Can someone please explain this to me?

[Daniel malack]

And your point is...? If you use an on line calculator for single phase voltage drop it is very likely that the X2 is already figured in. If you calculate it from first principles, i.e., starting from V=IR, then you MUST calculate the voltage drop in both current carrying conductors and add them together. Doubling the distance accomplishes this.

For example, if your conductors have a 1 ohm resistance each, your load is 8 ohms, and you apply a 10 volt source to the circuit, the current is throughout the circuit is 1A, right? Your 1 ohm resistors get 1V apiece and your load gets 8V. Your voltage drop is 10V - 8V = 2V, and (2V/10V)(100%) = 20%.

The voltage drop is across the residence measured at the load. Your counting the conductors twice . The answer too original question would be 70 amps.

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[480sparky]

The voltage drop is across the residence measured at the load. Your counting the conductors twice . The answer too original question would be 70 amps.

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You count two conductors because you HAVE two conductors in the circuit. You don't have a circuit with just one.

You've got three loads. Two of them are both 100-foot long resistors. The third is the load.

 

[Ingenieur]

The voltage drop is across the residence measured at the load. Your counting the conductors twice . The answer too original question would be 70 amps.

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Draw the ckt
a closed loop
source
2 lines (supply & rtn)
load
you have 3 v drops
the load and each line
(each line sees the same i and is the same R, hence 2 times)
the 3 v drops must sum equal and opposite sign to the source
kvl

 

[ggunn]

The voltage drop is across the residence measured at the load.

Precisely. My simple example shows that in a one ampere circuit with three resistances (two conductors and a load), the voltage drop in BOTH conductors equally affects the voltage at the terminals of the load. From a 0V reference point at the ground of the supply the voltage at the head of the load is reduced from 10V to 9V, and the voltage at the tail of the load is RAISED from 0V to 1V, so that the voltage across the load is 8V. One volt drop across one conductor and one volt drop across the other for a total Vd of two volts.

Do you have any background in electrical circuits? This is pretty simple stuff.

 

[Little Bill]

The Southwire calculator only asks if it is single ø or 3ø. so for 240V (or 230) you would think it knows there are two conductors.
It shows a #10 for 240V 30A for 100'.

I put in 200' ad it came back with #6.

So neither scenario came back with #8

 

[GoldDigger]

The Southwire calculator only asks if it is single ø or 3ø. so for 240V (or 230) you would think it knows there are two conductors.
It shows a #10 for 240V 30A for 100'.

I put in 200' ad it came back with #6.

So neither scenario came back with #8

What percent VD did you enter?

 

[Daniel malack]

What percent VD did you enter?

Have to assume 3% already on service feeder. So only alow for 2 % drop on branch circuit would give #8

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[Little Bill]

What percent VD did you enter?

3%

 

[Daniel malack]

Or 3% and using 60° column because it is under 100Amp. Which is more like the right answer. Just watched a Mike Holt YouTube video . Very informative.

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[480sparky]

Southwire's calculator states 1.1417 ohms/1000 ft (unknown if it's solid or stranded) for AWG 10. NEC does not show this number.

 

[480sparky]

Or 3% and using 60° column because it is under 100Amp.............

You're kidding, right? Where did you come up with anything about the 60° column or under 100 amps?

 

[GoldDigger]

Southwire's calculator states 1.1417 ohms/1000 ft (unknown if it's solid or stranded) for AWG 10. NEC does not show this number.

Southwire sells copper.....
They must be using "conservative" values.

 

[Daniel malack]

You're kidding, right? Where did you come up with anything about the 60° column or under 100 amps?

Equipment terminal ratings 110.14(C) 2014 code. 100 AMP or below 60° column, unless terminals at both ends (breaker, device and wire) are rated 75°. Then you can use the 75° column. From experience I have not seen a breaker under 100Amp rated above 60°. Most are rated 45°. Really! So the answer to the original question is #8

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[Daniel malack]

I am still studying for my WV Master Electricians Exam. I have been going along pretty good but I just hit a brick wall. Here is the question from my practice test.
A 30 amp 230 volt load is located 100 feet from the source. What is the minimum size branch circuit conductor required to operate within the limits for voltage drop that is recommended be the NEC?
a. # 10 conductor
b. # 8 conductor
c. # 6 conductor
d. #4 conductor

The book says the correct answer is B. I have no idea how to come to this conclusion. Can anyone help me?

Looks like you did the voltage drop calculations correct now have to apply 110.14(C) terminal ratings, 100 amps or less, or marked for #14 through #1 awg must use 60° column. Unless breaker and device terminal ratings and wire are all rated 75° (or above) you can use the 75° column. In my experience i haven't seen a 100a or less breaker rating above 60°. That would make the answer #8 awg.

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[oldsparky52]

In my experience i haven't seen a 100a or less breaker rating above 60°.

My experience has been that most breakers below 100A are rated for 60c or 75c terminations. Here is one example.

From page 7 http://download.schneider-electric....File_Id=7726421594&p_File_Name=0730CT9801.pdf

TerminationsThe 10–30 A circuit breakers have pressure plate terminals suitable for single or two-wire terminations.Copper or aluminum conductors may be used as outlined in Table 2. QO-GFI 15–30 A and QO-AFIcircuit breakers have pressure plate terminals suitable for single-wire terminations. These circuitbreakers are suitable for use with 60°C or 75°C conductors.

 

[Daniel malack]

My experience has been that most breakers below 100A are rated for 60c or 75c terminations. Here is one example.

From page 7 http://download.schneider-electric....File_Id=7726421594&p_File_Name=0730CT9801.pdf

Your example shows them rated at 40° C

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[Daniel malack]

Your example shows them rated at 40° C

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OK I had to read further down. If you do service calls and have several brands of breakers on your truck, you would be hard pressed to find 1 above 40°C. That example is a special use breaker how much does it cost? It may be worth paying more for a breaker and less on wire, but you have to take into account every terminal rating on the circuit. If you have 1 light switch with a terminal rating below 75° C you have to use 60° column, even if you use 90° wire, and 75° breaker.

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[oldsparky52]

Your example shows them rated at 40° C

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That was ambient temperature, not termination rating.

. Each pole has an individual thermal-magnetic trip elementcalibrated for 40°C ambient temperature.

On page 7 you will find

These circuitbreakers are suitable for use with 60°C or 75°C conductors.

 

[oldsparky52]

OK I had to read further down. If you do service calls and have several brands of breakers on your truck, you would be hard pressed to find 1 above 40°C. That example is a special use breaker how much does it cost? It may be worth paying more for a breaker and less on wire, but you have to take into account every terminal rating on the circuit. If you have 1 light switch with a terminal rating below 75° C you have to use 60° column, even if you use 90° wire, and 75° breaker.

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Daniel, I don't want to get into a pissing contest on this, but most breakers today (rated under 100A) are rated for both 60c and 75c terminations. I agree that other terminations need to be considered. Circuits less than 40-amps will rarely ever be used at ampacities that require a 75c termination, just the way it is. But ... it is not true that a circuit under 100A has to be calculated at the 60c termination because of the circuit breaker because most of them are also rated for 75c terminations (other current limiting terminations may be in the circuit but it's not usually the circuit breaker that needs to be considered).

 

[JFletcher]

Where did the .999 Ohms/1,000' come from?

I believe I pulled it from this on-line calculator:

https://www.cirris.com/learning-center/calculators/133-wire-resistance-calculator-table

The Southwire calculator only asks if it is single ø or 3ø. so for 240V (or 230) you would think it knows there are two conductors.
It shows a #10 for 240V 30A for 100'.

I put in 200' ad it came back with #6.

So neither scenario came back with #8

Ditto. Only way I got #8 was with Al.