120 percent rule for solar breakers

Most solar PV systems are connected to a property’s load panel or sub-panel. Solar inverters feed AC electricity to connected appliances, and any extra power is back-fed to the grid to power other appliances in the network. However, the Canadian Electrical Code limits the connecting capacity of multiple power sources feeding a distribution centre. In this article I will explain why such limitations exist and ways to increase a solar PV system’s capacity beyond CE Code 64-112’s 120 and 125 percent rules.

Why load side connections?

Load side connections are the simplest way to connect a solar PV system to an AC grid. The only upgrade required to a load panel is an installation of a bi-directional breaker as far away as possible on the bus from the other source breaker(s). However 64-112 limits the sum rating of the supply breakers at 125% of bus rating for homes, and 120% for non-dwellings, and I’ll give an example to show why.

What’s the point of CE Code 64-112’s 120 and 125 percent rules?

Consider a bus that has a rating of 225A and connected to a single source of energy (transformer, generator, or what have you) through a 200A breaker. No problem here. The bus is always protected because if more than 200A of current tried to reach it, the supply breaker will trip and interrupt the current. Is this still true when the bus is connected to another power sources? Below is a picture of a load panel being fed from two sources, namely a solar inverter and another power source, which could for example be a generator, a transformer or another panel board. Each power source is connected through a breaker that limits the current or power flow reaching the bus.

solar inverter interconnection — This is how most small grid-tried solar PV systems (<500kW) are interconnected to the grid. They are installed behind a breaker like any other appliance connected to the bus.

Suppose that a solar inverter is connected through a 100A breaker. Now if both sources were feeding the bus at their maximal capacity, they could be pushing a total of 300A into the bus, which exceeds the bus rating of 225A :

For a 225A bus in a home,CE Code 64-112 requires that the sum of the breakers feeding a bus can’t exceed (225+25%) 281A as per CE Code 64-112 (4). So if the main breaker that’s connecting the home to the grid has a rating of 200A, the solar breaker(s) can’t exceed a rating of 81A or 80A. So we can install one 80A breaker to tie-in one inverter or for example, 4x20A breakers to tie-in four separate inverters or distribution panels connecting several inverters.

CE Code 64-112’s 120 and 125 percent rules

64-112 (4) limits the sum of the sources feeding a bus to 125% of the bus’ rating for homes, and 120% for non-dwellings. If we want to install a greater output capacity, we can

buy a panel with a greater bus ampacity. This is the most expensive option, and its impact is limited. For example it’s not easy to find single phase panelboards with rating greater than 225A. We can also
reduce the rating of the main 200A breaker. This depends on whether or not the client can afford to reduce their loading capacity. Again this option is limited.

History of line vs load side connections in Canada

When Ontario spread-headed the adoption of distributed solar in 2009 with their microFIT program, they required output of solar inverters to be connected to a separate meter. A home would still have one run from a transformer, but it would split to two meters, one for consumption, the other for generation. Most jurisdictions today including Ontario are providing either net-metering or net-billing settlement systems that don’t require a separate meter. This load meter just has to be replaced with a bi-directional one to be able to measure exported energy.

Advantage of line side connections

Many installers and inspectors outside of Ontario are only familiar with load side connections because it is the most popular way of interconnecting relatively small systems. But CE Code 64-112 actually allows for line side connections (rule #1) before it allows for load side connections (rule #4). A line side connection will bypass any load-side connection limitation, including CE Code 64-112’s 120 and 125 percent rules.

CE_Code_64_112_Line_Side_Splitter_Solar_Grid_Tied — As we can see with a line side connection the panelboard is now fully protected and risks no overload. With a line side connection the panelboard no longer limits the interconnecting capacity of a grid-tied solar PV system. Furthermore, the line side equipment such as the meter socket doesn’t have to be upgraded at all, unless of course the capacity of the solar inverters exceeds the ampacity of the meter socket.

Line side connection on meter socket

If the bus bar on meter socket already has enough extra space for the new cables you want to install I would just install them there. It’s important to note to the inspector you are not actually increasing the capacity of power flow through the meter socket since solar is a negative load. So unlike a load side connection that increases the current through the bus, any current from the solar on line side will decrease the current from the grid and so the ampacity of the meter socket need not be greater than what it already is. Of course that’s assuming the size of the solar pv breaker is less than or equal to the rating of the main breaker on the home panel.