Wire size - who's right?

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  #1  
Old 01-24-08, 11:45 AM
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Question Wire size - who's right?

240 volt, 50 amp circuit, 3% max drop, 100 foot or less run length. Southwire calculator on the net says #6 wire and specifically says #8 cannot carry 50 amps current when you try to calculate the max length of a 50 amp circuit using #8. The sign at the Big Orange store says #8 for a 50 amp circuit. I just finished using #6 on my hot tub project and the inspector signed off. Could I have gotten away with #8? Just curious.
 
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  #2  
Old 01-24-08, 12:03 PM
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You have several different limits that come into play.

One is voltage drop. This depends upon the _resistance_ of the wire, which itself depends only on cross section and length. For a short enough length, 14ga wire could carry 50A with acceptable voltage drop.

The other is 'ampacity'. This is the maximum current that the wire can safely carry without overheating. Note the key phrase: without overheating. If the wire has a higher temperature rating, then it can safely carry greater current.

Still another limit is the temperature rating of everything that is connected to the wire, for example the breaker terminals or any splices. If the wire gets hot enough to overheat the connectors, then you again have an unsafe situation.

See http://www.houwire.com/products/tech...cle310_16.html Note that for each wire size you have different values for different temperature ratings.

If you have wire with a 75C or higher rating, and if all of the terminals that the wire connects have a 75C or higher rating, and if you are not using NM or UF cable (which are specifically restricted to use at 60C ampacity), then yes, 8ga conductors are suitable for 50A.

I am assuming _copper_ conductors here; 8ga aluminium is not suitable for 50A.

-Jon
 
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Old 01-24-08, 12:31 PM
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All of those statements are correct given the right circumstances. Yes, you probably could have used #8 copper THWN-2 for your 50A spa, but I think the #6 is a better choice given the 100' distance. The voltage drop on the #8 works out to 3.2%.

I really hate those charts at the big box stores; there are many situations where the chart is simply wrong. Wiring sizing is much more complicated than X gauge = Y amps.
 
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Old 01-24-08, 12:58 PM
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I agree with both of above with their comments and this is very important part is the type of wire[s] the amp rating will help some degrees but the type of wire and termation ratings now that is important part to use the proper cable / wire ratings.

it is more compated than the simple chart what the big box store prodive to read and very easy to misunderstood with it because the factors will involded with :

Distance
Amp
Tempture
Type of inslualtion matearls
Dry or Wet Location


Basically this common info as other and myself mention that how we can count to get the correct wire/cable size and some part of NEC have specal section related to welder and motors [ it kinda long winded but it was discussed before in this forum so goggle that word and it will come up plenty ]

and few other details can change the factor of using the wire/cables setup.

Merci, Marc
 
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Old 01-24-08, 05:41 PM
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My philosophy: If you can get by with a borderline 8g, put in 6. It never hurts to use materials that exceed the minimums.

Now, if only I could implement that philosophy ...
 
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Old 01-24-08, 06:08 PM
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I agree with what's been said above. Everybody is right ... depending on the details. Electrical work isn't as simple as looking up a number in a table. Very similar looking situations often differ by enough details to make a large difference.
 
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Old 01-25-08, 07:15 PM
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VD=2KIL/cmil, you have the knowns- 2(single phase load), K(constant-12.9 for copper and 21.2 for alum. conductors), I(current), and L(length) divided by cmil or cross sectional area of conductor or cmil+(2xKxIxD)/VD of course there is ohms law method as well. if it is a 50 amp breaker not 50 amp load assume 100% which is 40 amps the reciprocal of protection. 2x12.9(cu)x40Ax100'/7.2=14333cmil which would be a #8 conductor and at 75degree terminations you will not have 90degree term. the #8 conductor is fine.
Originally Posted by thomrose View Post
240 volt, 50 amp circuit, 3% max drop, 100 foot or less run length. Southwire calculator on the net says #6 wire and specifically says #8 cannot carry 50 amps current when you try to calculate the max length of a 50 amp circuit using #8. The sign at the Big Orange store says #8 for a 50 amp circuit. I just finished using #6 on my hot tub project and the inspector signed off. Could I have gotten away with #8? Just curious.
 
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Old 01-25-08, 07:41 PM
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Wow, in english next time..... Remember, this is a DIY website....I'm thinking only electricians or engineers would know what you just typed up.....


Originally Posted by sparky480 View Post
VD=2KIL/cmil, you have the knowns- 2(single phase load), K(constant-12.9 for copper and 21.2 for alum. conductors), I(current), and L(length) divided by cmil or cross sectional area of conductor or cmil+(2xKxIxD)/VD of course there is ohms law method as well. if it is a 50 amp breaker not 50 amp load assume 100% which is 40 amps the reciprocal of protection. 2x12.9(cu)x40Ax100'/7.2=14333cmil which would be a #8 conductor and at 75degree terminations you will not have 90degree term. the #8 conductor is fine.
 
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Old 01-25-08, 11:47 PM
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Originally Posted by HotinOKC View Post
Wow, in english next time..... Remember, this is a DIY website....I'm thinking only electricians or engineers would know what you just typed up.....

Sparky480 is a very good guy i talk to him from time to time anyway welcome here Sparky480.

Merci, Marc
 
  #10  
Old 01-26-08, 12:15 AM
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Originally Posted by sparky480 View Post
the #8 conductor is fine.
Thanks for the how the math works on this.
 
  #11  
Old 01-26-08, 03:02 PM
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i apologize for too much info, but im that guy instead of telling you what to do, or convincing you what is right. i would like to educate that person, so they understand why it is right. thats why i put everything in parenthesis instead on leaving it up to some one to figure it out. ty
Originally Posted by HotinOKC View Post
Wow, in english next time..... Remember, this is a DIY website....I'm thinking only electricians or engineers would know what you just typed up.....
 
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