Wire size calculation

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bgelectric

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So i have a welding plug in one location about 30 ft from the panel, and i want to relocate it to another location about 70ft away from the plug, the current feed is a 6/3mc cu 50A ocpd. Would it be a problem to continue from the plug 70ft to the other location with 8/3 mc since it is rated for 50amps. The total distance would be 100 ft or worst case scenario 120ft.
 
The wire would be protected at its ampacity, so that is not a problem. If this is a 480 volt system, the voltage drop would be under 3%, so that would also not be a problem. But if this is a 208 volt system, you might be over 4% VD, so it might be worth considering staying with the #6.
 
charlie b said:
The wire would be protected at its ampacity, so that is not a problem. If this is a 480 volt system, the voltage drop would be under 3%, so that would also not be a problem. But if this is a 208 volt system, you might be over 4% VD, so it might be worth considering staying with the #6.
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In 310.16 is MC 75 deg column? If so would you not have to tag the #6 as a 50 amp circuit since #6 could be fused as high as 70 and you have reduced the amacity with the #8? (65 amps next breaker up)
 
jwjrw said:
In 310.16 is MC 75 deg column?
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That is how I read 330.80.
jwjrw said:
If so would you not have to tag the #6 as a 50 amp circuit . . . .
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It is not uncommon to use a larger wire at the beginning of a long run, and connect to a smaller wire closer to the load. I know of no requirements to tag the system to alert a future electrician of the reduction in wire size.
 
jwjrw said:
In 310.16 is MC 75 deg column? If so would you not have to tag the #6 as a 50 amp circuit since #6 could be fused as high as 70 and you have reduced the amacity with the #8? (65 amps next breaker up)
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It would not be a problem to tag, I am more concerned about VD, Is there a way to calculate the combination of 6/3 and 8/3 to find VD For 120ft? I would think you just find vd for 6/3 then for 8/3 and the sum would give you total vd. Although you would have to use the voltage you came up with for the 6/3 after the vd was subtracted in the calculations for the vd of the 8/3? If that made any sense can someone confirm that would be the right way to go about it?
 
bgelectric said:
Is there a way to calculate the combination of 6/3 and 8/3 to find VD For 120ft?
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Here are my results:
  • With a 50 amp load, the VD along 30 feet of #6 is 1.28 volts.
  • With a 50 amp load, the VD along 90 additional feet of #8 is 6.09 volts.
  • That gives you a total VD of 7.37 volts.
  • If the system is 480 volts, that is a 1.5% VD.
  • If the system is 208 volts, that is a 3.5% VD.
 
charlie b said:
That is how I read 330.80. It is not uncommon to use a larger wire at the beginning of a long run, and connect to a smaller wire closer to the load. I know of no requirements to tag the system to alert a future electrician of the reduction in wire size.
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I was always told you could go bigger at the end but not smaller. For example running a # 12 20 amp circuit then extending it with a piece of # 14. (romex)
The 14 is not good for the 20 amps and required a tag that the circuit ampacity is not 20 amp anymore because the 14 is not good for 20 amps and someone could see #12 and think it could be on a 20 amp breaker. I could of swore I read it in the NEC.:confused:
 
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jwjrw said:
I was always told you could go bigger at the end but not smaller. For example running a # 12 20 amp circuit then extending it with a piece of # 14. (romex)
The 14 is not good for the 20 amps and required a tag that the circuit ampacity is not 20 amp anymore because the 14 is not good for 20 amps and someone could see #12 and think it could be on a 20 amp breaker. I could of swore I read it in the NEC.:confused:
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I'm confused... why would anyone put #14 on a 20 amp breaker in any normal situation? Do you mean a 15 amp circuit?
 
ericsherman37 said:
I'm confused... why would anyone put #14 on a 20 amp breaker in any normal situation? Do you mean a 15 amp circuit?
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What I'm saying is if you extend a circuit with a smaller conductor you now have a circuit that has to be fused no more than the smaller conductor's allowed ampacity. BUT say you were to come to a job and are working in the service panel. You see # 12 on a #15 amp breaker . You or someone else may think the 15 amp breaker could be increased to a 20 amp because the first part of the circuit is in #12 BUT you do not know the rest of the circuit is in # 14 and only good for 15 amps.

Example:

#12 in panel leaving panel going to point A. from point A to point B wire was changed to #14. The #14 can not be fused higher than 15 amps. The start of the circuit is in #12 which if you or I looked in a panel would think is a 20 amp circuit would not be able to be fused at 20 due to the # 14 connected to it.
 
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jwjrw said:
You or someone else may think the 15 amp breaker could be increased to a 20 amp because the first part of the circuit is in #12 BUT you do not know the rest of the circuit is in # 14 and only good for 15 amps.
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Then you or someone else would have failed to do your (or their) jobs properly. It is not up to the present day electrician to prevent a future electrician from making an error.
 
Wire Size Calc

Wire Size Calc

My calculations match "charlieb".

6.13V drop for 90' of #8 at 50A
1.29V drop for 30' of #6 at 50A

Actual performance should be better because you shouldn't have 50A continuous on a 50A breaker
 
bgelectric said:
Well my main question is would there be any issues with the welder if i had a voltage drop up to 3.5%?
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BG,
Way back, installed for a welder friend, with 45 years experience.
Used #6 for a long 100' one-way run through an attic.
Considered
(1) heat de-rating
(2) VD
(3) the low duty-cycle of the welder machine, aprox 30% depending.

Although the manufacturers states that it is a 225A welder,
I know from current measurements that the duty cycle in typical welding is way low
depending on exactly which metals and type of weld in progress.
A clamp-on analog meter and an O'scope are useful in observing this phenomena.

The point is, that this transformer-based equipment does not supply a constant load
and when the load is heavy it is still nothing like a 100% duty cycle.
The welder application is NOT comparable to a motor load

That said, the net effect of VD would be that under peak loads (which over a 60 second run are very much up and down spikey loads)
only the max spike loads would be effected by VD.
The current waveforms look like a series of "WWWWWWW", and only the peaks would suffer from VD.
The carry-over heat from one hit to the next would over-ride the minor effects of VD.

Mike makes his living from this set-up, TIG, MIG, etc.
HTH, :)
 
billyzee said:
My calculations match "charlieb".

6.13V drop for 90' of #8 at 50A
1.29V drop for 30' of #6 at 50A

Actual performance should be better because you shouldn't have 50A continuous on a 50A breaker
Click to expand...

Please, could you show me how to arrive to these vd calculations.

Thank you very much
 
If you have a welder rated for 240/208, and you are on 240 power, then I see no reason not to let voltage drop go to 6%. If you are on 208, then it may not be as smart to let it go that low, but many folks will tell you that most light commercial equipment is designed for voltage drop.
 
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