DC Breaker Load vs Line question

gkerlingkerlin Solar Expert Posts: 27
Hello,

As part of a solar install on an RV I am putting a DC circuit breaker on each side of the MPPT Controller

The circuit breakers are marked "Line" on the top terminal and "Load" on the bottom one.

With regards to the breaker between the panels and the Controller it seems that the Panel side is "line" and the MPPT side is "Load"

The breaker between the MPPT and the batteries seems a bit more confusing.... (and does it even matter?)

Would the batteries be the "Line" side or the "Load" side?

And does it matter? It would make my layout of the conduit between the breaker box and the MPPT much easier if the batteries were the "Line" side of that breaker.

Thanks in advance.

Comments

  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: DC Breaker Load vs Line question
    gkerlin wrote: »
    Hello,

    As part of a solar install on an RV I am putting a DC circuit breaker on each side of the MPPT Controller

    The circuit breakers are marked "Line" on the top terminal and "Load" on the bottom one.

    With regards to the breaker between the panels and the Controller it seems that the Panel side is "line" and the MPPT side is "Load"

    The breaker between the MPPT and the batteries seems a bit more confusing.... (and does it even matter?)

    Would the batteries be the "Line" side or the "Load" side?

    And does it matter? It would make my layout of the conduit between the breaker box and the MPPT much easier if the batteries were the "Line" side of that breaker.

    Thanks in advance.

    Some breakers are designed with an asymmetry between the line and load side and may not be able to interrupt current as well if the applied voltage is on the load side. There are also features such as shunt-trip (remote control) and GFCI which make the two sides different. But AFAIK for a DC breaker, unless it also shows a preferred current direction (which I have never seen) I would not expect it to make much if any difference.

    NEC requires breakers which have line and load labels on them to be installed as labelled, but for this case, since both sides are energized and a fault could occur on either side, the point seems moot. But FWIW, the battery is the "unlimited" energy source and would probably justify being the Line side for that reason.
    SMA SB 3000, old BP panels.
  • gkerlingkerlin Solar Expert Posts: 27
    Re: DC Breaker Load vs Line question
    inetdog wrote: »
    NEC requires breakers which have line and load labels on them to be installed as labelled, but for this case, since both sides are energized and a fault could occur on either side, the point seems moot. But FWIW, the battery is the "unlimited" energy source and would probably justify being the Line side for that reason.

    That NEC label thing is what got me to thinking about which side is really the load in this case. It's a 60amp Tristar controller, an 80amp breaker, and #2 wire.

    Normally the current should flow towards the batteries so they would be the load I guess, but then again like you said - the batteries pack a lot more power.

    I think I'll consider the batteries the line side unless I get some convincing argument to do otherwise.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: DC Breaker Load vs Line question
    gkerlin wrote: »
    With regards to the breaker between the panels and the Controller it seems that the Panel side is "line" and the MPPT side is "Load"

    Wrong. Its the other way.
    If you go to Midnite's web site and look for their E-panel instructions you will find a discussion and explanation. Your cable between the panels and the controller is sized so that it cannot be overloaded by the panels. The only source of current that could overload that cable would be the batteries. It would require a short in the panels and a fault in the controller (seems unlikely) but that's the way they recommend.

    gkerlin wrote: »
    The breaker between the MPPT and the batteries seems a bit more confusing.... (and does it even matter?)
    Would the batteries be the "Line" side or the "Load" side?

    Batteries are line side. Same reasoning.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • silvertopsilvertop Solar Expert Posts: 155 ✭✭✭✭✭
    Re: DC Breaker Load vs Line question
    vtmaps wrote: »
    Wrong. Its the other way.
    If you go to Midnite's web site and look for their E-panel instructions you will find a discussion and explanation. Your cable between the panels and the controller is sized so that it cannot be overloaded by the panels. The only source of current that could overload that cable would be the batteries. It would require a short in the panels and a fault in the controller (seems unlikely) but that's the way they recommend.




    Batteries are line side. Same reasoning.

    --vtMaps

    MIdnight Solar explanes this very well, In simple terms your are protecting the the controler from a battery meltdown, check out there combiner inst.
  • gkerlingkerlin Solar Expert Posts: 27
    Re: DC Breaker Load vs Line question

    OK guys.. thanks much. I've got it now. Line side in both cases towards where the real power lies... The batteries.
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