Unusual Power Distribution Setup

[mhulbert]

A few years ago I wired a camp with 3 remote cabins. The underground PVC was put in well before I became involved, and it "daisy chained" from my panel to each cabin (ie: Panel-->Cabin1-->Cabin2-->Cabin3). I ran #2 through this whole daisy chain, with a pull box mounted on the side or each cabin. In cabin 1 and 2, I used polaris blocks to tap the main line with a short run of #2 going into a 100A panel in the cabin. BTW, there's about 75' cable between each cabin, and the load in each is a 15A, 120V water heater, and some lights/plugs (about 2500W total per cabin)

What are the pros/cons of doing it this way, as opposed to the typical radial distribution method that we normally used (ie a feeder for each building, homerun to the panel). The job saved a lot of money doing it this way, but I have my worries about trying to find a fault or other problem down the road. The camp plans to add a 4th cabin, extending the 100A feeder past the last cabin.

Any thoughts, good, bad or insightful?

Mike

 

[ramsy]

I ran #2 through this whole daisy chain ..there's about 75' cable between each cabin, ..(about 2500W total per cabin)..The camp plans to add a 4th cabin, extending the 100A feeder past the last cabin.

My End-Of-Line check for that 4th cabin would look like this:

____ Target__Voltage ________ Table Data Or Field Measurement
1) I  = P/E (2.5k*4)/120 = 83 Get load Amps. or Clamp meter
2) Z  = TotFt/1k*Ohm = 0.1164 NEC Tbl.8/9 or Meter Z=(E-Ec)/I
3) Ec = (Nominal-Z*I) = 110.3 Get choked Volts
4) Pc = Ec*I ---Not Needed--- Get choked power
5) Ic = Pc/E ---Not Needed--- Get choked Amps
____ Check__Wire__Size ______
6) CM   = KIL/VD = 66495 (#2) K@75?C=12.9, L=600.Ft, VD=E-Ec 
7) De   = I/Iwire@Max?C ---- 83/130=0.64 Max Derating @90?C
8) PF   = Ec/E -Not Needed--- Tbl.430-150 See Motor or Meter
9) ASCC = E/Z --Not Needed--- Avail.Short.Ckt.Cur.

When all cabins are at full use, the 4th cabin remains above 110.vac (3), with enough derating capacity (7) to run that conduit through an ambient hot springs around 140?F, see Tbl.310-16.

Without an ambient you could stuff a 9-wire circuit through that PVC, see Tbl.319-15b2, but with those Polaris blocks tapping off the same 2-wire for each cabin, you won't need the available derating (7).

While your impedances won't slow down ASCC (9), for a 5th cabin 750 ft. Z=0.146 (2) would choke volts below 108.vac (3), where laptop computers would start crashing. This End-Of-Line check tells me using either a 3-wire ckt, or 208 or 240.vac water heating equip. would provide more room for future expansion.

 

[mhulbert]

Roger,
what did you use to produce those measurements/math? They look very interesting. I will have to tell them that if they want a 5th cabin, they'll have to run a new HR back to the main panel.

Hopefully, the cabins won't be simultaneously loaded...the water heaters are small 1 gallon units, so their cycle time is very short, and they supply only a small sink. Unfortunately, they couldn't get 240V units this small, I agree, it would make things easier on the system.

Thank you

 

[ramsy]

I will have to tell them that if they want a 5th cabin, they'll have to run a new HR back to the main panel.

Or, just pull another phase through that conduit and balance the 120.vac loads between 2 circuits, a 3-wire, which would increase your target voltage at EOL, by deviding the load in half. See line: (3) Choked Volts=(Nominal-Z*I).

Roger, what did you use to produce those measurements/math? They look very interesting.

The field calcs. come from troubleshooting motors with different test instruments, and Herman, "Delmar's Standard Textbook of Electricity" 1999. The conductor specs. came from NEC tables.

Using Z before VD, NEC 110.10, a debate over 110.3b, and the users bob and iwire inspired me to develop this Target Voltage worksheet from a recent thread. Line (3) Choked Volts=(Nominal-Z*I) is unusual, but it simplified the math.

People constantly ask about V-Drop, during wire size and derating calcs., so the traditional CM=(KIL/VD) (6) was added. Line (7) De=Ic/Max?C is also unusual, but shows how max derating is available from Ic (choked Amps), which came from Z. Our Engineer/Moderator Don pointed out PF adjustments in Tbl.430-150, which are added on line (&). The ASCC in line (9) is most usefull with impedance-grounded Delta's.

If more people tested this worksheet with their End-Of-Line checks and designs, they could post improvements and make it more accurate and usefull for everyone.

 

[LarryFine]

Or, just pull another phase through that conduit and balance the 120.vac loads between 2 circuits, a 3-wire, . . .

I'm curious about how you deduced the service was 120v and not 240/120v. I re-read the posts and couldn't tell.

 

[ramsy]

..how you deduced the service was 120v and not 240/120v.

That template was accurate for 1-phase only, so my figures were based on a single120vac run between cabins. Suggested pulling another phase for a shared neutral, 120vac, 3 wire (counting neutral). Wiring phases together may get 240 or 208vac, whichever it may be.

..load in each is a 15A, 120V water heater

 

[mhulbert]

My apologies, it is a 3 wire feeder (120/240V), all #2.

Wouldn't your calcs still be valid for a 120V load, regardless if there is another phase or not? Or does neutral VD decrease as the opposite phase of the load increases( ie the phases become "balanced"?)

Mike

 

[ramsy]

..it is a 3 wire feeder (120/240V), all #2. Wouldn't your calcs still be valid ..regardless if there is another phase or not?

1) Multi-phase load complications:
- The length would be the same (L), but if two phases evenly split the loads, your max load would be (I)=83/2 or 42A. At the 4th cabin 600.ft away, target voltage improves from 110 to 115vac, and max derating from 0.64 to 0.32.

2) Multi-phase Derating Complications:
Neutrals that only carry circuit imbalance don't count as current-carrying conductors, except where a phase is missing from a 4-wire-Y circuit, or a major portion of nonlinear load causes neutral harmonics. 310-15(B)4a-c.

3) Reactance (XL) complications: (Not an issue with PVC & unsheilded cables)
- I used NEC Tbl.8 here. These DC Resistance values ignore conduit reactance (XL). But, when needed, XL & Z. values in Tbl.9 are limited to 3-phase systems and a special formula must adjust these Z. values when PF doesn't match 0.85.

 

[ramsy]

..does neutral VD decrease as the opposite phase of the load increases( ie the phases become "balanced"?)

To the best of my knowlege you are right for a 3-wire ckt using 120/240, but not 120/208Y 3?. If loads are balanced and synchronous between all phases, the shared neutral current would be zero, (I) could be devided again for new target voltage and derating values.

Your cabin loads may be intermittent, but when shared neutrals between balanced phases are continueous or otherwise synchronous, current values in Ec=(Nominal-Z*I) could be adjusted accordingly. Many thanks for that correction.

The #2 wire, 2-phase shared neutral went to my spreadsheet with these [values].

 Chk EOL 1-Phase  3-Phase  NEC Tbl. Or  Field Meter
0) E  =      [120]   [120] Tbl.8/9 Z=ft/1k*ohm or VD/I
1) I  =       [42]    [42] Load Amps    = P/E
2) Z  =    0.1164   0.1420 [0.194] =Z,T9XL=  [0.045]
3) Ec =       115      114 choked Volts = Nominal-Z*I
4) Pc =      4835     4790 choked Power = Ec*I
5) Ic =        40       40 choked Amps  = Pc/E
 Chk Wire
6) CM   =   66495    54523 [12.9]   =K@75c ft=  [600]
7) De   =   0.323    0.323 [130]    =D@90c  #=    [1]
8) PF   =    0.96     0.95 Pwr Fact W/VA= Ec/E
9) ASCC =    1031      845 CM=KL/Z, De=I/Max?C, SC=E/Z