This can be done -- the subpanel still has load to spare, albeit only by an amp
Since you have a 100A service, I can use the simplified calculations from 220.84, which give you 3VA/ft^2 for the house, 1500VA per Small Appliance Branch Circuit (kitchen/laundry), and the fixed appliances at nameplate rating (it sounds like you have none though), as well as 100% of the nameplate rating of the A/Cs.
From here, we get:
- 1200ft^2 * 3VA/ft^2 = 3600VA for lighting and general receptacles
- 1500VA per appliance branch circuit (kitchen, laundry) * 4 (3 kitchen, 1 laundry) = 6000VA for appliance branch circuit loads
- 8kVA for the range, 1.2kVA for the DW, 1.5kVA for the microwave, and 5kVA for the dryer = 15.7kVA
Before the HVAC loads are taken into account, we apply a 40% demand factor to everything over the first 10kVA, giving us a total of 16.1kVA so far.
The HVAC is simple -- we are dealing with 3 11.5A (conservatively) A/Cs on 110V circuits, so that's another 3.8kVA at 100% demand factor, giving us 19.9kVA total load, or 82A on a 240V service.
Now, when we add the 4.5kVA water heater in, this is covered by the 40% demand factor as it's over the first 10kVA of non-HVAC load, so the resulting net load is 4.5kVA*40% = 1.8kVA, or another 7A on the service -- pushing your service to 89A, which is still manageable.
We then repeat this for the feeder to the subpanel, with the stove at 8kVA, the dryer at 5kVA, and the washer circuit at 1500VA, as well as a conservative assumption of 1440VA for the two rooms (180VA/outlet * 4 outlets per room * 2 rooms), to yield 12.4 kVA for the feeder load after applying the 40% demand factor. With a 240VAC feeder, this gives us a 52A feeder load, which has just enough room to handle the additional factored load from the water heater (again, 1.8kVA or 7A) -- that is pushing the feeder to 59A though.
Well, to simplify the maze somewhat, you have all the usual loads of an all-electric house.
- 50A furnace
- 50A range
- 30A air conditioning pack
- 30A dryer
- Eleven 120V branch circuits, some MWBC, many on 30A breakers (WTH?)
Obviously we can oversubscribe this somewhat, e.g. You're not likely to run heater and A/C at the same time.
The fine art of panel subscription is not a core skill for me, but hopefully this summary will make it easier for someone else.
That panel, though
While we're here, we gotta talk about that panel. First, panel B top-tapping panel A before the main breaker is very dangerous unless there is another main breaker before this. The meter is not a fuse, and there are no fuses between the transformer and main breaker. This is considered acceptable for a very short hop from the meter to the main, however, in your case this totally unfused feeder crosses your whole house. Nothing prevents a short in the cable, or in this panel, or summed loads on this panel, from setting that cable on fire.
Second, all 4 panels are Federal Pacific "StabLok". It has serious issues with panel fires due to bus defects, and unreliable trip. When feasible, all four should go.
The three subs will be straightforward to DIY, the main will be more troublesome unless there's an outside main breaker. I would DIY install a 200A main service panel right next to it as a subpanel, reuse that 70A breaker to feed it, and move all the 120V circuits out of the main into the new sub. Then bring in an electrician to cut it over to be the new main.
I see many branch circuits on 30A breakers. Those should be 15 or 20 depending on wire gauge.
Gaps in the panel covers should be patched so curious fingers can't touch buses, except finding either breakers or blanks to cover those spots will be a pain.
Best Answer
From the Rheem website for this model:
Most likely, that means it is using an SCR based controller (solid state power controller) to do variable power control of the heater elements and the phase-angle firing of the SCRs is causing extreme line noise and harmonics that is affecting other devices when it is on. Not a lot you can do about it, but in small doses as a HW heater would run, it's not particularly harmful. If it is causing severe negative effects that you can't stand, you could try having someone install a Line Reactor or an Isolation Transformer ahead of the HW heater. That will not be inexpensive though.