ButchNH Show full post »
Hmmmm, my installation passed with no load shed installed, and it's a 30Kw genset feeding a 400A service.
The 150A breaker on the genset protects the 4/0 feeder coming into the house, and the 400A main breaker protects the utility side.

Yes, my wife did prove once that the Evolution controller will shut the unit down if overloaded.
I had told her it would run anything in the house.  No more problems after I explained that I didn't mean she could run EVERYTHING in the house at once. LOL

BTW, when it shutdown I was working in my shop.  The genset is adjacent to the shop building, and I never heard the RPM drop even slightly before the shutdown, so load shed likely would have done nothing even if installed.  The diesel engine is rated at 50HP at 1,800 rpm, which is over 37Kw, so it would have to be severely overloaded before it would bog down.
Geoff Z
NEC Article 702 Optional Standby Systems
NEC 702.4(B) System Capacity
The calculations of load on the standby source shall be made in accordance with Article 220 or by another approved method

The way I have always understood it is that the system shall be sized to handle the entire connected load. Which is an Article 220 load calculation. That is the way our inspectors here enforce it and what they want to see. Anything connected to load shed does not need to be included in that calculation. I can't guess if that is the intent of the code making body. However that is how we do it here. I have always said shame on the code making body. I believe that every wire in the system must be protected at its proper ampacity. That being accomplished a homeowner should be able to choose the size generator they want. They have caused many many people to spend way more money on a larger system than should have been necessary. On the flip side of that thought is the elderly couple taken advantage of by an unscrupulous dealer. Part of their thought process may have been to protect them. We've seen the installation by others more than once where it was promised the generator would run the AC. Requesting a service call from us to determine why their generator won't function properly. Only to have the generator stall when the AC attempts a start. Addition of a sure start sometimes can solve that. However they should never have been put in that position in the first place had the installer done what they were supposed to.
Brian Baughman
Keep in mind that on an existing installation, 220.87 is an approved sizing method.  Using 220.87, you may be able to back up a 200A service with a 16 kW generator.  The bigger question then becomes, can the 16 kW generator start the motor loads.  The 2020 NEC requires ALL types of generators to be sized in accordance with NEC Article 220.   

The two biggest code violations that I see with generator installations are the lack of proper load management, and the improper installation of service rated transfer equipment.

I appreciate this forum and all the advice and idea’s I received. Thank you to Ziller for hosting and everyone that has contributed. When I’m doing a project I appreciate reading about others experiences so I’m going to share mine.   The solution we went with gives us the flexibility we wanted without having to replace the existing transfer switch or deal with load shedding modules at this time.  Our  17 year old 4390 is running well and we’d like to get all the life out of it we can.  We decided to add a sub panel fed off of the existing  Generac 12 circuit transfer switch.  It is located next to our 200 amp main panel.  I chose a 12 space 24 circuit Siemens load center as that is the brand of our main panel and was in stock at our local home center and should provide plenty of room. This version of the panel also came with the neutral and ground bars already installed.  The original wiring of the Generac Panel to the main panel was #4 wire. Although the panel is rated for 100 amps it was fed from a 60 amp breaker in the main panel. I changed that breaker out to 100 amps and ran all new #3 copper to the Generac panel.  The main  internal wiring of the Generac Panel was also #4 so I changed those feeds to #3 as well.    I needed to make some space in the Generac Panel to make room for the 100 amp breaker that is feeding the new sub-panel so I relocated 2 lightly loaded circuits on to other existing breakers in the Generac Panel.  The homeline breakers in that panel are rated to accept 2 wires under their screws.  I used the #4 wire I removed  to run a circuit back to the main panel that is controlled via a mechanical interlock so that if we choose to we can safely feed the main panel by manually selecting the circuits we want to feed, keeping in mind of course the limits of our current generator.  I was surprised to find that the existing neutral that I had planned on using  was not the same size as the #4 conductors (I think it was #6). My understanding was the neutral needed to be the same size as the conductors. The panel is 17 years old so maybe the rules were different then or there was some other exception in the code.  I used it for now but will replace it with #4.  I also learned after ordering my wire that I could have downsized the ground conductor to #8, since I had the #3 in green I used it.     At this point we have not relocated any additional circuits to the Generator protected sub-panel although we may.  I want to get a clamp meter to check the actual load on each circuit as I do my load calculations to be sure we are not anywhere close to our generators limit.  With this set up in a power failure we can flip a few breakers and pick what additional things we want to turn on.  I  made an error by only  cutting the  wire long enough  to replace the  feeds  to the Generac Panel.   In hindsight I should have relocated the “feed” breaker so that it could sit next to the breaker that would supply power to the main panel in an outage.  That breaker by design must be in the top left spot to work with the main  panels interlock.  With my current setup I have to remember to shut off  line power to the Generac Panel during an outage.  Since starting this project I have found an additional interlock device that would prevent both breakers from being on at the same time that I’d like to install. To be clear this is an additional interlock to prevent the transfer switch from switching back to “street” power if it saw power being fed from the generator. The main panel interlock prevents any back feed to the utility side.   We did purchase a nifty hole punch tool to make the correct size holes for the conduit.  Space was a little tight and punching out the metal with this tool was much easier than it would have been trying to drill  the full size hole.   I’ve attached a few picture to show what was done and some of the materials involved.  My clamp meter showed up today so I’ll be   checking the loads of the circuits to see what we can safely add to the generator in an outage.  In case anyone is concerned the main panel interlock is located on the cover that was off for the photo.  IMG_4557.jpg   141.jpg  142.jpg  140.jpg    141.jpg