Voltage drop and the 90 degree column

[KP2]

KP2
I did not mean taking 66% of the total bldg load.
I do this for the apartment feeder ONLY....meter stack to apartment.
If your adjusted load for the meter stack is 1083 amps you must use this for your calc.

So the term "Adjusted load" what does that mean; am I being to literal in hoping that the demand load and the adjusted load are not the same?

The demand load is 1083 amps, which is the result of the feeder calculation based on 220.84.

 

[ggunn]

I did start off this thread thinking I could adjust the R value since we will use XHHW-2 conductors and adjust the temperature by 15°C, but folks did not agree. I did not present this very clearly in the beginning, I know that did not help.

I hope you got your answer, which is "no". The resistance of a conductor does not depend on what type of insulation it has.

 

[KP2]

In addition to the nicely written description above, don't forget to put a 4/0 Ground conductor in each of the conduits per NEC 250.122.
Also, if the conduits are large enough, it would be more cost effective to run 75C, 350 MCM in 4 conduits which satisfies your 1200 amp breaker per NEC 310.13(A), Leave the two conduits spare if possible. A 3" conduit would satisfy for 350 MCM for 3 wire or 4 wire system including the 4/0 ground.
Assuming you are willing to use copper.

Thank you for this; the EOR has specified 6 sets of 600kcmil CU with a 300kcmil ground.

We reminded the GC that we qualified Aluminum, and here we are.

Compared to your 350's I see just how over engineered these feeders are in the first place.

Now to prove it, and not step on toes in the process.

Thanks Again everyone for the help.
Kevin

 

[KP2]

I hope you got your answer, which is "no". The resistance of a conductor does not depend on what type of insulation it has.

Correct, however the temperature of the conductors have everything to do with the resistance of the conductor.

 

[powerpete69]

Incidentally, demand load is how much amps you run on average over a 8 hour day for example.
Connected load is the load with everything on at the same time which is just an mathematical exercise and not reality.
Are you sure your average amps will be 1083 amps? I am doubting it. Also, your 1200 amp breaker is likely a 80% breaker which means that it will trip over 960 amps within an hour or two....or less. It will not trip right away. It will just trip when it gets too hot similar to your wires getting too hot.
Also, why are you using 2% voltage drop? Why can't it be 5% voltage drop? A 480 Volt system feeds 460 Volt loads, that's a more than acceptable 5%. 240V panel feeds 230V loads, again 5% voltage drop. Take a 110V receptacle fed by a 120V panel. That's 9% voltage drop and everything is just fine. Not sure what 208V loads are, but the point is that everything is built for voltage drop since most loads are some distance away.

If you really have a demand load of 1082 (again, I doubt), use a voltage drop calculator to figure your voltage drop. You may want to upsize a hair over 350 MCM using my 5% figure, but again, that breaker is going to trip a few times a day if you really have the 1082 A demand load. Which essentially means that (4 conduits) 350 MCM should work fine and meets code.

 

[ggunn]

Correct, however the temperature of the conductors have everything to do with the resistance of the conductor.

Of course, although it is a relatively minor effect and has nothing to do with which ampacity column the conductor type is in. The ampacity tables have nothing to do with voltage drop.

 

[MyCleveland]

I am a little confused as to what you are driving at, but the ampacity tables have to do with the stability of different types of insulation, nothing more. They all refer to the same internal metal wire and they have nothing to do with voltage drop. They do not imply that conductors from different columns will reach different temperatures while carrying the same amount of current. They only tell you how much current a conductor can carry before the insulation starts to melt; and the different columns refer to different types of insulation.

The different columns also represent the different max operating temps...60-75-90C.
Each temp selected will result in a different R value and therefore a different VD result.

 

[wwhitney]

Of course, although it is a relatively minor effect and has nothing to do with which ampacity column the conductor type is in.

Maybe not so minor. Suppose your initial design uses a conductor at its full 75C ampacity (based on 30C ambient), and you decide to use two sets to reduce voltage drop. If the heat dissipation is linear in temperature over ambient (may be an erroneous model), then to first order the resistance has halved, so if the operating temperature with one set is 75C, the operating temperature with two sets will be only 52C.

At 20C, the temperature coefficient of resistivity of Al is 0.0038. If the coefficient is close to that for the 52C to 75C range, then the resistivity at 52C will only be about 92% of that at 75C. So using 2 sets would reduce the voltage drop to 46% of original, rather than 50% of original. [To be more accurate, you'd have to iterate, as the lower resistivity would mean an even lower operating temperature, etc. That's why I rounded 52.5C down to 52C.]

OK, so maybe it is minor. But for extreme cases like in the OP, it may start to be significant.

Cheers, Wayne

 

[MyCleveland]

Thanks Wayne
I am terrible at this...trying to type out when I would rather talk it through.
My thought was because he has so much wire capacity, there must be some way to account for that in a temp adjustment of R giving a better VD result but I don't know how to guide KP2 in this calc.

I only chimed in when i could not duplicate KP2's VD calc without using a 60C R value...and rambled from that point.

 

[powerpete69]

Also, why are you using 2% voltage drop? Why can't it be 5% voltage drop? A 480 Volt system feeds 460 Volt loads, that's a more than acceptable 5%. 240V panel feeds 230V loads, again 5% voltage drop. Take a 110V receptacle fed by a 120V panel. That's 9% voltage drop and everything is just fine. Not sure what 208V loads are, but the point is that everything is built for voltage drop since most loads are some distance away.

Please disregard this portion of the post, my apologies. I was thinking that we were at the last load, but we are not. You will need additional distances from whatever gear we are feeding here. So the max 2% probably is realistic.

 

[ggunn]

The different columns also represent the different max operating temps...60-75-90C.
Each temp selected will result in a different R value and therefore a different VD result.

Not for the same amount of current. You can push more current through a 90 degree wire than you can through a 75 degree wire, true enough, but if you push more current through any wire, of course Vd is greater, even if R were to stay the same.

 

[KP2]

Hello folks, to give some closure on this thread the engineer held me to the 5% voltage from the Feeders to the meter center, to the unit feeder, to the AC unit on the roof. Here is the final approved set of Voltage drop calculations.

The only derating the engineer allowed was 85% of the unit Sub Feeder; he said since 310.15(B)(7) allows 83% reduction of the demand, he would allow me to use 85%.

At least we didn't have to go with Copper.

Thanks again for all those that supported this effort.
Kevin