless you're fishing the cable through a concealed space (in other words: treat it like fat NM).

Go big or go home!

One mistake many people make when putting panels in is trying to shave pennies by buying a panel with too few spaces in it. While you may only need a handful of breakers now, putting in a large subpanel now is far cheaper than having to rip it out and install the large subpanel later because you needed more circuits. Keep in mind that you can't really make use of double-stuff breakers these days due to AFCI requirements, as the 2017 NEC extends AFCI protection to all dwelling unit circuits (the only exception being a circuit feeding a fire alarm control panel as per NFPA 72).

As a result, the best bet is to buy a big subpanel now. Given that a main lug or convertible panel will suffice (as the main breaker in a subpanel that has one is simply a convenience shutoff for the rest of the panel, save for feeder tap cases you don't have to worry your little head about), I'd recommend a Siemens P3040L1125CU -- it's not terribly expensive, has plenty of spaces as far as subpanels go short of going to 200A bussing, and comes with all the accessories needed -- just mount, pull bonding screw, add breakers, and wire!

TORQUE ALL CONNECTIONS TO SPEC

No mechanic questions the importance of properly torquing the head bolts on a car's engine, and the worst that will happen if you don't is that the engine will break and leave you stranded. Considering that a mis-torqued lug on a panel or breaker can start your house on fire, why question the importance of properly torquing electrical connections? In fact, the 2017 NEC added a requirement to properly torque connection lugs and screws to manufacturer specifications (it's 110.14(D) in the new Code).

So, get an inch-pound torque wrench and/or an inch-pound torque screwdriver and make sure those panel and breaker lugs are torqued to the manufacturer's specifications (they'll be printed on the breaker or the loadcenter's labeling).

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