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Spa Project...Sub-Panel upgrade questions


Jon Burke's Avatar
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03-18-17, 09:50 PM   #1 (permalink)  
Spa Project...Sub-Panel upgrade questions

Hi All...new to this forum, but not to forums in general, so TIA for any help. This will probably get a little long.

Background: Not a professional, but plenty of experience doing my own upgrades/remodels/etc.

Existing: 200amp main panel that was upgraded about 18 months ago when I added 4kW solar panels. I also have a detached 375 sq foot 'studio' on my property that I'm thinking of remodeling into a spa. It has its own small sub-panel that existed before I bought this place.

The Spa project: I'm considering adding a hot tub, sauna and amenities (radiant floor heat, small on-demand water heater, AC and outlets) to this studio. The amp draw comes out to be around 129 amps if everything was on at once, which possible from time to time. The total wire length is around 110 feet with 40 feet underground in 1" EMT conduit. So this would require a 150 amp sub-panel upgrade using 2 gauge wire (according to wiresizecalculator.net).


Question 1) Is the above wire size calculations correct? I'd like to double check with a pro, but need to make sure it fits into the 1" conduit which is already buried in the yard (and under some concrete). If I need to remove some component, so be it.

I could always use a thicker wire from the panel to where the conduit starts and then go slightly thinner through the conduit to the studio if that helps(?)



Question 2) I've never heard/seen of a 150amp subpanel on a 200 amp main panel, any issues I need to address? My house itself is very efficient...mostly LED lights, range is gas, clothes dryer is gas...the only big draws are AC and an EV charger. House is 1920 sq feet so AC is about 2.8kW. I added it up and with everything on (rarely to never) its 63 amps. so by the numbers I should be good, but what am I not considering?

Question 3) My Square D main panel label says '100 amp branch maximum'...so I assume that means nothing over 100amp per breaker slot, so what about using this breaker for the 150amp?

https://www.lowes.com/pd/Square-D-Ho...reaker/3467881


Thanks again for the help. This is still in the 'design planning/can I even do this' phase so please let me know if there's anything else I should be considering.

 
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taz420's Avatar
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03-18-17, 11:37 PM   #2 (permalink)  
Actually Ray, that's a 4-stab breaker, so IINM it CAN be used since each leg is divided among two stabs. We'd need to know the panel part number to make sure it's listed for that panel.

The real issue here is that 1" conduit. He put something into that calculator wrong, because he needs 1/0 copper (2/0 if it's derated to 75C) for 150A service, not #2, which.. Yeah. No way. Even if it were #2 it would still be approximately 50% fill (not to mention a total bitсh of a pull). And you absolutely can NOT downsize the wire to fit the conduit. You either have to lay new conduit or use adequately sized direct-burial wire (you can use a water tunneler to go under the concrete if its not too wide). I know home depot carries a twisted 4/0-4/0-4/0-2/0 aluminum direct burial cable that's used for hooking up trailer homes. It's like $4 a foot, which will be a helluva lot cheaper than the 330' of 2/0 copper, 110' of #1 copper you'd need, plus the new conduit.

Jon - How did you calculate 129A? Just because, for example, your spa requires a 70A breaker doesn't mean it's drawing 70 amps.


Last edited by taz420; 03-19-17 at 12:25 AM.
 
Jon Burke's Avatar
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03-19-17, 12:38 AM   #3 (permalink)  
Hi Taz....thx, that's what I was thinking (hoping) w/the 4-stab breaker as well.

Re: wire size, I used that link but used 129amp as 'max amp' which outputs 2 gauge. So for 'max' should I have put in 150amps? Which then spits out 1/0 at 90 degrees C.

I calculated amps using the kW rating for each appliance and backed out the amps from there:
hottub: 5.5kW
Sauna: 9.0kW
Flooring: 1.8kW
H20 Heater: 9.0kW
AC Unit: 2.0kW
Outlets: 3.6kW*

total: 30.9kW @240V = 129Amps

*Edit...I just realized this number is way off, no way I'd have 3.6kW plugged into outlets so I can edit that down to 1.5kW which now brings total amps to 120 and gives me a bit more room.

So what's my wire size now?

Plan B would be to just drop the water heater....its a nice-to-have (indoor shower), but not necessary. Those tankless on-demand water heaters really pull the power.

 
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03-19-17, 12:46 AM   #4 (permalink)  
My incorrect post deleted. Thanks Taz.

Plan B would be to just drop the water heater
You have to dig a trench for the conduit anyway. Why not lay in a pex hot water line from the house.

I really think you need to do a load calc for the house to see how much spare capacity you have for the Spa.


I can explain it to you, but I can't understand it for you.

 
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03-19-17, 12:51 AM   #5 (permalink)  
You can only get four #4's in a 1" pipe which is nowhere near what you need.


~ Pete ~

 
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03-19-17, 01:35 AM   #6 (permalink)  
You have to dig a trench for the conduit anyway. Why not lay in a pex hot water line from the house.
sorry if I wasn't clear, the conduit already exists from the house to the studio, I'm trying to not dig up/excavate everything again, if possible.

I really think you need to do a load calc for the house to see how much spare capacity you have for the Spa.
not a bad idea...I have Neurio setup already, I can easily turn everything on and see the draw.

You can only get four #4's in a 1" pipe which is nowhere near what you need.
I'm planning three 2gauge (hot, hot, neutral) to feed the subpanel. I'd put in a grounding rod at the sub-panel, no need to run a 4th/ground. This is proper way to feed/ground a sub-panel, Yes?

Been using this for conduit fill table: Conduit Fill Table - Electrical References - Elliott Electric Supply

 
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03-19-17, 02:34 AM   #7 (permalink)  
This is proper way to feed/ground a sub-panel, Yes?
NO!

You need to carry an insulated ground for the spa panel.


~ Pete ~

 
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03-19-17, 02:47 AM   #8 (permalink)  
have Neurio setup already, I can easily turn everything on and see the draw.
Not how you do a load calc. Example: Service and Panel Size Calculator[
the conduit already exists from the house to the studio
The total wire length is around 110 feet with 40 feet underground in 1" EMT conduit.
It is too small to be used and EMT will rust out if buried. As PJ already wrote you need new PVC conduit at least two inches in diameter.
I'm planning three 2gauge (hot, hot, neutral) to feed the subpanel. I'd put in a grounding rod at the sub-panel, no need to run a 4th/ground. This is proper way to feed/ground a sub-panel, Yes?
NO. You need four wires. A GEC (Ground Electrode Conductor AKA groubd rod) can not be used instead of an EGC (Equipment Grounding Conductor). #2 is only good for 100 a. I thought you wanted 150a. 1" conduit is too small for three #2s and a #6 ground. You will need at least 1" conduit but pulling will be easier with 1" PVC conduit.

If they really ran EMT as you wrote it may already be rusted out in spots so it is no longer even continuous.


I can explain it to you, but I can't understand it for you.

 
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03-19-17, 01:44 PM   #9 (permalink)  
Thanks for the replies....overall it looks like I'm going to have to re-think this and re-budget since its going to require some trenching.


Not how you do a load calc. Example: Service and Panel Size Calculator[
I get 48amps using that tool. Lower than conservative estimate of 63 so I'm OK there.

NO. You need four wires. A GEC (Ground Electrode Conductor AKA groubd rod) can not be used instead of an EGC (Equipment Grounding Conductor).
OK, understood. thx.

#2 is only good for 100 a. I thought you wanted 150a.
Well, 'Plan B' was to not install the tankless water heater (9kW) which gets me down to ~80 amps and a 100amp sub panel, but that plan is busted anyway since the 1" won't even handle the #2 wire requirements.

And I probably incorrectly assumed its EMT, but its a moot point now anyway.

Thanks again...I'll be back. Eventually.

 
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03-20-17, 10:29 AM   #10 (permalink)  
I calculated amps using the kW rating for each appliance and backed out the amps from there:
hottub: 5.5kW
Sauna: 9.0kW
Flooring: 1.8kW
H20 Heater: 9.0kW
AC Unit: 2.0kW
Outlets: 3.6kW*
Is this a special water heater for the hot tub? Typical electric water heaters are only 4.5 KW.

How did you come up with 3.6 KW for outlets in a 375 sq ft structure?

You are adding both floor heating and air-conditioning loads. I'd probably drop the floor heat load and only run one at a time, either heat or AC, but not both at the same time.


Last edited by CasualJoe; 03-20-17 at 10:51 AM.
 
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03-20-17, 11:09 AM   #11 (permalink)  
You figure the larger of the loads lIke A/C and heat, not both.

 
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03-20-17, 05:14 PM   #12 (permalink)  
Is this a special water heater for the hot tub?
It's a tankless heater for the sink/shower - and that's a small one.

 
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03-21-17, 06:16 PM   #13 (permalink)  
How did you come up with 3.6 KW for outlets in a 375 sq ft structure?
How about this? How did you arrive at 3.6 KW? How many receptacles do you have?

 
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