Should I upsize the wire on a 250ft run?

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electrofelon said:
Disagree. Everyone does this all the time: run a standard sized feeder/service but use a lesser number to calc V drop. I'll run a resi service 200 feet in 4/0 AL and still call it a 200 amp service. We also use larger equipment sometimes just for the larger lugs.
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Just because everyone does it 🙄

I know it's "acceptable"
I never argued that it's not.

You could also "acceptably" run it in a 1" pipe.
But I wouldn't do that either.
 
Shak180 said:
I'm going to be running a 100 amp circuit over to a subpanel that's going to have (2) 50 amp 3 phase circuits coming out of it. I bid the job using 3 awg copper but now I'm wondering if I should use 2 awg copper. What do y'all think? Will the 3 awg copper be sufficient? There's a 2.2% voltage drop using the 3 awg which doesn't seem bad.

The loads being plugged in are an electric heater that pulls 36 amps, a welder that pulls 23 amps, and a plasma cutter that pulls 12 amps.
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What size EGC are you pulling?
 
I see where James L is coming from. Is it reasonable to call a circuit capable of delivering 71A with acceptable voltage drop a 100A circuit?

I would argue that in _common usage_ the answer is yes. As used in the electrical trade, the rating of a circuit is the 'trip rating' of the breaker. So if you have a feeder with conductors that have 100A ampacity, protected by a 100A breaker, it is perfectly normal and correct to call that a 100A feeder.

However we all have to recognize that this 100A number hides lots of details. The 100A number does not include voltage drop. The 100A number does not include the various 80%/125% rules. Heck, the 100A number does not even include the nominal voltage.

Consider a situation where you need a circuit (not a feeder) for an 80A continuous load. You would be required to use a 100A breaker with conductors of 100A ampacity. You would call this a 100A circuit and say that this load requires a 100A circuit...even though this circuit is only permitted to supply 80A.

If you want to be precise you have to add all those other things in. Here we have a 100A circuit with acceptable voltage drop for the installed 71A load. Calling it anything other than a 100A circuit would cause confusion because electricians know a 100A circuit means 100A breaker with wire suitably protected by that breaker and suitable for the load.

-Jon
 
winnie said:
I see where James L is coming from. Is it reasonable to call a circuit capable of delivering 71A with acceptable voltage drop a 100A circuit?

I would argue that in _common usage_ the answer is yes. As used in the electrical trade, the rating of a circuit is the 'trip rating' of the breaker. So if you have a feeder with conductors that have 100A ampacity, protected by a 100A breaker, it is perfectly normal and correct to call that a 100A feeder.

However we all have to recognize that this 100A number hides lots of details. The 100A number does not include voltage drop. The 100A number does not include the various 80%/125% rules. Heck, the 100A number does not even include the nominal voltage.

Consider a situation where you need a circuit (not a feeder) for an 80A continuous load. You would be required to use a 100A breaker with conductors of 100A ampacity. You would call this a 100A circuit and say that this load requires a 100A circuit...even though this circuit is only permitted to supply 80A.

If you want to be precise you have to add all those other things in. Here we have a 100A circuit with acceptable voltage drop for the installed 71A load. Calling it anything other than a 100A circuit would cause confusion because electricians know a 100A circuit means 100A breaker with wire suitably protected by that breaker and suitable for the load.

-Jon
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Agree. And also don't forget that if a fault occurs on that 80/100 amp feeder the breaker still needs to be able to trip in 5 seconds or less.
 
tom baker said:
Its easy to determine the power lost in volt drop and convert that into $$$. Run some numbers to find the sweet spot.
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mbrooke said:
Not how you determine wire size. Have a look at chapter 9 table 9.
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Tom Baker's approach is not the correct way to size the wire initially. However is is a very good way to answer if you should upsize the wire or not.

If, with the code minimum circuit, the cost of voltage drop is greater than the cost of installing larger wire, then you install larger wire.

This isn't the full picture, however, because 1) the larger wire might be needed for functional reasons such as actually getting the load to start or preventing lights from flickering even if by energy savings it isn't worth it and 2) the money for running the load often comes out of a different pocket than the money for installing the wire.

-Jon
 
winnie said:
Tom Baker's approach is not the correct way to size the wire initially. However is is a very good way to answer if you should upsize the wire or not.

If, with the code minimum circuit, the cost of voltage drop is greater than the cost of installing larger wire, then you install larger wire.

This isn't the full picture, however, because 1) the larger wire might be needed for functional reasons such as actually getting the load to start or preventing lights from flickering even if by energy savings it isn't worth it and 2) the money for running the load often comes out of a different pocket than the money for installing the wire.

-Jon
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Also, if it's an expanding business (why else would they be adding equipment), you might have to re-pull later, or add another faux 100 amp panel later
 
winnie said:
Tom Baker's approach is not the correct way to size the wire initially. However is is a very good way to answer if you should upsize the wire or not.

If, with the code minimum circuit, the cost of voltage drop is greater than the cost of installing larger wire, then you install larger wire.

This isn't the full picture, however, because 1) the larger wire might be needed for functional reasons such as actually getting the load to start or preventing lights from flickering even if by energy savings it isn't worth it and 2) the money for running the load often comes out of a different pocket than the money for installing the wire.

-Jon
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Right. But my point is chapter 9 table 9 has the resistance, reactance and impedance at 0.85PF listed for each wire size which is used in determining voltage drop, short circuit current, and disconnection time for any given circuit.
 
ActionDave said:
There is nothing improper or unsafe about calling a 100A feeder a 100A feeder and it is not more safe to call it something else.
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100 amps according to Table 310.16 and rules governing termination temperature limitations. However other factor come into play which may require that the breaker be reduced in size.
 
Can of worms time: Might the same conductors on an 80a breaker require a larger EGC than on a 100a breaker?
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electrofelon said:
Why upsize the wire from the size necessary? Not sure what is going on in this thread. Did I take a wrong turn driving home from work and end up on Mars?
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If it was unnecessary I'd agree with you. However it is necessary.

1) Voltage drop 2) disconnection time during a short circuit.

Smaller wire = higher Z

#2 in particular is crucial to life safety.
 
mbrooke said:
If it was unnecessary I'd agree with you. However it is necessary.

1) Voltage drop 2) disconnection time during a short circuit.

Smaller wire = higher Z

#2 in particular is crucial to life safety.
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Well put in a new NEC proposal that smallest ocpd possible be used, and clearing time be calculated for circuits. Be prepared to back up your claims that this is necessary
 
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