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Hi I'm adding a 200 capable transfer switch that will draw power from my 200 amp main. I will move some circuits from the main to the transfer switch. I know I wouldn't be moving all the circuit over, so I'm trying to determine what breaker size and service wiring size would be appropriate to power the transfer switch. I'm trying to do an article 220 calculation to determine this. I want to err on over sizing the breaker in case I want to add more circuits in the future. When on generator, I know I'll have to manage my loads but I don't want to under size the supply breaker so I have to manage the loads when on utility power. For my 220 calculation I've looked at a couple of web sites that explain how to do this and ended up using this one

https://www.ecmag.com/section/codes-standards/residential-calculations-estimating-elements-electrical-system

General Lighting and Receptacle Loads

The square footage supplied by the transfer switch is 1,728 so multiplying that by 3 gives me 5,184 VA for general lighting and receptacle load.

Small appliance loads.

The transfer switch will supply the kitchen but not the laundry room So this means I need to add 2 x 1,500 VA for a total of 3,000 VA.

So for these 2 groupings there is a total 8,184 VA. Taking into consideration demand factor the first 3,000 VA are at 100 %. The remaining (8,184 – 3000) at 35% for 1,814 VA. So I'm now at 4,814 VA(3,000 + 1,814).

Now I need to add in the special appliance load that I calculate at a 75% demand factor.

Dishwasher 1,500 VA Microwave 1,460 VA Refrigerator 609 VA Freezer 575 VA Grbg Disp 600 VA

Total 4,744 VA and at 75% = 3,558. So now I'm at 8,372. = (4.814 + 3.558)

Next I need to add in my 1 HP submersible well pump 4,000 starting watts? and 5,500 Watt HW heater. I wasn't sure of the demand factor for these 2 items, so I used 100% to err on the safe side. So that would be 9,500 VA for this grouping. And it brings my total to 17,872 VA(8,372+9,500).

So for 240 service, a 75 amp breaker should do and using a 100 amp breaker would give me a good safety factor. Does this seem reasonable. I can do the math but don't have much experience to determine if I'm in the ball park. Feed back would be much appreciated. Thanks

brucewol
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  • Is there a reason you need to run your water heater on the generator? – ThreePhaseEel Jun 19 '21 at 20:45
  • Ok you have a 200 amp transfer switch but what size is your generator? When we size the loads we do not use the standard or optional calculations listed in the code we calculate on the generator size and what loads will be connected. There are multiple ways to achieve the sizing you want a simple way is to use actual load testing, or adding up the va values of each device that will be used while on the generator. Is your switch automatic or manual? – Ed Beal Jun 19 '21 at 21:07
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    i guess i didn't make it clear enough when i said i would manage the loads on generator. My generator is 7000 watts and will feed the transfer switch through a 30 amp breaker, enough to run just my hw heater stand alone and probably nothing else. when i'm using the generator i'm ceratinly not in standard ope rating mode. my plan is run the well pump afternoon and build up pressure in the tanks to last me a day. then turn off the well pump breaker, turn on the hw heater breaker and and get a hot tank that should carry me through to the next day. turn off the hw breaker. then use other stuff. – brucewol Jun 19 '21 at 21:41
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    ThreePhaseEel I run the hw heater on the generator because i don't like cold showers;>) – brucewol Jun 19 '21 at 21:43
  • My switch is manual because my generator is portable and i would need to disable certain loads before starting it. – brucewol Jun 19 '21 at 21:45
  • I'm confident my 7000 watts will run the well pump and hw heater separately. as for my other stuff, i've gone three outages and run my critical other stuff - some lights, internet modem, laptop and cell charges on a honda 2000 watt using extension cords. – brucewol Jun 19 '21 at 21:49
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    You don’t need a 200A transfer switch. Just a sliding interlock plate that interlocks your existing 200A main breaker with a 30A breaker. For modern panels those are as little as $50. – Harper - Reinstate Monica Jun 20 '21 at 02:10
  • i have a 35 year old siemens main - i think, that has a 200 main and 2 100 circuit breaker that feed other panels. Not something that would be easy to replace. I've contacted siemens and they don't offer anything like that. To pass inspection, the sliding interlock can not be attached to the breaker cover where it could be defeated by removing the cover. – brucewol Jun 20 '21 at 05:52
  • You say you "will move some circuits from the main to the transfer switch", do you mean to a new panel fed by new transfer switch? Why can't you put an interlock breaker in that panel and skip the transfer switch? – NoSparksPlease Jun 20 '21 at 15:24
  • Explained this in a prior comment. Extensively researched this and there aren't any for my 35 year old main. possibly I could retrofit a generic one but I want to have this work inspected and my county will not pass inspected if it isn't UL listed for my main panel. Also I might want to add circuits in the future and with the new AFCI requirements, I will soon run out of room in my main panel. – brucewol Jun 20 '21 at 15:47
  • My new transfer switch Reliant TTV2012DR includes a panel that has 36 full width slots, so plenty of room for expansion if needed. If my calculations for article 220 are correct, i would probably add a larger service feed to the transfer switch, maybe 150 amp to allow the most flexibility for expansion. I could go to 200 amp but would need to use UL listed lugs. I like being able to shut off the utility power to the transfer switch and still have my existing main with power. Haven't seen a 200 amp breaker that connects to the panel bus bar. – brucewol Jun 20 '21 at 16:00
  • Yeah, we were trying to just tell you to build that for half the price, or better still, convert one of your 100A subpanels so that it became that. You are correct that 200A branch circuit breakers are not made. Highest made is 125A. – Harper - Reinstate Monica Jun 20 '21 at 23:47
  • Well, I'm trying to tell you that I can't legally add an interlock kit to my main, so any suggestions along that line are dead end. I know it wouldn't pass inspection in my county and moving my house to some other county is a bit impractical. The two other 100 amp are on the other side of the house and running my existing branch circuits to them where possibly i could add an interlock would be way more expensive then adding a UL listed transfer switch adjacent to my main. – brucewol Jun 21 '21 at 04:10
  • Well, my initial question was how to size the service feed wiring for a sub panel with an included transfer switch. Got a lot of suggestions about the transfer switch, none which would work in my situation. – brucewol Jun 21 '21 at 04:13
  • But I did eventually decide what to do as to the size for my service feed. Just go as big as I can. If I want to use lugs, that can be 200 amps. For a breaker, 125 amp is fairly common. I did find a few 150 amp breakers that connect to the breaker bus bar, UL listed but concerned that they might not meet code now. So for now the plan is to supply my transfer panel with 125 amp. – brucewol Jun 21 '21 at 04:18
  • Appreciate all the comments and in a round about way, made me think through what i was trying to do, and came up with an answer to my problem. – brucewol Jun 21 '21 at 04:19

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