add hot tub to exisitng A/C subpanel but only 3 wire feed

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  #1  
Old 06-01-13, 11:34 PM
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add hot tub to exisitng A/C subpanel but only 3 wire feed

I'm trying to add a hot tub near where the A/C has a sub-panel. On the main there is a 90 amp breaker. Wire to the sub-panel is 2 hot and a bare aluminum neutral. Looks like it is a 3 wire feed, not a 4 wire. The sub-panel has a 2 pole 40 amp breaker for the A/C and an extra 60 amp breaker that isn't attached to anything. This is an older house, built around 1985, so I guess they weren't required back then to run a 4 wire feed. I need to see what the wire size is for the sub-panel but it looks a lot bigger then 8. Right now there's a dual unit A/C and propane heat. Maybe at one time there was also electric heat. But in any case, it looks like my feed is big enough to handle the addition of the spa.

The spa can run either 120 or 240 and the maximum amps needed are 40 with quick heat configured. The manual says even when configured as 240, the electronics run on 120.

So my question is - do I need a ground? If so, can I just add a ground rod and isolated ground bus in the sub-panel? I think you can do this for separate buildings. Does the same also for older construction where you only have a 3 wire feed.

And if I add the ground rod, can I then have a 120 GFI receptacle. Thanks for any help.
 
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Old 06-01-13, 11:44 PM
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Your sub-panel is likely a 240 volt only panel and the bare wire is an equipment grounding conductor. If it has a 90 ampere circuit breaker on the feeder then the wire size should be no less than #3 if copper or #1 or #1/0 if aluminum. If the hot tub requires 120 volts for any purpose you may not use this sub-panel as a source without additional equipment.
 
  #3  
Old 06-02-13, 12:03 AM
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Welcome to the forums!

To help us understand the requirements for the supply to your hot tub, please post a link to the installation instructions. Most require a 3-conductor, 4-wire supply.

do I need a ground?
Yes, but you already have that. What you don't have is a neutral.

can I just add a ground rod and isolated ground bus in the sub-panel?
The ground isn't isolated; it's bonded to everything including the panel enclosure. The neutrals are isolated in a subpanel.

if I add the ground rod, can I then have a 120 GFI receptacle.
No. You need neutral to supply any 120V load.
 
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Old 06-02-13, 12:47 AM
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Originally Posted by Furd View Post
Your sub-panel is likely a 240 volt only panel and the bare wire is an equipment grounding conductor. If it has a 90 ampere circuit breaker on the feeder then the wire size should be no less than #3 if copper or #1 or #1/0 if aluminum. If the hot tub requires 120 volts for any purpose you may not use this sub-panel as a source without additional equipment.
90A on AL needs #2 per 310.16/75[SUP]o[/SUP].. 100A is #1. Copper would be #4.

OP: What size breaker does the tub manufacturer require? Since you have to replace the feed to the subpanel anyway might as well make sure it's sized properly to handle the added load.
 
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Old 06-02-13, 09:43 AM
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Thanks all. Now that I think of it, the neutral has to be isolated from the ground and cabinet. I'll look at the installation manual for the spa this evening. It's at another house. Is there any way to just add a neutral? There is a wiring diagram included in the manual. Maybe I can separate the 240 for the pump and heater from the 120 for the electronics. Then power the electronics from a nearby 120 circuit. That would be a lot easier then running a new 40 amp feed and new sub-panel.

The existing sub-panel has a 90 amp feed and currently only a 40 amp breaker for the A/C. The spa requires a max 40 amp circuit. So the sub-panel has enough capacity, it just lacks the neutral.
 
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Old 06-02-13, 10:58 AM
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Is there any way to just add a neutral?
If it is conduit you can pull a neutral from the source panel if the conduit is large enough.
 
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Old 06-02-13, 02:19 PM
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If it's not conduit, you will have to replace the entire feeder. You can add a ground in cases where one is not present in cable, but not a neutral - and a bare conductor can not be used as a neutral.

You can get 2-2-2-4 AL XHHW from Blue or Orange for a dollar something a foot.
 
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Old 06-02-13, 10:59 PM
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yeah, pretty much figured you couldn't use a bare ground in a cable to act as a neutral. With a 120 circuit, it would be carrying current with only the outer cable casing acting as insulation. There's no conduit, so looks like I'll need to run another feed for the spa. Not looking forward to getting into the crawl space but that's what I'll need to do.

I looked at the wiring diagram for the spa and it doesn't look like there is anyway to separate the 240 pump and heating from the 120 control electronics.

Thanks all. learned some things from this forum. I think I've progressed from dangerous to just incompetent.
 
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Old 06-02-13, 11:08 PM
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I think I've progressed from dangerous to just incompetent.
From unknowingly incompetent to knowingly incompetent?

Thanks for seeking input when you think you need it.
 
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