#6 Ground Wire? Spa wiring requirements

   #1  

5030tinkerer

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My house is pre-wired for a spa. Trouble is, that the wiring is 6/3 and behind sheetrock (of course it is). With 6/3 comes 6 gauge hot in red, 6 gauge hot in black, a 6 gauge neutral, and a bare 10 gauge ground. The installation directions (http://www.olympicho...manual_2013.pdf) for the spa indicate #6 wire for all four of these wires for wire distances of up to 100'.

I measured my wire length from my main electrical service panel to where the spa panel would be and came in at 34 feet. From http://www.calculato...es=50&x=64&y=13, it would seem that the electrical carrying ability of #10 wire is greater at this distance than #6 wire has at 100 feet since the voltage drop is half a point less with the 50 amp load I popped in there.

Am I stuck ripping open my wall or would this be a safe install as is?

Adding confusion is the following verbiage from p 14 of the install manual (when the diagram on the following page calls for #6): 'An electrical subpanel containing two GFcI breakers is included with each spa. We recommend that this subpanel be used to supply power and protect the spa. This subpanel requires a 70 amp, single phase, 230 volt, four wire service (two line, one neutral, one ground). The ground wire must never be less
than #10 AWG. use Nec 250-122 (table) and local codes for more information. A minimum #6 AWG solid copper bond wire is also required.'

The NEC table can be found at http://www.safecoelectric.com/images/resources_pdf/NEC2008 Table 250.122.pdf and indicates that #10 wire is good for up to 60 amp service (but again this is designed for 100' wire lengths).
 
  
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5030tinkerer

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You are correct, but these ratings are generally based on a 3% maximum voltage drop at a given amperage for a 100' distance. If you have a 1' long section of 10 gauge wire, for example, that wire can pull 1650 amps at a 3% voltage drop. Crazy, heh?

The open question is whether there is something 'special' in a grounding application and, especially, in a grounding application with a spa.
 
   #4  

Western

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Yeah I wouldn't know that, but would assume since it is a appliance with water and people, all precautions would be prudent. Personally I would follow the install to a "T"
 
   #5  

s219

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It's common for the ground wire to be a size or two lower. If I recall code correctly, ground does *not* need to be sized for the current carrying loads of the hots and neutral. So the distance/drop calculations aren't exactly relevant.

Also, the max current ratings of wire have nothing to do with voltage drop or length. It's all about temperature and capacity. You cannot alter the rating of a wire just because it's short. No way you will ever see a 1650A rating for 1 foot of #10 just because it's only 1 foot.

The normal procedure is to choose gauge based on the current load. Then upsize as needed to offset voltage drop. As an example, my barn branch circuit (for a 120V receptacle and lights) is 15A, which otherwise needs #14 copper to handle the max current. However, due to the distance, I upgraded to #6 (AL) so the voltage drop would not be a problem. We needed to be able to run power tools during construction.

Same for my pier circuit to a boat lift. It only pulls 12A, but we ran #10 copper because of the distance.
 
  
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5030tinkerer

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It's common for the ground wire to be a size or two lower. If I recall code correctly, ground does *not* need to be sized for the current carrying loads of the hots and neutral. So the distance/drop calculations aren't exactly relevant.

Also, the max current ratings of wire have nothing to do with voltage drop or length. It's all about temperature and capacity. You cannot alter the rating of a wire just because it's short. No way you will ever see a 1650A rating for 1 foot of #10 just because it's only 1 foot.

The normal procedure is to choose gauge based on the current load. Then upsize as needed to offset voltage drop. As an example, my barn branch circuit (for a 120V receptacle and lights) is 15A, which otherwise needs #14 copper to handle the max current. However, due to the distance, I upgraded to #6 (AL) so the voltage drop would not be a problem. We needed to be able to run power tools during construction.

Same for my pier circuit to a boat lift. It only pulls 12A, but we ran #10 copper because of the distance.

No doubt that the normal procedure is to choose gauge based on load, based on the Code. What's interesting is that if you pop even a 900 amp load into the calculator at Wire Size Calculator, you'll see that it specifies a 12 gauge wire. From this, it would seem that the electrical carrying ability of wire (ampacity) IS indeed based on distance, but that National Electric Code doesn't want people choosing wire size on distance when the run is less than 100', but on their resultant thoughts on allowable voltage drop. That same calculator, when presented with a 50 amp load, specifies a #10 wire for wire lengths up to 28 feet, but doesn't specify a #6 wire until the 47-73 volt range. Sadly, as my wire length is going to be closer to 34', I'm in the 8 gauge range (smaller wire than I have now).

I might as well rip the wall and ceiling open and put in the #6 wire that they specified UNLESS someone can tell me why it is that it is normal for the ground wire to be allowed smaller.
 
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   #7  

Inspector507

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The ground wire does not need to be full sized based on the load. Load current and voltage drop have little to do with it. The grounding conductor is there to create an effective ground-fault current path back to it's supply source to facilitate opening of the circuit breaker in a fault condition. Short circuit current and and load current are different.
 
  
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5030tinkerer

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The ground wire does not need to be full sized based on the load. Load current and voltage drop have little to do with it. The grounding conductor is there to create an effective ground-fault current path back to it's supply source to facilitate opening of the circuit breaker in a fault condition. Short circuit current and and load current are different.

Are you saying that the #10 grounding wire is fine in this application, then, and I *DO NOT* need to rip open my wall and ceiling to gain the access?
 
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teejk

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Are you saying that the #10 grounding wire is fine in this application, then, and I *DO NOT* need to rip open my wall and ceiling to gain the access?
Back up a bit...the instructions said the "bonding wire" needed to be #6, everything else minimum #10. We need a few "sparkies" to explain "grounding" vs. "bonding". It's confusing to me especially with that "single point grounding" change but I think it describes the wire from your main panel to its grounding source.
 
  
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5030tinkerer

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The wiring diagram on page 15 (attached in line here) contemplates the run from the main panel to the spa panel as needing to be #6 wire, but then discusses a #10 as a requirement elsewhere (where the 'converted' install isn't performed).
 

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