Feeder for Hot Tub Subpanel Load Calculations

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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?
 
Most hot tubs I've seen have the 125 percent added in for the breaker size and wire so you can probably just get a 70 to 90 amp feeder to it and the 2 outlets I'd treat as "lighting Loads" if a residence since that's what the convenience outlets are in the calc even . The hot tub I'd put in with the other appliances like WH, Dry, Dish, Disp, and compactor. But I might be wrong on the last one. A 200 amp residential service will almost never see near that and the heater in the hot tub cycles. On the feeder like a service. What is a service? It's a feeder from the utility transformer. You can have a feeder to a panel with loads and an additional Feeder from that one. The load calcs help account for load diversity and non coincidental loads so see how small wire and breaker you can safely go. After That it's a design question outside the code.
 
The service is the easiest, just list the items being served by the feeder as if they were just branch circuits on the service. This will get you reductions for demand factors or other factors at the Service level.

For the feeder, you're basically stuck with using the nameplate of the hot tub as the primary load. Whether it is a continuous load or not could be debated and may depend on the design of the tub. They can certainly run for more that 3 hours with the heater on, but I'd argue that is atypical. Heating and motor loads usually have a 125% factor though. Is there any ampacity value on the nameplate? They could have added 125% factors to the watt rating, but since it comes out nicely to 48 I doubt it (and the probably want it on a 60A circuit). So multiply by 125% if you want to be sure. Someone discounting the portion above 10 KW at 40% is probably mis applying the Optional calculation rules. Not sure you can do that. Maybe if the feeder is 100A or more you could.

I don't see a load requirement for the receptacles since they are outside and related to a dwelling. But common sense says to add something for those. I usually add about 10 amps on a 240V feeder to cover that. I'd put them at 0 though in the service calculation. Not all circuits have load values in the service calculation (e.g. the mandated outdoor receptacles, bathroom receptacles) as they get covered in the general lighting square footage value.

How large of a feeder were you planning (or wanting) to install?
 
The service is the easiest, just list the items being served by the feeder as if they were just branch circuits on the service. This will get you reductions for demand factors or other factors at the Service level.

For the feeder, you're basically stuck with using the nameplate of the hot tub as the primary load. Whether it is a continuous load or not could be debated and may depend on the design of the tub. They can certainly run for more that 3 hours with the heater on, but I'd argue that is atypical. Heating and motor loads usually have a 125% factor though. Is there any ampacity value on the nameplate? They could have added 125% factors to the watt rating, but since it comes out nicely to 48 I doubt it (and the probably want it on a 60A circuit). So multiply by 125% if you want to be sure. Someone discounting the portion above 10 KW at 40% is probably mis applying the Optional calculation rules. Not sure you can do that. Maybe if the feeder is 100A or more you could.

I don't see a load requirement for the receptacles since they are outside and related to a dwelling. But common sense says to add something for those. I usually add about 10 amps on a 240V feeder to cover that. I'd put them at 0 though in the service calculation. Not all circuits have load values in the service calculation (e.g. the mandated outdoor receptacles, bathroom receptacles) as they get covered in the general lighting square footage value.

How large of a feeder were you planning (or wanting) to install?
Thanks for the replies so far. I never knew that the branches off the feeder can be treated as branches off the service and therefore are part of the service demand factor. That's very good to know. Honestly, the feeder and hot tub are the more confusing and frankly more ambiguous part of the NEC as far as load calcs.

The hot tub can definitely run for over 3hrs and has motors so I did treat it as a cont. load. To be safe, I simply added 14400 for the hot tub (11520 *1.25) + 2400VA for the 2 20s which = 16800/240=70A. The feeder breaker and subpanel terminals are all rated for 75deg so #4 wire gives me 85A max and we always use 90deg THHN cable. It's 3 live wires in conduit buried 18" so no derate for over 3 needed and the ambient temp in Vegas gets hot but 18" down in soil won't get anywhere near 100deg. Even if I use the 105-113 deg row on the derate table at .87 gives me a max adjusted amps of 80A. CONCLUSION: A 70A feeder breaker with #4 THHN should be more than enough.

I certainly welcome anyone knowledge and/or expertise here if I'm missing something. Thanks.
 
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