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Existing Subpanel Wire Size & New Subpanel


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  • Existing Subpanel Wire Size & New Subpanel


    I am a newbie to this forum so hopefully I am following protocol here. I have searched this forum extensively before deciding to post my questions here.

    I purchased a home in 2007 that was built in 2004. Main Breaker panel is in a "finished" garage with 200A service. All 240V circuits (2 AC Compressors, Dryer, Range) and a handful of local 120 circuits are in this panel. There is also a 125A breaker that than feeds the main house distribution sub-panel out of the basement (which is completly full, 120V breakers only, maxed out with tandem breakers as well). The basement is unfinished and I need to add a new subpanel - was hoping to tap off of existing panel but feel that I may need to run an entire new cable from the main panel (external conduit in the garage due to fully finished drywall in place).

    First question is regarding the cable & size and capacity rating they used to feed the basement subpanel from the main panel.
    The cables markings are: E32071 (UL) 3 CDR AWG 1 1 CDR AWG 3 COMPACT AL -- ALUMAFLEX (TM) AA8176 TYPE SE CABLE STYLE SER TYPE XHHW-2 CDRS 600 VOLTS MADE IN USA and the run is approximately 50 ft.

    From some of the calculators on the Southwire site it seems to indicate that the max load ths cable is rated for is 75 - 85A. not sure about overall loading requirements. The house is approximately 5400 sq ft divided evenly among 3 stories. Can anybody comment first of all that the original SE cable used to feed the original subpanel is adequate and legal? If so, would I have some recourse to go back to the builder or inspector to push for getting this done correctly to code?

    Second question is - if everything seems OK with this existing cable - should I have enough capacity to add a sub-panel from the existing basement panel or should I run a second cable (same type) to the new subpanel.

    Thanks for any help here, I really appreciate it.


  • #2
    any sub panel derived from the main panel should be SER not type SE. The neutral bar in the sub panel should not be bonded to the equipment ground. An extra grounding bar should be installed in the sub-panel and be fed by the bare ground wire. The white goes to the neutral bar and the two hots [black and red] goes to the sublugs in the panel. you could piggy-back from the sub panel to your remote location but the amperage wouldn't even approach the 125 amp on the first sub-panel. If you wish another 125 amp panel then run a new line from the garage main to wherever you want. Again, use SER not SE!


    • #3
      First off, I am surprised that a home that new, and that BIG, got away with only a 200A service. Anything I have done in that size range required a 320/400A service.

      The #1AL SER cable you have was rated for 100A as a branch circuit or feeder. It was never big enough for a 125A feeder. Would you have any recourse NOW to have it changed? Not a chance in he** if you ask me. ESPECIALLY since you did not have the house built.

      Is it enough to support that existing panel and another sub? Only a load calculation or site evaluation can tell this. Simply saying it is filled to the brim with single pole breakers really doesn't tells us at this point. Sorry.

      Personally, no way I'd run a second sub off that chock-full existing one.


      • #4
        Thanks for the speedy feedback. I believe the cable is SER given that it says both "TYPE SE CABLE STYLE SER ..."

        I will likely do a fully load calculation and inspection but I am currently thinking about placing a new panel adjacent to the existing one in the basement using a similar type of feeder from the garage. Perhaps I can move some breakers / load from the first panel over to the new one and then just use 2 separate 100A breakers in the garage (ditch the 125A). I realize I do not have all the load data yet but was wondering if my logic made sense.

        If I was the original owner I would hav demanded 400A service. Curious how hard that is to get upgraded? My garage panel is pretty much right behind the outside meter, everything is underground. Not sure if my service cable to the secondary transformer is rated for 400A or if that would have to be upgraded as well? I supposed I could call Xcel and find out the options. I may want to pick up a new table saw and and a few other high-current toys.



        • #5
          by "load calculations" I hope you are not just counting the amperages of individual breakers. there's a section in the code that gives a watts per square foot for general lighting, 180 va per outlet, and adding the loads in watts or kilowatts to get total load. then you divide by 240 volts to get amps. there's more to electrical that meets the eye!


          • #6
            Hello HayZee, thanks for the reply. Yep, I definitely know there is more to load calculations (and I do need to study up on that more). Definitely know I can not just add up the amperage on my breakers (I would probably need 600A service if I counted them all )

            Follow-on question if you don't mind:

            I may wire up some temporary can lights in the basement with or without the trim kits (that I can reuse when I finish) just to get things brighter. I plan on using the same type as I have on the finished levels (Preferred Industries 108ra7x-ic) that I can fully cover with insulaiton later (I will do that in the basement ceiling as part of a soundproofing effort)

            Anyways, I thought I heard from someone that you could have up to 20 of these can lights on a single breaker (seems high to me)? I believe this model can take a maximum 75W bulb (I would likely use 65W Par 30) so even if they were all on at 75W max the load should be 1500W or 12.5A - OK for a single 15A breaker I assume? Could I wire these from the one breaker (already supporting a handful of ceramic string pull fixtures that I would replace) via a triple gang switch box supporing 3 series of say 6 - 7 can lights each? Could this all be done with 14/2 NM since they are for permanent lighting and less than 15A total?

            Thanks for any additional help from the forum team. Definitely appreciate it and look forward to contributing in areas where I can in the future.



            • #7
              Technically, 20 65 watt fixtures would work on a 15 amp circuit. but for the sake of argument I wouldn't go more than 15 on one circuit, that's 3/4 of the rating for a 15 amp circuit. this is on your judgement of using 65 watt par30 lamps. the bulbs have an aluminzed internal reflector to direct the heat and light downwards, but you will get some upward radiated heat.


              • #8
                Smaller Subpanel

                Thought about this more. I really do not need another 125A service panel for just my basement. The 125A fed subpanel today that is maxed out with breakers does include about four 20A breakers for outlets (including sump that is now sealed due to Radon Mitigation system) and a couple 15A breakers for lighting. I just want to make sure I have some expansion capacity before closing up the basement and will want a few new dedicated breakers for audio and home automation systems. I am thinking about just running a new NM 6/3 with Bare Ground from the main garage panel - likely with a 50 or 60A breaker to a new subpanel? Does this sound acceptable? (I assume the typical disclaimers about load calculations, etc. - but this is really just for a few additional circuits in the basement). One other question is can I use a much larger rated breaker panel (100 - 150A, etc.) - will have lots of blanks but I am having a tough time finding something small with a sufficient number of breaker positions.

                Thanks for any additional info!



                • #9
                  whatever you decide a 6/3 ser will have an undersized bare [wrapped] ground wire. any panel derived from a main panel is a sub panel, even one right next to a main panel. for your branch circuits - something to consider - is classed as new wiring and as such is subject to the new code regulations. ALL your new branch circuits have to be protected by a 15 or 20 amp AFCI not a regular circuit breaker. An AFCI is an arc fault circuit interrupter. They cost from $40 to $55 each although I've seen some from lowes at $39 and some change.


                  • #10
                    Thanks for the info HayZee! I was at Home Depot earlier, boy those new AFCI breakers are pricy - all wrapped up individually in those impossible to open security packages.

                    I see they had some AL 6/3 with a 6 AWG Ground as well:

                    "Alcan STABILOY AA-8030 AL Type SE Cable Style R XHHW-2 600V 3 CDRS 6 1 CDR 6 SUN-RES (UL)"

                    I would think this woudl do the trick? I don's suppose you need a special 50A breaker for AL vs. Cu (for expansion/contraction)? I would assume a std 50A double pole breaker could handle 6AWG as well?

                    Thanks so much for everyones help. I am going to get this project going. I was about to order a couple dozen Recessed Light Fixtures (108ra7x-IC from Preferred Industries because that is what is used on the upper levels) for about $15 each but noticed that Home Depot and others carry other brands of 6 in can lights including Halo also IC rated and new construction models for about $5-6 each. Not sure why the big price difference (this should probably be a different post but it was on my mind).



                    • #11
                      breakers manufactured these days are Cu-Al rated. any good anti-oxidant used with aluminum will work. 6/3 with ground copper romex will work with a 65 amp breaker, 6/3 aluminum will work with a 50 amp breaker.


                      • #12
                        Ground Wire Size

                        One more quick question, did I hear that my ground wire must be the same size because this is for a subpanel? Or can I use a 6/3 Romex with a smaller bare ground wire (8 or 10) for the 50A subpanel?

                        Thanks again,


                        • #13
                          6/3 romex has a reduced size earth ground. It attaches to an auxillary ground bar that you buy for the panel. The neutral is full sized and connects to the neutral bus with no bonding screw.