In this discussion board thread, Sean White explains why installers need to put the solar breaker on the opposite side of the main breaker. Please share your experience in the comments section below.
Student:
Could you please explain the reasoning behind your recommendation to put the solar breaker on the opposite side of the main breaker or as far away as possible on the busbar?
Sean White:
The reason it is better to put the solar on the opposite side of the busbar, which is required when we invoke the 120% rule and a good idea for other applications, is as follows:
If we are feeding more current to the load side of the main breaker, then we are adding to the current from the main breaker. If the solar plus the main breaker exceeds the busbar and the solar breaker is next to the main breaker, then when there are excessive loads in a worst case scenario, we can have too much current flowing to the loads from the spot where the solar and main breakers are feeding the busbar. It would be like having one big breaker if the solar and the main breakers are next to each other.
If we are feeding the busbar from the opposite side of the busbar from the main breaker, then we would have solar currents coming from one side and the utility currents coming from the other way. This way the currents will not be coming from the same place and the solar will actually make it easier on the busbar near the main breaker by relieving the main breaker side of the busbar from excess currents.
In theory, but not in practice, if we had a 100A busbar, a 100A main breaker, 100A of PV on the opposite side of the busbar from the main breaker and then we had 200A of loads, there will be 100A on each end of the busbar and yet 0A in the middle of the busbar. (this would not be to code in the US, however in some countries it is allowed).
In this scenario above, if we had both breakers next to each other, then it would be like having a 200A breaker feeding the busbar and would cause a hot spot on the busbar, which is a fire hazard.
I hope this helps and please ask more questions.
Learn more with Sean and connect with likeminded installers in his 40-Hour Advanced Solar PV Installer Training.
with respect, you have no clue what you are talking about – hot spot? bus bars are fully rated for the the loads they are to carry regardless of where you put the breakers. you should educate yourself because it is apparent you know nothing of how electrical gear is rated, designed and applied. if you are putting 200a of demand you have to have busing rated for 200a, i.e. a 200a panel. if you put 200a of demand (vs. connected load) on a 100a panel you screwed up and it doesn’t matter WHERE YOU PUT THE BREAKERS …a failure in the busing could occur ANYWHERE THE BUSING IS AT ITS WEAKEST!! not to mention it would never pass plan check if permitted by the county or city. fire hazard? lol…I could go on and on…if the bus fails (and melts) it will ground out and open the main. but a bus would never in a million years melt if you designed it properly. not only that, busing is ‘braced’ for different levels of AIC, that is AVAILABLE INTERRUPTING CURRENT. Usually the lowest is 10,000a on up. the busing is rated to take an influx of current (spike) from the utility company when surges and spikes in current due to failures and re-energizing the grid after an outage. this is the actual ability of the actual bus to take a great amount of current through ALL THE BUSING regardless if it’s a stand up board or just a panelboard. I wrote this off the cuff waking up seeing this email about breaker placement.
“With respect, you have no clue what you are talking about” – Wow! What an intro. Actually, Dr White is correct, based on Kirschoff’s Law of Currents. Rather than suggesting that others “know nothing”, and they should “educate yourself”, you might want to avoid epithets and go for argument – and not respond “off the cuff waking up”. Location of the back-feed breaker is important enough that is gets enumerated in NEC 690.64(B)(2), or 705.12(D)(2) if you’re using NEC 2014. It might be a good idea to hold your tongue and get some learning.