On the heat pumps I get it. With out actual numbers using an educated guess is the way to go.
I used 80 since it reciprocates with 1.25.
Either way it does not change it a whole lot. Just good to know what he used as a base line.
No way I could do so quick on paper off memory.
For the talk I will change the heat pump to MCA. I agree that would the correct way to present it.
Those heaters as mentioned by other very seldom run at full capacity. By including them in the General load. I would look at the General load for the dwelling as whole. I have to wonder if by water heater they were thinking about tank type that is usually around 4500 watt not 36,000 watts which would not have much impact. However that is not what is says. This is where common sense kicks in.
Fedder service conductors need be as large as the largest branch circuit. Which in this case is the water heater @ 150 amp. So one in each panel if you use two. Then break out load from there. Since the first ten kva is at 100%. And the rest at 40% one can look at that heater in that panel as149.9 amps. The thing of it is the the water heater is feed with 4-40 amp breakers so the it is not the largest in that panel. I would say the 10kw is in #1and 15kw in the other. So the largest branch if a single circuit to the aux heat would be. 62.5 amp load. With the demand on C -3 for aux heat @ 65% or 40.6 amps the demand load on the panel is low compared to looking at connected.
Looking at it that way the calc makes since.
Now would I do it.
So if I was the owner as far sy HVAC goes I would be sure to have outside air temp sensor installed on both outside air units for starters. Then the aux heat would only run for a short period on the first sequence. Then the heat pump would load ahead on full aux heat load.
So I would put one heat pump, one air handler with aux heat and one water heater in each panel. Then balance in the lighting loads along with the other required loads.
I would run a panel connected load sheet to help balance.
I would consider if load centers are used two 225 amp or one 400 panles board. With single runs to the water heaters your going to need the space.
I would also consider since it's 3500 sf ft. Adding two 100amp panel in mech rooms for additional branch circuits helping with any voltage drop and added space. Arc/GFCI does not like voltage drop. This will also remove these from the panels with the surge loads which increases nuisance tripping.
My big concern is the poco trans and utility meter.
Many areas have one trans four many houses. Get with your utility and give connected load and have them run the numbers based on all secondary loads from a shared transformer and the load on the primary into the sub division.
As a rule I take total connected and cut in half. That most likely will be what utility will do for your secondary to the meter. You can pay the difference and have them do a larger service drop and help with voltage drop during peak load demand.
As we all know voltage drop effects the out put of those fixed resistive loads. In turn the ampacity also drops. For a water heater upon demand is used. Voltage drop can bring on additional demand for the unit. Then instead of one or two sets of sequencers it may bring on three or four to try to keep up with water demand and increase run time before the sequencers start to drop out.
Again the AFCI and GFCI combo breakers do not like this drip. While on this subject do not forget about the no load current draw of the combo units. This would be I recommend heavy load in one panel and combo branch circuit out of another.
Then there the option of a 600 amp and ct meter in transformer or however your area does it.
If I find time today after yard work. I will work on panel schedules and connected load for fun based on custom 3500 SF house(dwelling).
