What happends when you back feed a panel with to much AC?

rollandelliott
rollandelliott Solar Expert Posts: 834 ✭✭
I've never understood this 125% limit rule the NEC has. If you have a 100amp breaker box that max you can backfeed that box with a maximum of 25amps (20amp breaker).

Since the Solar energy is decreasing the load via back feeding why is there a limit on how much energy you can back feed? Theoretically can't you back feed 100% or 100amps in this theoretical example?

Is there some safety issue that occurs when you backfeed more than 125%?

I've read that one can get around this by installing a "gutter" before the circuit breaker box. Seems like there is not a limit to the gutter. Why's that?

I've also read that some utilities don't approve of this method.

A brief reason for this would be appreciated.

Peace, Rolland

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: What happends when you back feed a panel with to much AC?

    The bus bars you are connecting to have a current limit. Exceed the current limit of any conductor and it heats up - and melts.

    I don't know what this "gutter" is of which you speak. Something to catch the metal of the melting bus bar perhaps. :p
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: What happends when you back feed a panel with to much AC?

    For the first 1/2 of your questions... My understanding would be:
    • Connections in box are rated for XXX amps maximum. Feeding the top and bottom of the bus bars would allow 2x XXX amps to be pulled from the box. That would allow people to pull too much current through various breakers and/or would in the case of a short circuit, possibly damage bus bar connections with 2x too much current.
    • More or less, one of the famous engineering rules of thumb is the square root of 2 (1.414)... Basically anything smaller than 1.414 is considered to be the "same" and anything over is "different". So, 1.20 is a number that was "picked" to be used.
    • Remember that heat goes with the square of current (P=I^2 * R)... So 2x current means the box would have 4x the amount of heat. 1.20^2=1.44x that amount of heat (see sqrt 2 number above). That is a significant amount of additional power to be dissipated by the box and you run a real chance of serious risk of fire/damage.
    • And most wires increase resistance as they get hot--As the box heats up, the wire resistance will increase, which will even cause more heat that needs to be dissipated).
    Regarding the Gutter Question--I don't know enough about power systems to make any guess on what all is involved in their ratings...

    However, I am sure they have some sort of maximum ratings.

    For example, pole transformers for home 120/240 VAC distribution are rated at 10,000 amps maximum surge. Sort everything coming from the pole needs to "break" 10,000 amps minimum--Your main breakers.

    For office parts, the resistance of the power drops from the distribution transformers come into the calculations too--For meter boxes very close to a distribution transformer, they have to use special fuses that can interrupt >10,000 amps, or whatever their requirement is, because surge current exceeds "normal" breaker's interrupt capabilities.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rollandelliott
    rollandelliott Solar Expert Posts: 834 ✭✭
    Re: What happends when you back feed a panel with to much AC?
    The bus bars you are connecting to have a current limit. Exceed the current limit of any conductor and it heats up - and melts. :p

    I understand the principle, but here is how I think.

    If you have 20amps of solar back feeding the box doesn't that decrease the need to pull more current from the grid? thus the amount of current draw is the same, now it is just a mix of solar and grid current together instead of just grid power.

    the only way this rule makes sense is if the grid power and the solar backfeed power are additive so that more current goes through the box. The grid power goes directly to the circuit box and powers the home, the solar power goes through the box and directly to the grid. Is that how it really works?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: What happends when you back feed a panel with to much AC?

    The problem is that all the current doesn't flow from the panels to the grid. There are load points connected along the way. It is therefor possible for the loads to pull from both the grid and the inverters at the same time, making the current potential on the bus bars the sum of the GTI output plus the maximum of the main breaker. This is why one of the "solutions" is to put in a smaller main breaker.

    Like Bill said, if the GTI feed is at one end of the bus and the grid at the other then there theoretically is no point where that "double draw" of current can happen, which allows for the 2X connection rating.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: What happends when you back feed a panel with to much AC?

    The Grid and Solar actually "share the loads" and any excess flows back to the grid... For all the world, this looks like a giant version of the car's electrical system. From a very simple home owner's point of view, the GT Inverter is actually supplying local loads (sharing current from the grid/battery) and recharging the battery (Grid).

    Now, lets look at your main panel... It was designed to carry (for example) 100 Amps at 120/240 VAC... By you feeding in power from the opposite battery bus, you are now supplying (up to) another 100 Amps at 120/240 VAC. And this "box" that has been designed to supply 100 Amps maximum to a bunch of internal circuit breakers can now supply up to 200 Amps--where does all that extra heat go?

    Remember, for home breaker panels, you can install more circuit breaker capacity than the main breaker rating... So even if "you" try to draw more than 100 Amps, the main breaker will pop and stop that from happening.

    The whole NEC is really designed to "idiot proof" home wiring systems as much as possible. If the "licensed electrician" and "inspector" have done their jobs, then anybody working on the system later can assume that basic safety requirements have been made and no matter what, anything they do down stream (adding more branch circuits for example) will also be "safe" (if done otherwise to code).

    If, on the other hand you have installed 200 Amps of current sources in a 100 Amp box--then adding more than 100 amps of total circuit breaker is no longer safe... So, assuming that anyone can do that, then every time anyone went to work on a home power system, they would have to work they way back out to the pole transformer to see if they can "engineer" the extra branch circuit the husbanded wanted at the garage work bench.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: What happends when you back feed a panel with to much AC?

    A few "current flow" scenarios:

    1). GTI inverter outputs nothing; all power comes from grid as with any household.
    2). GTI at full power, no loads on branch circuits; all power flows to grid.
    3). GTI at full power, some loads on branch circuits not exceeding GTI potential; power flows from GIT to loads, "surplus" GTI production flows to grid.
    4). GTI at full power, loads on branch circuits exceed GTI's output; loads consume all of GTI's power, additional power needs drawn from grid.

    It's that last one that can cause the problem: there are two sources of power sharing the same circuitry. If their current potential exceeds the circuitry rating (as can be the case when GTI is added to an existing system) then you could have overheating and all its resultant disasters. That's the reason for the limitations.

    I have to point out that I deal with off-grid systems as a rule and may not be 100% up on grid-tie intricacies. However if it were me I'd favour the line side tap install, wherein the GTI output goes through its breaker to the feed between the meter and the main panel breaker. All the power comes from the same end, as it were. In some cases a separate "outgoing" meter is required. In some places line side taps aren't allowed. Why? I don't know. For all the science that is supposed to be behind the rules every now and then you come up against something that just doesn't make sense.

    Ah, the joy of being off grid in the middle of nowhere! The rulebook is common sense and good practice because there aren't any inspectors. ;)
  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    Re: What happends when you back feed a panel with to much AC?
    I've never understood this 125% limit rule the NEC has. If you have a 100amp breaker box that max you can backfeed that box with a maximum of 25amps (20amp breaker).

    Since the Solar energy is decreasing the load via back feeding why is there a limit on how much energy you can back feed? Theoretically can't you back feed 100% or 100amps in this theoretical example?

    Is there some safety issue that occurs when you backfeed more than 125%?

    I've read that one can get around this by installing a "gutter" before the circuit breaker box. Seems like there is not a limit to the gutter. Why's that?

    I've also read that some utilities don't approve of this method.

    A brief reason for this would be appreciated.

    Peace, Rolland

    It is actually 120% (NEC 690.63B.2) for residential.

    The principle is you have a breaker box with a certain rated amperage bus bars feeding the branch breakers. It is theoretically possible to turn on a bunch of stuff in your house that would exceed the bus bar rating by 20%. If you did not have the solar feed the main grid breaker would pop, but with the backfeed from solar the maximum bus current could be grid breaker plus the solar feed breaker max., in summation.

    You can replace the main grid breaker with a smaller breaker and that would allow a larger solar backfeed breaker but you have to be sure that the grid breaker is large enough for your typical max loading, assuming no solar supplimenting.

    The pistol part of it is when I look at the GE panels of 150 amp versus 200 amp service, the bus bar is exactly the same thickness, just made longer to allow for more branch breakers. But specs are specs.
  • solarix
    solarix Solar Expert Posts: 713 ✭✭
    Re: What happends when you back feed a panel with to much AC?

    The backfeed rule is pretty much just a theoretical possiblility, but needs to be adhered to for the tiny fraction of times where it might light a house on fire. I've never even heard of a situation where the main breaker trips do to an actual overload of the panel. The other practice we follow here is to position the solar backfeed breaker on the opposite end of the bus from the main breaker, thus minimizing the chance of both feed currents adding together.