What is the significance of NEC 690.64 and maximum allowable back-feed?
I need to put 3 circuits of 20A (Enphase) into my 200A distribution panel, but my installer is saying I can only put in 40A due to 690.64 requirements.
Comments
Martha,
Thanks for your question. This is a very common occurance in the installation of larger residential PV systems. I wrote an article a little while ago that covers this topic in depth.
The main thing that it is meant to do is to protect your service panelboard's bus bar from being overloaded. For a quick example a 200A service panel can accept as much as 120% of its rating. That means that it can take up to an additional 40A of input. Since you have 60A of PV output you would be oversupplying the bus bar which could pose a fire hazard.
So, what can you do? You could downsize your main breaker if it is 200A to 175A. This would require an electricain to inspect the connected loads and determine that doing this will not cause unecessary tripping of the breaker.
The other option is to have the installer perform a supply-side tap. I detail this proceedure in another article. Supply Side Tap
I hope that answers your question and thank you for posting this.
Comments
Martha,
Thanks for your question. This is a very common occurance in the installation of larger residential PV systems. I wrote an article a little while ago that covers this topic in depth.
The 120% Rule - “What is it and why it is important?”
The main thing that it is meant to do is to protect your service panelboard's bus bar from being overloaded. For a quick example a 200A service panel can accept as much as 120% of its rating. That means that it can take up to an additional 40A of input. Since you have 60A of PV output you would be oversupplying the bus bar which could pose a fire hazard.
So, what can you do? You could downsize your main breaker if it is 200A to 175A. This would require an electricain to inspect the connected loads and determine that doing this will not cause unecessary tripping of the breaker.
The other option is to have the installer perform a supply-side tap. I detail this proceedure in another article. Supply Side Tap
I hope that answers your question and thank you for posting this.