Sizing bussing and panel

[rollin772]

Hi everyone, had a quick question. My father-in-law asked me to come over and look at installing a 2/30 for a hybrid water heater. When I got there it was a whole can of worms. He sometime recently had solar panels installed on his house. I think he needs a panel upgrade so I had to start doing some reading, and now I have some questions hopefully you all can help me with.

To start he has a 100A panel /MCB that is full with what I believe is a 2/20 back feed breaker from the PV system. My question's are in regards to 705.12 (B)(2)(3). From reading this it sounds with:
a) I would add up the MCB and Back feed breaker *125% = 150A min for the bussing.
b) if the bussing is 100A then it's over the 120% rating.
c) it sounds like I have to add up all breaker handles minus the MCB and if it's over 100A worth of breaker handles then the bussing is too small.

The people who installed his solar told him they sized it to be able to operate everything in his panel including adding a 2/30 for the water heater and a 2/40 for his dryer, and suggested putting in a sub panel. It sounds like that his current panel is already out of compliance.

Please correct me if I'm wrong, any assistance is appreciated.

 

[suemarkp]

What is the brand and model number of the panel? Many 100A panels have 125A bus ratings. But even if it is 100A, a 20A breaker from the solar system should already have the continuous factor in it (so solar should not be delivering more than 16A of solar). This implies a solar system of around 3.5KW. The 100A main breaker should also have already factored continuous loads. A 100A bus w/100A main + 20A solar puts you right at the 120% limit.

Adding up breaker handles is not how you calculate load. Do a load calc with what is there (and the Optional method generally produces lower loads). Most houses can run on a 100A service if the air conditioning isn't huge (MCA of 25 or less) and you only have two of the following three as electric - water heater, clothes dryer, kitchen range. Clothes dryers are generally two pole 30's. Never seen a 40A dryer.