wheisenburg
Independent Solar Consultant
First of all you look at the net power running on each hot leg. If you are have -100 on both L1 and L2 that results in 0 amps on the neutral. The -means the current is flowing out of the house against the voltage so you are exporting power. If you then add a +200 amp load on L1, the current flow on L1 reverses for a net of +100 amps on L1. This load is not balanced so the entire 200 amps will need to return on the neutral. This doesn't have anything to do with L1 and L2 adding together. If you put a 200 amp 120 volt single phase load on a system, it will have a 200 amp return current on the neutral. What does get added together is power that is produced locally with power that is imported. The power flow then becomes this. 12,000 watts exported on L2. 12,000 watts imported on L1. You are consuming 24,000 watts on L1. So the power produced locally on L1 never get exported. It is consumed locally and never flows out of the house. The neutral is at zero volts so it is just a return path for the current. Watts = volts * amps, so 0 * x = 0. There is no power flowing over the neutral.What happens if you have 100A of backfeed at 240v and 200A consumed on one L1? 100A two pole OCPD
That would be 100A from Utility on L1
100A to Utility on L2
200A to Utility on N
So it looks like this:
Net Power Flow:
L2 = -100 *120 = -12,000 watts (exported power on L2)
L1 = (200 - 100) * 120 = +12,000 watts (imported power on L1)
Net Exported is 0 watts.
Power Production:
L1 & L2 = -100 * 240 = -24,000 watts.
Power Consumed:
L1 = +12,000 watts produced locally +12,000 watts imported = +24,000 watts.
So then we get down to the following. If you are back feeding on the supply side of your main breaker the 200 amp load will trip the breaker. If you are back feeding into a breaker in the main panel, then by code, that breaker would be limited to about 20% of the panel rating.
Now if you installed this in such a way that you allowing the system to accept unlimited locally produced power and then to also pull 200% of the panel rating out of a single leg, then yes you could overload the neutral wire. Such an installation would not be code compliant. The bus bars in the panel are generally only rated for a 20% rating over the panel rating. So you would also be overloading the one leg bus bar and the neutral bus too. By code a 200 amp panel would allow about 40 amps of back feed into a breaker installed in the panel. You can go bigger, but only by lowering the rating of the main breaker such that the combined input current would not exceed the rating of the bus bar. This is why a solar installation is required to either be installed on the supply side of the main panel or to tie into a breaker in the main panel that limits the current that is back fed into the main panel.
So if the question is can an improperly designed system overload the neutral and/or the panel bus bars without tripping the main breakers, then the answer is yes. This is why there are code provisions that limit the way multiple sources of current are allowed to be be combined together.
Last edited: