100 amps at 565'

[wyliesdiesels]

Prior voltage drop calculations in this thread are being calculated using only the dc resistance value (0.061 ohms/1000ft for 350MCM AL, for example) listed for the conductor in Table 9.
Vdrop = 2*100A*0.061ohms/1000ft*565ft = 6.893 volts >>>> 6.893V/240V * 100% = 2.88%.

Another site referenced in an earlier post also erroneously uses only the dc value of resistance.

This is an ac circuit and the inductive reactance in an ac circuit also creates additional voltage drop. Table 9 in NEC also provides the effective impedance(Z) for 3 conductors in various types of conduit at a common power factor of 0.85 for 60Hz operation.
(This site will quickly explain the impedance components: http://ecmweb.com/content/calculating-voltage-drop-power-distribution-systems )

Southwire is calculating the voltage drop of a conductor based upon initial voltage, load amps, Cu/AL, distance, impedance(DC resistance plus AC reactance) and permitted voltage drop (%) based upon user inputs.
In order to stay at or below required % Vd, the impedance(Z) required to meet those requirements is calculated and then the conductor with that impedance value (or next lowest) is able to be looked up and the conductor size is pushed out of the app. It is the impedance of the conductor that determines the minimum conductor size.
The conductor ampacity listed in 310.15 (B)(16) actually has nothing to do with the result.***

For 350 mcm AL…..Vdrop = 2*100A*0.073ohms/1000ft*565ft = 7.3 volts >>>> 7.3V/240V * 100% = 3.44% (This fails the 3% limit.)

For 500 mcm AL…..Vdrop = 2*100A*0.057ohms/1000ft*565ft = 5.7 volts >>>> 5.7V/240V * 100% = 2.68% (This passes the 3% limit.)
Southwire app reports back needing 500 MCM AL.

***There is a note in the Southwire app and online calculator that the 60C table is used for conductor sizes in branch circuits. Since 334.80 specifies that NMB, etc….can’t exceed the 60C ampacities, Southwire references the 60C ampacities assuming we are using NMB. NMB is available in #14 - #2 Cu. From #1 and larger, they jump to 75C ampacity references.
They have added confusion and created skepticism by forgetting that branch circuits and feeders can be run with 75C rated insulation in this range without a good explanation of referencing 60C vs 75C ampacities.

It doesn’t matter. Follow the example below:

Situation: 240 vac supply, single phase, conduit, 3% max drop desired, 25amp load, Cu, 250ft distance
From Southwire online calc:
“1 conductors per phase utilizing a #6 Copper conductor will limit the voltage drop to 2.31% or less when supplying 25.0 amps for 250 feet on a 240 volt system.
For Engineering Information Only:
55.0 Amps Rated ampacity of selected conductor
0.4662 Ohms Resistance (Ohms per 1000 feet)
0.051 Ohms Reactance (Ohms per 1000 feet)
7.199 volts maximum allowable voltage drop at 3%
5.523 actual voltage drop loss at 2.31% for the circuit
0.9 Power Factor”

A quick calc by hand:
Vdrop(#6 wire) = 2*25A*0.45ohms/1000ft*250ft = 5.63 volts >>>> 5.63V/240V * 100% = 2.34%

Since we are concerned about eliminating voltage drop, #6 Cu wire must be installed and in order to get ~3% or less Vd, it must have ocp at 30A to maintain this criteria. It doesn’t matter if nmb(55a) or thhn(65a) is used - #6 Cu is #6 Cu.

Where conductor insulation rating and ampacity would matter is in derating for number of conductors in conduit and temperature correction for elevated temps. Elevated temps would increase the dc component of the reactance and effect the Vd. Southwire app makes no adjustment for these two situations.

Thx for reminding me about reactance and skin effect and the need to change the resistance for 2/0 and larger wire....i forgot about that...

I lost one leg of my under ground service to my back garage due to a lightning strike. If I do it again I would use conduit unless the electric company was responsible for the service.

Direct bury wire is NEVER a good idea. It may be cheaper in the short term(no conduit) but it will almost always bite u in the ***.

 

[kaiser715]

So...the other day, I met the engineer from the power company at the site to talk about running a second meter to the shop, and other options he may have.

If I do my own power run from the house, it will run about 75', turn about 60 degrees, run 325', turn 90 degrees, then 100' to the shop, along the existing woods road. If in conduit, I'll put a pull box at each of those turns.

The power company will run overhead lines. Straight line from the nearest pole (on neighbor's road), run just past the house, to the garage. A 30' wide cleared swath thu the woods. Shop would be visible from the house then.

That kind of killed it for me. Plan now is conduit (4"?), and my own wire (run from outside disconnect at meter base).

 

[mm08822]

So...the other day, I met the engineer from the power company at the site to talk about running a second meter to the shop, and other options he may have.

If I do my own power run from the house, it will run about 75', turn about 60 degrees, run 325', turn 90 degrees, then 100' to the shop, along the existing woods road. If in conduit, I'll put a pull box at each of those turns.

The power company will run overhead lines. Straight line from the nearest pole (on neighbor's road), run just past the house, to the garage. A 30' wide cleared swath thu the woods. Shop would be visible from the house then.

That kind of killed it for me. Plan now is conduit (4"?), and my own wire (run from outside disconnect at meter base).

Not sure what your method/plan is for pulling or the final conductor sizes, but you may want to consider another pull box in the middle of that 325'. Also consider pulling from the middle each way.

I would also leave as much of the conduit exposed until after conductors were pulled. Backfill around both sides of pull boxes and vertical 90's before pulling. (Just in case!)

 

[ard]

So...the other day, I met the engineer from the power company at the site to talk about running a second meter to the shop, and other options he may have.

If I do my own power run from the house, it will run about 75', turn about 60 degrees, run 325', turn 90 degrees, then 100' to the shop, along the existing woods road. If in conduit, I'll put a pull box at each of those turns.

The power company will run overhead lines. Straight line from the nearest pole (on neighbor's road), run just past the house, to the garage. A 30' wide cleared swath thu the woods. Shop would be visible from the house then.

That kind of killed it for me. Plan now is conduit (4"?), and my own wire (run from outside disconnect at meter base).

Whacking trees and overhead lines would be a non-starter for me too.

I am sure they would go underground- if you paid for trenching, conduit. AND still get you for the $26 a month.

Happy trenching!

Oh, a neighbor who built 23 years ago when we built is having 'wire issues'. I did my own electric, everything in conduit. Main line to pole; line to well...all landscape lights..all low voltage to barn/shop...Neighbor just went with 'whatever the contractor said'. So everything was UF, DB in the ground. So far he has had to dig up his asphalt driveway twice (take out one tree) to repair the main line (PGE fixes for free, but doesn't patch asphalt nor replace trees). He also had to replace the line to the well. 400 feet. He was 'surprised it was only $4500'.

Not a fan of DB (I know you are not doing DB, just dropping the story if others read it)

Keep us updated!

 

[kaiser715]

But I really *want* to do direct burial....easier, cheaper, etc. But I know better.

I'm gonna price out the CiC (cable in conduit), though. Looks like that might be about as easy...dunno about price, though. Might be harder to handle for DIY. Pulling 350 or 500kcmil will be hard enough.

 

[myredracer]

I'd be concerned about voltage drop from motor inrush current on startup like from the compressor. Inrush current (or LRA) can be up near 6 times FLA and then you'd add the full load amps of anything else running (lights, or whatever).

 

[kaiser715]

I'd be concerned about voltage drop from motor inrush current on startup like from the compressor. Inrush current (or LRA) can be up near 6 times FLA and then you'd add the full load amps of anything else running (lights, or whatever).

So.... what would you suggest??

House (meter base) will be about 100' from transformer...I am only house on that transformer. Then 500' to shop.

 

[walrus]

But I really *want* to do direct burial....easier, cheaper, etc. But I know better.

I'm gonna price out the CiC (cable in conduit), though. Looks like that might be about as easy...dunno about price, though. Might be harder to handle for DIY. Pulling 350 or 500kcmil will be hard enough.

Tech cable, wire in what looks like Liquid tite metallic conduit. Thats what I used rolled right in trench. Mine was only 180 ft though. That roll was plenty heavy but you can order in any length you want I think?
BTW direct burial is done all the time, prep the trench right and it works fine. Sand, under, around and over cable and it works fine

 

[reader2580]

Tech cable, wire in what looks like Liquid tite metallic conduit. Thats what I used rolled right in trench. Mine was only 180 ft though. That roll was plenty heavy but you can order in any length you want I think?
BTW direct burial is done all the time, prep the trench right and it works fine. Sand, under, around and over cable and it works fine

Wire & Cable Your Way sells Teck 90 cable, but only in 10 AWG to 14 AWG. They also sell a similar Interlocked Armor cable, but it would probably be at least $8 a foot for the OP for the size required.

I've looked at at the Interlocked Armor cable for my garage power feed, but it isn't cheap. I assume it is considered direct burial so would need to be 24" deep.

 

[braidmeister]

Do it right now...or do it right later.

If you're planning on staying for a long time, do it right so you hook yourself up. None of it is cheap...but the last thing you want to do is redo it. If you're planning on moving at some point - do it right so you're not nailed with a code violation or some other deal breaker that prevents the property from being sold. This is something your next of kin might have to deal with if you leave it to them and they need a cert of occupancy or sell it.

Something to think about...

 

[myredracer]

So.... what would you suggest??

House (meter base) will be about 100' from transformer...I am only house on that transformer. Then 500' to shop.

Without looking at your estimated loads and equip. specs. in detail, say you had 40 amps of "continuous" load of say lights + welder and added 6x22 amps of compressor inrush current (at 240 volts), you'd have 172 amps of total momentary current versus basing calcs. on a 100 amp rated panel, feeder & breaker. Then consider whether or not the compressor will start okay with the voltage drop based on the 172 amps. Google "motor startup inrush current and voltage drop" for detailed info.

Could be possible that you need to increase the feeder size by paralleling aluminum with more total ampacity or going with copper instead and maybe paralleling the copper. I used to spec. 2" thinwall duct a lot because it was the cheapest option of wire & conduit. You could possibly put a spare duct in the ground for a minimal additional cost in case you find you need it to upsize the feeder ampacity later on. Then there's the option of stepping the voltage up/down or a primary line to the garage/shop and separate meter. If you do need to allow for inrush current, I think increasing the feeder ampacity will be the way to go. Sorry I can't offer more detail/info. - I *used* to deal with this stuff on a regular basis as an EE but it's been too many years I've been retired.

 

[reader2580]

BTW direct burial is done all the time, prep the trench right and it works fine. Sand, under, around and over cable and it works fine

Power companies do direct burial of feeds to house all the time and they rarely fail. My parent's house was built in 1979 with underground feed and they had one failure caused by an animal years ago. Power company ran a temporary from the neighbor and eventually buried a whole new line. My house was also built in 1980 and has underground service, but no idea if the line has been replaced or not. I have aluminum direct burial to my garage that has been there since 1980.

I am pretty sure power companies are not using Teck cable due to cost.

 

[walrus]

Paralleling smaller conductors is a good idea.

Anyone who thinks that if you have a fault in a 500 ft conduit that pulling in new wires is going to happen after you get old ones out is never has tried it before . Maybe you would get them out but maybe you won't ? Especially if that yellow wire lube that ideal makes dries out in pipe.

 

[Cmreschke]

Paralleling smaller conductors is a good idea.

Anyone who thinks that if you have a fault in a 500 ft conduit that pulling in new wires is going to happen after you get old ones out is never has tried it before . Maybe you would get them out but maybe you won't ? Especially if that yellow wire lube that ideal makes dries out in pipe.

Parralelleing smaller conductors us not compliant with the nec. (Depending on the size of conductor and how small) 2/0 is the smallest that your allowed to parallel.

 

[walrus]

Parralelleing smaller conductors us not compliant with the nec. (Depending on the size of conductor and how small) 2/0 is the smallest that your allowed to parallel.

Yeah, I'm not sure what sizes folks were suggesting for a 565ft run

 

[wyliesdiesels]

Whacking trees and overhead lines would be a non-starter for me too.

I am sure they would go underground- if you paid for trenching, conduit. AND still get you for the $26 a month.

Happy trenching!

Oh, a neighbor who built 23 years ago when we built is having 'wire issues'. I did my own electric, everything in conduit. Main line to pole; line to well...all landscape lights..all low voltage to barn/shop...Neighbor just went with 'whatever the contractor said'. So everything was UF, DB in the ground. So far he has had to dig up his asphalt driveway twice (take out one tree) to repair the main line (PGE fixes for free, but doesn't patch asphalt nor replace trees). He also had to replace the line to the well. 400 feet. He was 'surprised it was only $4500'.

Not a fan of DB (I know you are not doing DB, just dropping the story if others read it)

Keep us updated!

What was the cause of the failures?

 

[ard]

What was the cause of the failures?

Dont know.

The one time for the main service he reported to me that as they were digging down he asked 'are you sure this is the spot?', and the rep pointed out that they could see smoking coming up from the ground a few feet down, and the dirt appeared discolored. On the line to the well it just stopped working. He said the cable just looked damaged and melted at the spot.

No tractor work in the area, nothing of note.

My guess- knowing his contractor- is the line got buried roughly with the dirt that was excavated- no 'lining of the trench with sand then 4 inches of sand cover over top'....

The well line was replaced with PVC conduit