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400 Amp service with a standby Generator

bozobozo

bozobozo

Bronze Member
Joined
Jan 21, 2012
Messages
36
Location
Building in Central PA
There a couple of questions in this post

As we plan our electrical requirements and layout for the new house, it looks like I'm rapidly heading toward a 400A service (2 200A panels) due to a number of factors but mainly caused by my woodshop and my wife's stained glass studio plus the rest of fairly good sized house. I will have this confirmed with a formal load calculation. In the mean time let's assume that I'll need the 400A.

Question 1. Since the house is isolated and the power grid in the area is subject to outages during storms, I will be installing a propane fired stand-by generator. Preliminary size is 19-20kW. I would like to use the "smart" transfer technology that allows the transfer to pretty much manage itself and not lock myself into specific circuits prewired to cut over. I realize that this is going to get complicated real fast, but, in concept, can this work in a 400A/2 panel installation? There will be circuits in both panels that need to be supported and to put all of those in one panel would significantly increase my "heavy wire" cost.

Question 2. One of the 200 Amp Panels will be in the basement mechanical room. This room is adjacent to the planned woodshop. Is there any code issues with putting the subpanel for my shop directly on the wall behind the main panel. In essence the main panel and subpanel would be back to back on the same wall.
 
Well first off unless you shed the load thru some energy management system a 20kw generator will not handle 400 amps. 20kw/ 240= 83 amps. The nec will not allow automatic transfer of the entire service unless the generator is rated for the calculated load or you have an energy management system which will control the loads as I mentioned above.

Question 2- As long as the main panels are grouped and location adjacent to the service or directly inside then the other panels(sub panels) may be installed anywhere. You cannot have one main 200 amp panel inside and the other main 200 amp panel outside.
 
Not sure i follow your answer to #1. Set aside the 400AMP for now, say it is the typical 200A. If I follow you correctly then neither GE nor Generac could sell any units with automatic transfer handling unless people purchased a generator capable of handling a 200 amp load. That is not how the technology works. Not sure I understand how it works but it goes something like this. They market a transfer capability with "smarts". The "smart technology" some how senses when the demand starts to exceed the rated capacity of the generator, it then start to shut down circuits (in the main panel(s)) starting with the lowest priority and so on. I do not have a clue on how it works. Maybe this is the load management system you mention.

Based on my preliminary calcs, my total load will be around 285A.

With respect to #2 Both panels would be inside (back to back on an each side of an interior partition (panel will be grounded via a UFER).
 
So how does this transfer switch know what loads to shut off. It sounds like an energy management system built in but there is more to it then just installing the switch.

Back to 2-- I am still unclear. If these inside panels are the main disconnects for the house they must be grouped together. You cannot have one on one side of a wall and the other on another side. Maybe I am not understanding correctly.
 
I agree. You have to group both main panels together (both 200 amp boxes side by side) then all other panels will be sub panels. Main disconnects must be grouped.

If you have a 400 amp service you will need a 30 kw or more generator to run it all. I don't know squat about the smart load boxes you are talking about.

Advice... If you are using propane shop around on generators. Some models/brands vary greatly on btu usage for the same kw rating. They all use a lot of gas. If I could get for instance 20 kw at 265,000 btu's on one unit or 20 kw at 310,000 btu's on another the first unit will save a lot of $$ on propane bills!!
 
If you put the wood shop, the glass studio, the dryer and range in one panel you could probably run the other panel on a 20kw.
 
Daddy-0- said:
If you have a 400 amp service you will need a 30 kw or more generator to run it all. I don't know squat about the smart load boxes you are talking about.
Click to expand...
30kw would only do 125 amps however the OP is using an energy management system which will control the load. Basically it will keep the generator from being overload by only running the loads that were prioritized. If you have a manual transfer switch then you don't have to shed the loads automatically as you can manually do it.
 
you could also install a metered dosconnect outside , which would allow to put anything, anywhere after that. i dont know why more people dont use them. it's a meter xcan with a main disconnect right below it. wonderful thing. everything down line is a sub panel
 
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