220.30 Optional Calculation

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suemarkp

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Kent, WA
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Retired Engineer
I used the Optional Calculation for dwellings for a 400A service. I further used this same calculations for the split 200A panels on that service, and a 100A subpanel off of one of the 200A panels, so I could determine the load on each panel.

The load calculated on the 100A panel comes to less than 100A (actually about 65A). Can this method be used for feeder loads under 100A? The code says "total connected load served by a single set of service or feeder conductors with an ampacity of 100 or greater", and I've wired that subpanel for 100 amps. I just don't understand if this means the resultant amp calculation must be 100A or more, or I just need to wire it for 100A or more. The optional method produces a substantially smaller amp demand than the classic calculation.
 
Re: 220.30 Optional Calculation

Dwellings connotes multi-family. If this is the case then the optional calculations depend on the HVAC (supplemental heating) usage. [220.31], [220.32] I am not sure what you have, sorry for the vague answer.

rbj, Seattle
 
Re: 220.30 Optional Calculation

Sorry, I think I just used the words as they appear in the code book. This is a single family dwelling. All of the space heating is on one of the 200A panels. My basic question is can I use this calculation method if the calculated load on a panel comes to less than 100A. I think I can since the feeder does have an ampacity of 100A, but the load is less than 100A.
 
Re: 220.30 Optional Calculation

Hi Mark,

You are in good shape as far as feeder's go if you wired for full capacity. What my next question would be is do the panel schedules balance out between the two 200 amp panels.
The only time I use an optional load calc is to qualify the load capacity within the panel rating. If the space heating loads and the 100 amp sub are wired to the same panelboard then the loading would lean toward a max-out. So that's where optional calcs for the space heating will help in that panel. (I think the factor is 40% if four separately controlled heater units, 65% if central electric space heat.) The other panel I'd use standard method unless it's tight there also. Another question is if the space heating is split up..then the panels can be balanced out as the best approach for a total optional calculation. Your heating is going to be around 8 to 12 btu-h/sf for the State's Climate Zone 1 if in the Puget Sound area. I hope this helps.

rbj, Seattle
 
Re: 220.30 Optional Calculation

It was an interesting exercise to calculate. The total service comes to about 350A overall (standard method) or 290A (optional method). However, getting the 200A panels to work out is difficult, as each is in the high 190's using the standard calculation and in the 160A to 180A range using the optional calculation. The 100A panel is 85A standard and 65A optional. This value was plugged in as one of the loads to 200A panel #1. Space heating is on Panel #2 and is around 130A (two heat pumps).

I wonder if one reason I have such a large discrepancy between the standard and optional calculations is because someone told me motors don't count for the 75% rule for 4 or more appliances fastened in place in the standard calculation. I have a lot of motor type loads (pool pumps, table saw, dust collector, pool heat pump), so each of those is in at 100%. For the optional calculation, all of those ended up in the 40% after the first 10KVA section.
 
Re: 220.30 Optional Calculation

Mark,

I wonder if one reason I have such a large discrepancy between the standard and optional calculations is because someone told me motors don't count for the 75% rule for 4 or more appliances fastened in place in the standard calculation. I have a lot of motor type loads (pool pumps, table saw, dust collector, pool heat pump), so each of those is in at 100%. For the optional calculation, all of those ended up in the 40% after the first 10KVA section
Click to expand...
First..a couple of questions. Did you mean pool water heater circulation pump or ground water source heatpump using pool water? Help me out with a little more detail. How many stages and size elements are in each unit? House sf? Tons of @ H/P? Are H/P's package or split systems?

The fastened-in-place(four or more) with heatpump [220.30(C)(2)] loads apply with the 100% NPR's in the 75% standard calc, and only 65% for central heating with supplemental elements in the optional method. The two heatpumps do not qualify as four separately controlled units.[220.30(C)(5). If there are two separately controlled baseboards elsewhere, then they would technically qualify.

rbj, Seattle
 
Re: 220.30 Optional Calculation

There is a pool circulation pump motor (1/2 HP), a pool spa pump motor (2 HP), an air-to-water heat pump for the pool (27A MCA). This is all pool equipment and I would have liked to consider it all fastened-in-place appliances with a 75% factor along with the power tools (table saw (1.5 HP), dust collector (1 HP)) and normal fastened-in-place appliances (like water heater, dish washer, garage freezer). But the pool motors, heater, and tools were calculated at 100%. Can all of these be used at 75% in the standard method?

The house has two split air-to-air heat pumps, a 4 Ton w/15KW resistance heat air handler, and a 2 Ton w/5KW resistance heat air handler. The strip heat and compressor can both run at the same time, so I used these at 100% for both calculation methods. I have no baseboard or other separately controlled electric heat.
 
Re: 220.30 Optional Calculation

I posted a new topic "Two heatpumps" in the NEC section. The idea is that the heatstrips are separately controlled at the stat for override. I do not see anything that disqualifies the two heatpumps as 4 separately controlled entities. This will allow 40% option calcs...hopefully.

rbj, Seattle
 
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