ElictriCalc Pro problem

[royta]

I'm getting different voltage drop calculations depending on conductor (60C, 75C, or 90C) setting.

2002 NEC setting
1ph
60C
200 feet
120V
20A
5% VD
= #8 copper ** Actual VD is 4.9% with an allowable 202 feet before I reach the full 5%.

2002 NEC setting
1ph
75C
200 feet
120V
20A
5% VD
= #6 copper ** Actual VD is 3.3% with an allowable 305 feet before I reach the full 5%.

2002 NEC setting
1ph
90C
200 feet
120V
20A
5% VD
= #6 copper ** Actual VD is 3.4% with an allowable 291 feet before I reach the full 5%.

​

Why is this? Shouldn't it be the same regardless of the conductor temperature rating?

 

[charlie b]

Insulation material has no effect on voltage drop. You are right about that. What method, or what formula, or what software are you using to come up with your results?

 

[royta]

Insulation material has no effect on voltage drop. You are right about that. What method, or what formula, or what software are you using to come up with your results?

I'm using an ElectriCalc Pro. I did not calculate long hand to verify.

I will also get a different number if I change between 1ph and 3ph. That shouldn't make a difference either, should it?

 

[charlie b]

I'm using an ElectriCalc Pro. I did not calculate long hand to verify.

Sorry, I should have seen that in the title of this thread.

I do not know how it works, nor how it takes insulation ratings into account. But I will reaffirm that the insulation system has no impact on voltage drop.

I will also get a different number if I change between 1ph and 3ph. That shouldn't make a difference either, should it?

That absolutely does make a difference.

On a single phase system, the driving force (i.e., the supply voltage) will push current down one wire, through the load, and back along a second wire. On a three phase system, the driving force will push current down one wire, through the load, and back along one or the other of the two remaining wires. When it heads back, it will have shifted in "phase" (meaning the points in time at which it reaches peaks and valleys). The paths are different, and the voltage lost along any single path will be different.

 

[royta]

I'm using an ElectriCalc Pro. I did not calculate long hand to verify.

Sorry, I should have seen that in the title of this thread.

I do not know how it works, nor how it takes insulation ratings into account. But I will reaffirm that the insulation system has no impact on voltage drop.

OK, I just got off the phone with Calculated Industries. They told me it takes into consideration the heat the wire can take. It won't make a difference when I'm only doing 15, 20, or 30A circuits, as the allowable ampacities of the 60C column in Table 310.16 are already more than the allowable OCPD in 240.4(D). In that situation, I should use the 60C setting. But...when I'm using starting wire sizes of #8 or larger, I can then play around with the 75C and 90C columns and get the maximum out of my wire. Anyway, it made sense on the phone.

I will also get a different number if I change between 1ph and 3ph. That shouldn't make a difference either, should it?

That absolutely does make a difference.

On a single phase system, the driving force (i.e., the supply voltage) will push current down one wire, through the load, and back along a second wire. On a three phase system, the driving force will push current down one wire, through the load, and back along one or the other of the two remaining wires. When it heads back, it will have shifted in "phase" (meaning the points in time at which it reaches peaks and valleys). The paths are different, and the voltage lost along any single path will be different.

I was thinking of when I'm doing a voltage drop calculation with a 120V circuit on a 208Y/120 panel. I guess that regardless of the service, it is still a 1ph circuit, and I would need to use the 1ph setting on the calculator. By setting it to 3ph, and doing a 120V voltage drop calculation, the calculator thinks I am using three conductors for the a 3ph circuit, not a single conductor with a grounded conductor for a 1ph circuit. Thanks for clearing that up.

 

[royta]

OK, I just got off the phone with Calculated Industries. They told me it takes into consideration the heat the wire can take. It won't make a difference when I'm only doing 15, 20, or 30A circuits, as the allowable ampacities of the 60C column in Table 310.16 are already more than the allowable OCPD in 240.4(D). In that situation, I should use the 60C setting. But...when I'm using starting wire sizes of #8 or larger, I can then play around with the 75C and 90C columns and get the maximum out of my wire. Anyway, it made sense on the phone.

Here is an example:
100A
240V
60C column requires #1 (110A max)
75C column requires #3 (100A max)
90C column requires #3 (110A max)

Now, I need to figure what size wire I would need if using wires from each of the three columns.

100A
240V
300 feet
3% VD

0 AWG 60C wire gives me a voltage drop of 2.9%. I can go up to 310 feet and still be within the 3% VD limit.

00 AWG 75C wire gives me a voltage drop of 2.4%, but I can go up to 372 feet and still be within the 3% VD limit.

00 AWG 90C wire gives me a voltage drop of 2.5%, but I can go up to 355 feet and still be withing the 3% VD limit.

There's no reason at all to use 00 AWG, since I don't care if I can go 372 feet with it. I only want to go 300 feet, so 0 AWG will work fine.

I think where the different settings come into play, is when I'm looking to use the absolute smallest diameter wire necessary but I'm not sure if the length of the circuit will exceed the wire of choice. i.e. I need to get a 100A 240V circuit from point A to point B. Should I use #1 (60C), #3 (75C), or #3 (90C)? The maximum circuit length of 60C #1 is 245 feet before increasing for voltage drop. The maximum circuit length of #3 75C is 146 feet before increasing for voltage drop. The maximum circuit length of #3 90C is 140 feet before increasing for voltage drop. ETA - And when using the 60C wire to it's full 60C potential, the 75C wire to it's full 75C potential, and the 90C wire to it's full 90C potential.

 

[bob]

I think where the different settings come into play, is when I'm looking to use the absolute smallest diameter wire necessary but I'm not sure if the length of the circuit will exceed the wire of choice. i.e. I need to get a 100A 240V circuit from point A to point B. Should I use #1 (60C), #3 (75C), or #3 (90C)? The maximum circuit length of 60C #1 is 245 feet before increasing for voltage drop.

Use the 75C rating. Unless the equipment has a 60C rating( new rated 75C )
you can forget the 60C. The 90C rating can only be used in conductor ampacity adjustments due to temp or per table 310.15(B)(2)(a).

I agree with Charlie regarding insulation temp rating and VD. The program maybe adjusting the conductor resistance to correspond with the insulation
temp rating which does not sound correct.

 

[beanland]

Wire resistance, too

Wire resistance, too

Wire resistance, and thus heating, increases with temperature. #12 copper operated at 75C has higher resistance than #12 operated at 90C. Higher resistance mans more heating for the same current.

 

[bob]

Wire resistance, and thus heating, increases with temperature. #12 copper operated at 75C has higher resistance than #12 operated at 90C. Higher resistance mans more heating for the same current.

That's true and if you have a conductor operating in a 75C environment the resistance should be adjusted. However, how many time have you installed a 75C conductor in 167F environment? The calculation should not be made
arbitrarily assuming every installation is 75C or 90C. Thats what the OP seems
to be indicating. My software program is doing the same thing.

 

[beanland]

Yep

Yep

I totally agree. There is complex interaction between wire sizes, insulation rating, the ability to dissipate heat, and the resultant allowable ampacity.

 

[royta]

That's true and if you have a conductor operating in a 75C environment the resistance should be adjusted. However, how many time have you installed a 75C conductor in 167F environment? The calculation should not be made arbitrarily assuming every installation is 75C or 90C. Thats what the OP seems to be indicating. My software program is doing the same thing.

That's why, even though I primarily use 90C wire (THHN), I will still be doing my voltage drop calculations in the 60C setting. If I am using 90C wire because I have to due to the environment, than I will most certainly change the setting on my ElectriCalc Pro.