Direct buried 600 kcmil parallel run

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Id as USA

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Hey everyone!

Getting ready to order material for is 140kw solar project.

Do my parallel runs need to maintain spacing in the trench if I’m using USE-2 direct buried cables? Spacing is required to dissipate heat, since this is buried are we concerned about heat?

Our trencher cuts a 4” trench and we are looking at a 500’ distance. Trying to determine if I need 2 trenches separated by a few inches or can I run both sets in the trench?

Thanks for your help
 
Yes, you need to be concerned about heat in underground installations as the earth acts as an insulator. You need to use two trenches and depending on the thermal resistivity of the soil, you may need larger conductors even with two trenches. There are some tables in Annex B that give some guidance, but all are based on a minimum 7" center to center spacing of the underground raceways.
 
Thank you for the responses….

Are you aware of a table with details for parallel run spacing?

Looks like annex B gives spacing of 7.5” for single conductor and 24” for 2 circuits.

Still unclear for spacing for direct buried parallel runs.

Either way… looks like I’m renting a mini!
 
It is about 480 V three phase system of about 170 A [140 kW?]
 
Actually started out that way, but the engineers had the voltage wrong… the service is a 208v high leg, so now we’re pushing 500 amp.
 
I don't think you have problems with cable ampacity, but you have 5.6% voltage drop [if the load it is 140 kVA [673 A ] and 0.85 power factor.
See IEEE 835/1994 page 20
[Tables from NEC art.310.60 C are calculated according to IEEE 835/1994.]
0.6 to 5 kV Unshielded Single Conductor Extruded Dielectric Cable Direct Buried - Triplexed - Double Circuit
25ºC Earth Ambient
90ºC - Copper Conductor - Concentric Strand
90 Rho 100% LF
600 kcmil
Iadmiss=523 A
However, it is for 6 cables and you have only 4. I think you may multiply by 6/4=1.5
You have to keep 7.5" distance [in height]
 
LarryFine said:
Welcome to the forum.

If you start deep enough, I would think vertical spacing would work.
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But because the thermal characteristics are (slightly) different at different depths, you would technically not have your parallel sets installed in identical configurations. I would not be able to argue successfully with an inspector who decided not to pass it.
 
GoldDigger said:
But because the thermal characteristics are (slightly) different at different depths, you would technically not have your parallel sets installed in identical configurations. I would not be able to argue successfully with an inspector who decided not to pass it.
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While the code does not address the ampacities of circuits 2000 volts or less as to the change the burial depth makes, the tables for the higher voltages do. The underground details specify the maximum burial depth for a duct bank to be 30" and 36" for directly buried cables. It goes on to say that where the burial depth is greater, you reduce the ampacity by 6% for each additional foot of depth.
Not sure why this does not apply to lower voltages as the physics would be the same and often the current flow in the lower voltage circuits is greater than that in the high voltage circuits creating more heat.
 
GoldDigger said:
But because the thermal characteristics are (slightly) different at different depths, you would technically not have your parallel sets installed in identical configurations.
Click to expand...
By that logic, two indoor parallel conduit runs of the same length, one of which goes through thermal insulation, and one of which doesn't, would also be a violation. Seems like a stretch to me, although technically correct.

To the extent that one parallel conductor will have higher resistance due to higher operating temperature due to worse heat rejection, the feedback is negative--it will carry slightly less current and so the temperature disparity will decrease (compared to the equal current case).

Cheers, Wayne
 
wwhitney said:
By that logic, two indoor parallel conduit runs of the same length, one of which goes through thermal insulation, and one of which doesn't, would also be a violation. Seems like a stretch to me, although technically correct.

To the extent that one parallel conductor will have higher resistance due to higher operating temperature due to worse heat rejection, the feedback is negative--it will carry slightly less current and so the temperature disparity will decrease (compared to the equal current case).

Cheers, Wayne
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I agree. The OP is about a 500ft run at that length I'd be worried less than 50ft becuse the long distance will allow for more evening out of the resistance and will have more room for error without significantly changing the resistance of each cable vs the other.
 
I'm comparing digging one slightly deeper 500' trench to digging two slightly shallower 500' trenches.
 
Id as USA said:
Actually started out that way, but the engineers had the voltage wrong… the service is a 208v high leg, so now we’re pushing 500 amp.
Click to expand...
I know different regions have their own slang for these services, but the NEC suggests using ANSI terminology. They show this as a 240/120V delta. Personally, I like to add the suffix 3PH 4W.
 
The ampacity is o.k. in my opinion.
First, since the overall cable diameter is about 1.13" then the centre-line to centre line
of twins it is 4-2.26=1.74". Let’s take it 1.5”
If we take 30 fts depth for the upper twin [2*600 kcmils copper ] and 37.5 the deeper
twin and 150 Rho [not 90, since the nativ soil is drying due to heat dissipated from cables [see IEC 60287-2-1] the calculation result it is 535 A ampacity for the lower row cables.
 
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