hitehm
Senior Member
- Location
- Las Vegas NV
My company is installing a residential outdoor subpanel fed from a feeder circuit off the house main panel. The subpanel will be 4 position main lug only and will have 1 60A breaker for the hot tub and 2 20-amp breakers for lighting and general use receptacles. I have several questions about load calculations to properly size the feeder's breaker in the main panel and wiring:
As for the feeder circuit load:
1 – Since there is no building or structure fed by this feeder, how do I calculate the 2 20A general use branch circuits in the subpanel? At 180VA each? I think I understand they are essentially part of the load for the service based on square footage of the property but I assume they must still be added to the feeder load calculation since they are loads on the feeder wiring. What am I missing here?
2 – What type of load is the hot tub? An appliance, A motor, other? It has 3 motors and a heater all totaling 11,520 max watts. Are they separate loads?
3 – The hot tub has a max wattage of 11520 watts. Is the load added at 100% or is there any demand factor that would apply? I read on another forum that someone calculated the wattage over 10k of a hot tub at 40%. I don’t see anywhere in the NEC220 or it's tables where the hot tub would qualify for this. Regular appliances (not small appliance ckts for kitchen) only qualify for demand of 75% after 4 units and this feeder is not supplying a separate building over 100A so it doesn’t qualify for the “optional methods” 40%.
As for the service load:
1 – Since we are essentially adding to the service load, is the feeder added to the service load as 1 singular load or are all the individual branch loads on the feeder’s subpanel treated separately on the service just like its main panel branch circuits? Or is it not part of the service load calc at all? My confusion is that NEC chapter 220 seems to treat feeders and services as equal as far as load calcs. But how can you have a feeder that’s not part of a service? Can someone explain this, hopefully in simple terms?
As for the feeder circuit load:
1 – Since there is no building or structure fed by this feeder, how do I calculate the 2 20A general use branch circuits in the subpanel? At 180VA each? I think I understand they are essentially part of the load for the service based on square footage of the property but I assume they must still be added to the feeder load calculation since they are loads on the feeder wiring. What am I missing here?
2 – What type of load is the hot tub? An appliance, A motor, other? It has 3 motors and a heater all totaling 11,520 max watts. Are they separate loads?
3 – The hot tub has a max wattage of 11520 watts. Is the load added at 100% or is there any demand factor that would apply? I read on another forum that someone calculated the wattage over 10k of a hot tub at 40%. I don’t see anywhere in the NEC220 or it's tables where the hot tub would qualify for this. Regular appliances (not small appliance ckts for kitchen) only qualify for demand of 75% after 4 units and this feeder is not supplying a separate building over 100A so it doesn’t qualify for the “optional methods” 40%.
As for the service load:
1 – Since we are essentially adding to the service load, is the feeder added to the service load as 1 singular load or are all the individual branch loads on the feeder’s subpanel treated separately on the service just like its main panel branch circuits? Or is it not part of the service load calc at all? My confusion is that NEC chapter 220 seems to treat feeders and services as equal as far as load calcs. But how can you have a feeder that’s not part of a service? Can someone explain this, hopefully in simple terms?