Load calculation for multiple hot tubs - showroom

[buzzbar]

Hi all,

I'm bidding on a TI for a retail hot tub company, and they want to have (15) display units in the showroom. I was told by the GC that they want a 50A 240V circuit to EACH hot tub, as they want ALL of the tubs to be fully-functional, meaning that the water needs to be heated up.

How would you go about doing a load calculation for such a scenario? I can't find anything in the NEC that allows derating, so I assume that it'll be 100% of nameplate, or perhaps 125%, since the tubs will likely run past three hours.

Lastly, the building has 480V, and the hot tubs will most likely need 240V (not 208V), so I'm thinking that we need to either go with a 120-240V 3-phase delta step down transformer, or one or several 120/240V single phase step down transformers.

Thanks in advance!

Andy

 

[JFletcher]

Dont know about diversity here, but the limiting factor may be the plumbing - how would you fill 15 hot tubs at once to even be able to need to heat them all at once from a cold start? Sure, prolonged power failure and restoration would kick them all on at once...

btw, there are a good # of tubs that require 60A+ discos now. 50A may not cut it. the last one I installed required a 60A GFCI disco, and a few models in the same brochure needed 70A.

While the heater(s) and recirc pump may run 3+ hours, it is unlikely showroom models would use the jet pumps on for any amount of time. Once they are up to temperature, if left on, the 15 units could conceivably draw w/e the heaters and recirc pumps need simultaneously.

Wouldnt at least a few of the hot tubs use gas heat?

 

[electricguy61]

Dont know about diversity here, but the limiting factor may be the plumbing - how would you fill 15 hot tubs at once to even be able to need to heat them all at once from a cold start? Sure, prolonged power failure and restoration would kick them all on at once...

btw, there are a good # of tubs that require 60A+ discos now. 50A may not cut it. the last one I installed required a 60A GFCI disco, and a few models in the same brochure needed 70A.

While the heater(s) and recirc pump may run 3+ hours, it is unlikely showroom models would use the jet pumps on for any amount of time. Once they are up to temperature, if left on, the 15 units could conceivably draw w/e the heaters and recirc pumps need simultaneously.

Wouldnt at least a few of the hot tubs use gas heat?

I've wired probably a 100 hot tubs in my career, and I've never seen one with a gas heater

Learn something new every day.

 

[Dennis Alwon]

I don't know of any code rule that allows you to use a demand factor for the tubs. IMO, you have to use them at 100% get your total for the building and then use the demand factor allowed by art. 220

 

[drcampbell]

I don't have a complete answer without diving in, (and billing for it) but here's my three cents' worth:
- 208 volts probably suffice for a showroom.
- Autotransformers could be used to step 277Y480 down to 240Y416, and MWBCs could be used for each group of three receptacles.
- Be sure the HVAC contractor gets a copy of your load calculation.

 

[charlie b]

I've wired probably a 100 hot tubs in my career, and I've never seen one with a gas heater.

I have heard of hot tubs that get their heat from a wood fire. Sounds like too much work for my tastes.

 

[charlie b]

Don't know about diversity here, but the limiting factor may be the plumbing - how would you fill 15 hot tubs at once. . . ?

I would expect them to be filled sequentially.

 

[buzzbar]

Thanks all. It turns out that they only need (5) hot tubs powered up, which obviously helps with the electrical load. And Charlie is correct, the tubs will be filled and heated up sequentially.

 

[JFletcher]

I would expect them to be filled sequentially.

as would I, thus sequential loading - i.e., not all 15 (now 5) tubs would be loaded at once.

even with 5 tubs, I would expect one of those a 'premium' (upper end) model requiring a 60A service and disco. I havent priced 50A vs 60A GFCI discos lately, but if the cost is relatively negligible vs o/a job cost, installing the 60s would make a happier customer imo (showroom versatility vs 'we have to put that tub in x spot because of the electrical').

afaik, there is no NEC exception to spas/tubs that wont actually be used (like a showroom). You'd have to wire it like any other indoor spa/tub. I would not have a problem with sizing the load based on 5 heaters and recirc pumps running simultaneously vs full nameplate amperage. Then again a showroom may want full function (jet pumps too) 8-14hrs/days (which afaik is incompatible with spa settings (15min max to keep you from dying) but I suppose they can be over-ridden).