On this project I was hired as a sub-contractor to install the electrical panel for a larger well known solar company. This panel had to be upgraded due to the large 13kw ground mount solar PV system being installed. This was the first time I ever replaced a electrical panel directly next to a window. The original electrical panel (200 amps) was installed at the time the house was built – the panel was installed in-between the wood studs and the siding. The electrical panel has a 1-2″ metal flange that was nailed into the wood studs, then siding was installed over the flange and nails. To remove, I must cut and remove the siding that covers the flange, then pull nails. In this case the new 225 amp electrical panel was physically slightly smaller than the old panel.
In this case the main breaker in the new electrical panel had to be de-rated to 175 amps to allow room for the solar power. What is de-rating? De-rating a electrical breaker is a term for reducing the amperage size of a electrical circuit below the maximum intended size. What does this mean to the end user? It means the home now has the ability to draw 175 amps of electricity from the utility company instead of the original 200 amps. Draw is another word to use or take. You can now take up to 175 amps of electricity continuously. Why would I want less?
I will try to simplify the answer – How electricity is calculated when building a home. Continuous loads and non-continuous loads – Continuous loads are loads that run more than 50% of the time, for example HVAC, water heaters, lights, pools. Non-continuous loads like receptacles, ovens, washer, dryers anything on less than 50% of the time. Both these loads are calculated to determine the electrical load of the home. When the calculations are performed they come up with a number. For example – 150 amps.
Residential electrical panels are sold in different sizes / ratings. Small to large to huge. Small panels are 60 amps, medium are 100-125 amps, large are 200-225 amps, and huge are 400 amps. When a electrical panel is installed it is always larger then the power calculations. The amperage rating corresponds to the largest size main circuit breaker allowed to be installed.
Circuit breakers are a type of over current protection device that protects a circuit from using to much electricity. For example you have a 200 amp circuit breaker in your 200 amp electrical panel, if you take more than 200 amps of electricity at ant time continuously the circuit breaker will get hot and trip to the off position to protect the wiring.
Example if the power calculations come up with a number like 126 amps the smallest allowable electrical panel would be 200 amps. The panel rating always has to be bigger than load (126 amps)
The smallest allowable circuit breaker would be 150 amps ( the breaker size has to be slightly larger than the actual load). When you purchase a 200 amp electrical panel it comes with a 200 amp main breaker installed not a 150 amp. If you remove the 200 amp circuit breaker and replace with a 150 amp breaker you have de-rated the main breaker. In this case there would be 50 amps of capability not being used. This unused space is required to back feed medium to large solar PV power systems. The idea is if the main electrical panel and breaker are rated for 200 amps so in theory if you plugged in every device you could find and turned on all the stoves, HVAC, washer dryers etc and then turned on a solar power inverter the electrical power could exceed the 200 amps and overheat the electrical panel. Most electrician believe these are only extreme theories and not very practical for most people that work during the days and use power in the evening. Furthermore with all the energy conservation devices like energy star appliances and LED lighting power consumption is being reduced below levels in the past.
We can help you determine if you need a new electrical panel for you new solar system. We have legal cost effective ways to eliminate electrical panel upgrades in most cases.
Why not de-rate save money, and electricity. – Solar Electric Pros . com