Voltage drop and the 90 degree column

[kwired]

This quote applies as follows: Imagine you have a bunch of 20A circuits, which typically are run using 12ga wire, protected by a 12ga EGC. Say you had a bunch of 75C rated wire and had to run 9 'current carrying conductors' in a single conduit. Derating rules would push you up to 10ga wire and you would also be required to use a 10ga EGC. But if instead you used 90C wire you would be derating from a 30A ampacity and you could stick with 12ga wire.

Note however that the voltage drop in these hotter 12ga conductors would be greater than the same conductors operating at a lower temperature.

The quote is not saying that voltage drop is the same in conductors of the same ampacity but different temperature ratings, rather it is noting that you can use the higher temperature rating to reduce the conductor size for purpose of derating, and thus avoid having to increase the size of the EGC.

Reducing conductor size will always increase voltage drop.

-Jon

But you are trying to find a way to reduce conductor size so it will fit in your raceway.


Yes higher temp insulated conductor will have more ampacity per the tables, but at same time will increase VD if you use that additional capacity. Same conductor will still have same VD characteristics regardless of what insulation is on it, all the higher temp rating does is allow higher insulation temp before compromising insulation integrity becomes a concern.

You are adding one benefit but taking another away. May not be 1:1 difference so details are important.

 

[iceworm]

Winnie, kw -
Thank you. I don't have a handbook. I had no idea what to make of the quote.

Never really trusted the handbook anyway. All that extra is just an opinion

 

[wwhitney]

A few comments:

1) If voltage drop depends strictly on copper/aluminum area, then my previous suggestion of using 12 sets instead of 6 sets was a step in the wrong direction, as with a fixed insulation thickness, smaller conductors are less efficient in terms of copper/aluminum area vs insulated conductor area.

2) If 1082.7A is the NEC calculated load, then I would think there is a reasonable argument for reducing that figure for voltage drop calculations. "Everyone" knows the NEC calculated load is a significant overestimate. You still have to meet it for ampacity, but as voltage drop is not an NEC issue, you can use other procedures for determining the real load for voltage drop.

3) 2014 Table C.9A says that (5) 750 kcmil compact conductors with THWN or XHHW insulation will fit in a 4" PVC conduit, Schedule 80 (worst case, as you haven't specified the conduit material). Is that not adequate for the original parameters of your question? With 6 sets, I get that 250 kcmil is the smallest aluminum conductor in each set, and for 1200A, 250 kcmil aluminum is the smallest EGC. So your EGC will have to match your ungrounded conductor size in each set.

4) How does neutral size figure into 3 phase voltage drop calculations? If it doesn't, then you could shrink the neutral and fit in (4) 800 kcmils (ungrounded and EGC) and a smaller neutral. At least 400 kcmil should fit per the tables, but you might fit 500 kcmil. I haven't done the calculations.

Cheers,
Wayne

 

[synchro]

3) 2014 Table C.9A says that (5) 750 kcmil compact conductors with THWN or XHHW insulation will fit in a 4" PVC conduit, Schedule 80 (worst case, as you haven't specified the conduit material). Is that not adequate for the original parameters of your question? With 6 sets, I get that 250 kcmil is the smallest aluminum conductor in each set, and for 1200A, 250 kcmil aluminum is the smallest EGC. So your EGC will have to match your ungrounded conductor size in each set.

Cheers,
Wayne

Agree with Wayne that these should fit based on the table. If you are not comfortable being this close to the edge on fitting the conductors, I wonder if there would be enough benefit by going copper just for the EGC. Of course going all copper would help but be big $$.

 

[KP2]

Thanks again for keeping this moving folks; the PVC runs are SCH40.

I realize that VD is not required; but in multifamily most EOR seem to require something, and this project is strict.

So far, I am up to 750kcmil feeders with a 600kcmil EGC and they won't fit in the 4" PVC

I see 3 potential possibilities remaining;

1. If there is a way to reduce the EGC 1 size it will fit, or perhaps I made a mistake and did the increase wrong.

2. I was not provided with the full calculation, so I do not know what the neutral load is, but reducing the neutral down a size will seal the deal as well.

3. Compact conductors.

Thanks Again
Kevin

MC-S2	(6) sets of 4" C with 4 - 750kcmil & 1 - 600kcmil EGC
Supply Voltage 3Ø (V)	208
VD=(1.732xLxRxI)/1000
1Ø=2 3Ø=1.732	1.732
Resistance	0.0282	750kcmil AL
Length (ft)	480
Demand (A)	1082.7
Number of sets	6
Voltage Drop (V)	4.23
Percent of Voltage Dropped	2.0
Increase for EGC
Details	Feeder Ampacity (A)	# of sets	Wire size (CM)	Total CM
Feeder before adjustment	1200	4	500000	2000000
Adjusted Feeder		6	750000	4500000
Size Ratio for increased EGC	2.25
EGC before adjustment (CM)	250000		Wire size (CM)
Adjusted EGC	562500		600000
Conduit Fill	# of conductors	Area (in^2)	Total Area (in^2)
750kcmil	4	1.0532	4.2128
600kcmil	1	0.8709	0.8709
			5.0837
		4" PVC equals	5.022

 

[KP2]

Looking again at my steps for the increase in the EGC, perhaps I should have just used 1 set of conductors; so 750/500 = 1.5 for a size ratio.

That would equal 250,000 CM X 1.5 = 375,000cm --> a 400kcmil would be the increase this way. that helps too.

I read 250.122(B) to equal the circular mill of the ungrounded conductors as the sum of the CM of feeders, because I was going from 4 to 6 sets with the increase.

Is this the correct increase method for this situation?

Thanks
Kevin

 

[winnie]

1) I read 250.122(B) the same way you do. IMHO you go by the _total_ change in cross sectional area. So if your baseline is 4 parallel 500s to get the required ampacity and you change to 6 parallel 750s then the increase in cross sectional area is 2.25x.

However_ code doesn't actually specify the baseline that you need to use. For your installation, 12 parallel 1/0s, 6 parallel 250s, 4 parallel 500s, 3 parallel 900s or 2 parallel 1750s are all 'reasonable' in the sense that they get you to more than 1200A using NEC tabulated ampacity. Doing the calculation from a different baseline might help, though I wouldn't use conductors that you can't actually purchase or install in your conduit as a reasonable baseline.

2) Check the area of the conductors; I thought that most aluminium conductors were supplied as compact stranded. http://www.cmewire.com/catalog/sec12-lvi-al/lvi-al-05-rw90-xlpe.pdf gives a diameter of 1.08in for 750s, which is an area of 0.916 square inches Sizes listed at 'wire and cable your way' are also smaller; using their conductor sizes you could just barely fit 4 900s and a 500 EGC in each of your conduits.

3) You could use copper for your EGC to make it smaller.

-Jon

 

[Knightryder12]

To the people who are saying that VD is not an NEC requirement, that may be true, but if the area he is in has an energy code, them he has to follow that. In FLA where I am from the energy code dictates the VD requirement. It says in our energy code that a maximum of 5% VD is allowed and does not say it has to be 2/3, 3/2, 4/1, 1/4 etc. Just 5% max. Is the 2% a job specific requirement?

 

[KP2]

Hello folks, I submitted my voltage drop calculations (using 600kcmil at about 2.5% voltage drop) to the EOR; and this is what he cam back with;

"
Kevin,



Need to carry calculations to next level . meaning, calculate voltage drop from meter center(s) to unit load centers; and from unit load center to furthest device on a branch circuit.

Total drop for entire run (service to MC to load center to outlet) should not exceed 5% total.

Thanks,"

I just don't see how this is supposed to produce a realistic voltage drop result.

I understand if this was for Site Poles, or a large feeder to multiple motors; but these are apartments with loads that will never reach 100% of the calculated demand load of 100% of the units on the feeder.

Even with a motor, we use the name plate amperage not the FLC at 125% like the conductors; it's like I need to prepare for Armageddon.

Am I to calculate this voltage drop request the following way;
1. To get started I would use 100% demand of 100% of all the units to calculate the voltage drop on the feeders to the meter center.
Approximately 2.5%

2. Calculate the voltage drop of each Units sub-feed at 100% of the demand for each unit; the largest voltage drop of this set of feeders will have it's percentage added to the percent of drop from step 1. I should have this below 3% already since the unit sub feed is typically my only voltage drop concern.

3. Calculate the Voltage drop of each branch circuit of each units loadcenter; then take the largest percentage of voltage drop within that loadcenter and add it to the voltage drop of that unit's sub feed (should be less than 3%) and add it the 2.5% drop on the meter center feeder and pray they are all less than 5%.


This sounds as silly as how I started this tread, but I think it is what is expected of me.

Any thoughts would be greatly appreciated, this just seems so ambiguous to me.

Thanks
Kevin

 

[MyCleveland]

KP2
.....it's like I need to prepare for Armageddon. I like that.
How did you come up with the 2.5%VD using 600 AL ? Did you use 60C for the wire R values ?
I get the following results (% VD) using your description of the service and load amps.
60C 75C
PF 1.0 2.47 2.60
PF .99 2.84 2.97
PF .98 2.98 3.11
You see where this is heading....
Which brings up the question, can you use the 60C R values if you are using the 75C ampacity values, I think not.
In your case though, your service wire is so oversized (230%), there must be another way to calculate because conductors will never come close to rated ampacity.

Is the Service point where all the meters are located ? or are there feeders to various meter stacks ?
Not sure you will be able to meet the 5% total if you are already at 2.5 - 3.0 at the service.

When I calculate the apartment feeder VD I use 66% of the unit load. If I used 100% I would be up sizing quite a bit.

 

[david luchini]

Which brings up the question, can you use the 60C R values if you are using the 75C ampacity values, I think not.

The resistance value of the conductor is unrelated to the ampacity selection.

 

[KP2]

Thanks MyCleveland,

KP2
How did you come up with the 2.5%VD using 600 AL ?

I used the resistance values Table 8.

	Meter Center Feeder Voltage Drop
	MC-S2 - (6) sets of 600kcmil Aluminum Conductors
	VD=(1.732xLxRxI)/1000
	1Ø=2 3Ø=1.732	1.732
	Resistance of 600kcmil AL	0.0353
	Length (Ft)	480
	Demand (A)	1082.7
	Number of sets	6
	Voltage Drop (V)	5.30
	Supply Voltage 3Ø (V)	208
	Percent of Voltage Drop (%)	2.5

Did you use 60C for the wire R values ?

No, if I can, how?

I submitted using 600kcmil which gave me 2.5%, my previous example posted was with 750kcmil; I figured I would go in with the 600's since I feel they are adequate.

I get the following results (% VD) using your description of the service and load amps.
60C 75C
PF 1.0 2.47 2.60
PF .99 2.84 2.97
PF .98 2.98 3.11
You see where this is heading....
Which brings up the question, can you use the 60C R values if you are using the 75C ampacity values, I think not.

Not exactly, other than I am ignoring PF.
Can I use 60C R values, if how?

In your case though, your service wire is so oversized (230%), there must be another way to calculate because conductors will never come close to rated ampacity.

I sure hope so.

Is the Service point where all the meters are located ? or are there feeders to various meter stacks ?

This is for a feeder to a meter stack; there are three in total.

When I calculate the apartment feeder VD I use 66% of the unit load. If I used 100% I would be up sizing quite a bit.

Any chance you have documentaion to lower the ampacity for the voltage drop calculation; 66% of the demand would be a huge help.

Thanks
Kevin

 

[KP2]

Yeah, at 66% of the load, the Voltage drop is at 1.6%

That would save the day for sure, if I could validate it.
Thanks
Kevin

 

[MyCleveland]

The resistance value of the conductor is unrelated to the ampacity selection.

David
Tables 8 & 9 in Chapter 9, values are based on 75C wire. Table 9 gives a correction formula for adjustment to other temps.
Ampacity tables give us selections of 60-75 or 90C values.
My point was if you are using ampacity from say the 60C column, you then adjust values in table 9 to 60C, or corrected R value.
Agree or Disagree ?

 

[MyCleveland]

Yeah, at 66% of the load, the Voltage drop is at 1.6%

That would save the day for sure, if I could validate it.
Thanks
Kevin

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.

 

[david luchini]

David
Tables 8 & 9 in Chapter 9, values are based on 75C wire. Table 9 gives a correction formula for adjustment to other temps.
Ampacity tables give us selections of 60-75 or 90C values.
My point was if you are using ampacity from say the 60C column, you then adjust values in table 9 to 60C, or corrected R value.
Agree or Disagree ?

I do not agree. The Table values are based on a 75C conductor operating temperature, not a 75C conductor insulation rating.

If you had identical 200A loads with 3/0 feeders of identical length in identical ambient temperatures, one feeder with THWN (75C insulation) and one feeder with THHN (90C insulation), the voltage drop would be the same.

 

[powerpete69]

VD compensation is not required by the NEC for this feeder and is a design issue. VD aside you'll still need to have 1200 amps worth of conductors. 6 sets of 250 kcmil aluminum (205 amps*6 = 1230 @ 75° C) is the minimum size conductors required by the NEC for a 1200 amps OCPD. Since that is the minimum required to next step is to find 6 sets of a certain size that will satisfy your 2% VD. According to my Southwire VD calculator you cannot get down to 2% with only 6 sets @208 volts.

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.

 

[MyCleveland]

I do not agree. The Table values are based on a 75C conductor operating temperature, not a 75C conductor insulation rating.

If you had identical 200A loads with 3/0 feeders of identical length in identical ambient temperatures, one feeder with THWN (75C insulation) and one feeder with THHN (90C insulation), the voltage drop would be the same.

I agree with your statement.
My point was if you are using the 60C amapcity you can then use an adjusted R to a 60C operating temp, or if using the 90C ampacity you then need to adjust to a 90C R value.

 

[ggunn]

I agree with your statement.
My point was if you are using the 60C amapcity you can then use an adjusted R to a 60C operating temp, or if using the 90C ampacity you then need to adjust to a 90C R value.

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.

 

[KP2]

David
Tables 8 & 9 in Chapter 9, values are based on 75C wire. Table 9 gives a correction formula for adjustment to other temps.
Ampacity tables give us selections of 60-75 or 90C values.
My point was if you are using ampacity from say the 60C column, you then adjust values in table 9 to 60C, or corrected R value.
Agree or Disagree ?

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.