Energizing both sides of a load center.

New_Mexico_WillNew_Mexico_Will Registered Users Posts: 107 ✭✭
I'm using a single phase inverter, currently wired to one leg of a load center.

I'd like to use both legs of the load center.  Is there a best practice for this?

I've seen people use a jumper from one leg to the other, and Ive seen a double pole breaker used.

Is one better than the other?  Are both/neither safe?  Other ideas?


  • BB.BB. Super Moderators, Administrators Posts: 31,629 admin
    If your AC inverter's output is less than the load center's rating (i.e., a 200 amp load center and an XX Amp inverter output), then an extra breaker from bus bar A to bus bar B is not needed... (two pole breaker might nicer "wiring" and simple On/Off switch for service).

    The only issue with using both bus bars in parallel for a 120 VAC system... In a 120/240 VAC split phase system... The neutral current is the difference between the L1 and L2 current (20 amps on L1 - 10 amps on L2 = 10 Amps in neutral).

    With a single 120 VAC phase system--The typical dual pole breakers (120/240 VAC) split phase connection--L1 and L2 current adds up for neutral current (20 Amps L1 + 10 Amps L2 = 30 Amps Neutral current)...

    So the use of (for example) 3 Wire + Ground ROMEX cable (Black + Red + Neutral + Ground) used for 120/240 VAC circuits that share a single 3 wire ROMEX cable cannot be "safely used" to carry power from the L1+L2 breakers with the single neutral. All wiring (L1+Neutral, L2+Neutral) need their own Return (Neutral) so you don't over current the Neutral (for larger AC inverters---Typically over 15 Amp output for 14 AWG ROMEX.

    That is the only "big gotcha" that you need to be careful of when wiring an existing home or new circuits with using both L1+L2 bus bars on a single phase 120 VAC distribution system (wiring a separate Neutral for each 120 VAC circuit).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mcgivormcgivor Solar Expert Posts: 3,795 ✭✭✭✭✭✭
    edited November 2020 #3
    Wether using a jumper or a double pole breaker the net result would be the same, it's the placement of the jumper itself, either before the breaker, or in its absence across the live terminals. Good practice in any electrical installation is to not supprise the next person who may work on a system in the future  by using unorthodox methods. The question is can it be done? The answer is yes. Should it be done?  The answer is no.

     There are rules and regulations outlined in electrical codes which are primarily adhered to to maintain conformity, it could be argued that you may be the only one who will work on the system, however that doesn't come with a gaurentee. As an electrican I personally don't recommend straying from the rules and regulations, which are there to protect fellow workers.

    Should you decide to to jumper the live terminals, at very least provide  labels to identify what has been done, it may seem trivial, but a heads up is  welcome to the next person who may be working on the system, and it may be someone other than your.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • New_Mexico_WillNew_Mexico_Will Registered Users Posts: 107 ✭✭
    I didn't do a great job asking that question, so let me clarify some things.

    This is my weekend getaway cabin, so nobody touches anything except me.  Regardless, I always label things properly, and always think about either the next guy or when I have to go back to repair or upgrade something and my old brain might not remember everything (mostly the latter).  Not just with electrical, but with everything I install.

    So, for the purposes of this discussion, when I say "safe", I mean electrically safe, rather than circumstances that may or not ever exist and determining their safety.  Again, I clearly label everything where necessary.

    I have a 6,000/18,000 watt single phase 120 volt inverter wired only to a load center in my powerhouse.  It is a 200 amp panel.  From that, there is (among other circuits), a 50 amp single pole breaker that feeds my cabin subpanel (150 amp) through 40' of  6awg THHN individual stranded copper wires in 2" conduit.

    It is the subpanel in the cabin where I would like to use both legs, but only at 120 volts.  I'm not trying to get 240 volts out of it.  No main breaker in the subpanel.  I just need a couple more circuits available.  

    I think it makes the most sense to use a jumper wire between the live terminals because it will be more obvious what has been done.  Although it will be labeled, most people don't bother to read things.

    So, electrically safe?
  • BB.BB. Super Moderators, Administrators Posts: 31,629 admin
    Just make sure that every branch circuit that leaves the panel has its own dedicated neutral return. Do not use 3 wire+ground ROMEX (or other Red+Black+Neutral wiring) (black+red+white+ground) for carrying two 120 VAC circuits as the Black+Red currents will add on the neutral (15a+15a on L1+L2 = 30 Amps on Neutral--Unlike where a 120/240 split phase where 15 amp L1 - 15 amps L2 = 0 amps on Neutral).
    • 6,000 Watts / 120 VAC = 50 amps
    So the 200 amp panel is way over your inverter's output current capabilities.

    The 40 feet of 6 AWG THHN wire should be fine with 50 amp breaker, and ~1.58 volts drop (at 50 amp @ 40 feet) is well within the 3% suggested max voltage drop:


    Voltage drop: 1.58
    Voltage drop percentage: 1.32%
    Voltage at the end: 118.42

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • New_Mexico_WillNew_Mexico_Will Registered Users Posts: 107 ✭✭
    Gotcha!  Thanks a million Bill, you do a great job of explaining things. 2 wire + ground is all thats being used, and I am very conservative with my circuits.  Much appreciated.  Happy Thanksgiving 🦃

  • BB.BB. Super Moderators, Administrators Posts: 31,629 admin
    You are welcome Will...

    Sorry to be repetitive--I was not sure you saw my first post about the 3 vs 2 wire circuits.

    I would suggest stepping back and make sure that your wiring system is simple to convert to 120/240 VAC if ever "upgraded" in the future and you don't accidently bake in the 120 VAC assumptions (i.e., in theory, you could wire your subpanel for 120/240 split phase--I.e., pull the extra wire in conduit and even use a two pole 50 amp breaker in the main panel to "future proof"). If you ever end up with 120/240 VAC split phase needs (new 120/240 VAC inverter, new 120/240 VAC genset, possible 240 VAC well pump, tools for shop/etc., and/or if the "Grid" ever reaches your place).

    Another suggestion would be to look at a 6 kWatt 120/240 split phase inverter and how you would "balance" loads on the L1/L2 bus bars (and if the subpanel would ever be wired for 120/240 circuits--as suggested in previous paragraphs)... Load balancing between L1 and L2 is not needed for your single phase 120 VAC inverter (an example of "baking in" the 120 VAC assumptions). But could be an issue if ever a split phase energy source is installed/connected.

    A 6 kWatt inverter would probably output around 30 amps max per phase (as an example)--So "balancing" loads now between L1 and L2 bus bars now for future split phase power, rather than simply filling L1 first then adding a few circuits to L2 ("overflow") is another example of future proofing and not baking the 120 VAC @ 50 Amps of "today's" energy sources into your system.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • New_Mexico_WillNew_Mexico_Will Registered Users Posts: 107 ✭✭
    Great points as always Bill.  However, I am 100% off grid and have no use for 240, other than my welder, which I always run from generator.  

    I honestly doubt I would do it, but if I used a 50 amp 2 pole breaker in the main panel, would I then run each hot line to the 2 terminals in the sub panel? 🤔  Of course, I can't use a 2 pole in the main panel without a jumper either.  Or are you suggesting just pulling the wire and leaving it unconnected for future possible use?

    This is how I labeled the sub panel.  Sufficient? 

  • BB.BB. Super Moderators, Administrators Posts: 31,629 admin
    Looks to be very clear and concise.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 31,629 admin
    Regarding the 120/240 options... Yes you can pull a spare wire and tape/wire nut it off for future needs (sometimes difficult to pull wire through a conduit with existing cables inside).

    Also--If the subpanel is also a dual bus bar (120/240 VAC split phase)--You would jumper your main panel, install a 50 amp breaker pair, and wire all the way to the subpanel and wire up L1 & L2 as "normal"... That way, you only have one panel (your main panel with "jumpered" L1 & L2 buses--And the downstream panel(s) are all wired normally--Again making it easier for future upgrades to split phase (only the main panel would need "de-jumpering" and nothing "forgotten" elsewhere downstream).

    I know you don't plan on ever needing 120/240 VAC split phase--But who knows the future (some governor or president decides to make electric vehicles mandatory 10 years down the road--Just long enough to put the "mess" in the next politician's lap--As well as everyone else when the grid goes down from lack of generation & distribution resources).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • New_Mexico_WillNew_Mexico_Will Registered Users Posts: 107 ✭✭
    Oh yeah, you live in California, where such a mandate already exists. 

    It might not actually be that difficult right now to pull those wires.  I'm going to take your suggestion.  Thanks again Bill!
Sign In or Register to comment.