voltage drop question

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bark

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I have a 240 volt single phase 400 amp service that was to be about 75 feet from the power pole, but that has changed to approx 600 feet. I've tried a few calcs for voltage drop but wanted to ask here and get some opinions. I figured a parallel service feed of 250 MCM. But with the added length I've done some work and come up with size 1000 MCM alum. But I don't think the parallel feed came into play. Would it be two conduits of 500 MCM wire? That's what I'm thinking but want some other thoughts. Any help appreciated. Thanks in advance
 
iceworm

iceworm

Curmudgeon still using printed IEEE Color Books
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bark said:
I have a 240 volt single phase 400 amp service that was to be about 75 feet from the power pole, but that has changed to approx 600 feet. I've tried a few calcs for voltage drop but wanted to ask here and get some opinions. I figured a parallel service feed of 250 MCM. But with the added length I've done some work and come up with size 1000 MCM alum. But I don't think the parallel feed came into play. Would it be two conduits of 500 MCM wire? That's what I'm thinking but want some other thoughts. Any help appreciated. Thanks in advance
Click to expand...

240V, 400A, 600' (1200' rountrip)
Table 8
Al 1000kcmil is .0212 ohms/100' --> ~ 4% VD

Al 500kcmil is .0424 ohms/1000' so, parallel 500s give the same VD

same for quad 250s

ice
 
Julius Right

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
In my opinion, the Table 9 it is more suitable for a.c.
Effective Z at 0.85 PF for Aluminum wire in p.v.c. conduit for 1*250 MCM =0.094 ohm/kft
So VD=2*400*0.094*0.6=45.12 V. 2*4 parallel cables VD=45.12/4=11.28 V [4.7% for 240 V rated]
For 1*1000 MCM =0.039 ohm/kft .VD=18.72 V [7.8%].
It seems to me 2*4*250 mcm will be better.:weeping:
 
iceworm

iceworm

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ActionDave said:
What's the calculated load?
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luckylerado said:
Is this really a 400 amp circuit or is it 2 @ 200 amps?

I second the need to do a load calculation first because nothing says you need to use 400 amps for VD. That is a long way
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kwired said:
That is most important factor here. Just because you have 400 amps of overcurrent protection doesn't mean you have 400 amps of load.
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Just thinking - not telling:
We don't know if the customer is large residential or small commercial. Either way, I'm thinking the customer is not too interested in the lights blinking when large loads start. I'd likely be sizing the feeders to account for starting current voltage dips - not as small as I could get away with. Then again, with the information we have, I can't even say that.

edit to add - Slow poster. We have more information now.

ice
 
iceworm

iceworm

Curmudgeon still using printed IEEE Color Books
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EE (Field - as little design as possible)
bark said:
The load calc's were around 330 amps give or take.
Click to expand...

Table 8
1000kcmil gives about 3.5% VD.

Considering the customer is office trailers, maybe no big startup loads - maybe don't have to consider light blinks.

ice
 
kwired

kwired

Electron manager
Location
NE Nebraska
Well if it is about 330 amps that is about 82% of the original 400, so some reduction in the end result can be expected but certainly not a huge difference.


Amount of VD that can be tolerated really depends on what is supplied, and even then you still get short transients when starting a larger motor or HVAC compressor, both of which may be able to be reduced with soft start equipment though you don't really find such equipment intended for single phase induction motors either.

One maybe can see about getting 480 or even 600 volt service and a transformer to 120/240 at load end, but also need to consider cost of this vs cost of heavier conductors, or see if you can get POCO to pull a medium volt cable through a raceway (even if you have to provide the raceway) and install pad mount transformer closer to the load. I know MV cable is sort of expensive but probably is considerably less then needing 1000 or more MCM of low volt cables.
 
Julius Right

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
iceworm said:
Julius - It is single phase.

ice
Click to expand...
Thank you, iceworm. You are right. Table 9 it is for three-phase system. However the difference is small. For instance, if the resistance is the same-skin effect will be the same and proximity effect is very close- then only reactance differs a bit. For 2*250 mcm it will be 0.032 ohm/kft instead of 0.041 and for 2*1000 MCM it will be 0.028 instead of 0.037.
In this case 600 ft 400 A 4*250 MCM the VD it is only 4.45% for 2*4 *250 MCM Al with respect to 2*1000 mcm= 6.8%.
 
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