NEC load calc for load relocation from main panel to a feeder panel

[MSI]

I am trying to relocate a few breakers in a residential house from the main panel of a new construction. The loads include four 60A furnace breakers, two 30A water heaters, two 50A AC units, and a 30A dryer breaker, all of which I plan to move to a new panel. My goal is to keep the feeder breaker size to 150A. However, when I perform the load calculations for these appliances without knowing their actual wattages, I find it difficult to keep the total below 200A. The load calculations exceed I have a 200A service panel, but I'm only using certain appliances without including other general loads. Here’s how I can resolve this issue:

**Step 1:** Determine the larger of the two loads: 65% of the heating load or 100% of the AC unit.

**Step 2:** Consider general lighting usage as 0 since I am relocating no loads to the new panel.

**Step 3:** Small appliances also count as 0 because I am not relocating any loads to the new panel.

**Step 4:** For fixed loads, I have two water heaters at 4,500W each, totaling 9,000W, and one dryer at 5,000W.

**Step 5:** From Step 1, use the highest value. For Step 2, 3, and 4, take 100% of the fixed loads and the rest at 40%, which totals 9000W.

**Step 6:** Add the results from Step 1 and Step 5, then divide by 240.

The result is above 150A; I am sure this is not true as the actual demand is definitely less than 150A, but how can we prove on paper that the loads I am trying to relocate will be less than 150A?

 

[jaggedben]

First of all, it doesn't seem at all prudent to put all that load on a 150A breaker unless you know some of it is basically non-concident (either per 220.60 or per common sense). Seems like the *actual* load may be above 150A, and the calculated load even higher.

Are you getting the 65% from the optional calculation? Arguably the code does not say you can use that option for a feeder that serves only part of the load of a dwelling. However if you calculated the entire load of the dwelling (including small appliance and lighting, etc.) according to 220.82 then a case can be made that no sub-portion of the load should require a larger feeder. But I still think that will be larger than 150A.

Otherwise I think you have to size your feeder for 100% of the nameplate load unless some of it (e.g. heat and A/C) is definitely non-coincident. See 220.60. I see no other allowances in 220 you can take advantage of.

(Must be an expensive house. That amount of load is pretty gobsmacking to me. I imagine there are ways to be more efficient. But I digress...)