Burial of multiple UF cables in the same trench

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MikeFromBowie

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Caifon, NJ
I have an application where I want to run 5 microinverter branch circuits (approx. 17 amp peak 240 VAC each) from a ground solar array to a house. These circuits will be terminated in a subpanel with 20 A breakers and then fed (via a disconnect switch) to a 100 A breaker in the main. I am familiar with trench depth requirements (18 inch in conduit, 24 inch without) and with rules concerning no dc or control circuits in the same conduit. However, my questions are these: (1) can I place 5 cables UF 6-3 WG together in a 24 inch trench? (2) Can I put my 5 cables in a single PVC conduit (and if so, what size conduit)? (3) For either of these cases, is any derating required because of the multiple cables? (4) Would it be feasible and more economical to run a single much heavier subpanel feed from the main 100 A breaker to an outdoor subpanel at the solar array end?
 
Julius Right

Julius Right

Senior Member
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Electrical Engineer Power Station Physical Design Retired
According to NEC you may use Art.340.80 Ampacity. “The ampacity of Type UF cable shall be that of 60°C (140°F) conductors in accordance with 310.15.”
Table 310.15(B)(16) UF 3*6 AWG 55 A.
From Table 310.15(B)(3)(a) for 10-20 loaded conductors a 50% derating it is recommended.
The cable cross section is 1.223”*0.361”= 0.441503 sqr.inches.
5 cables total cross section 5*0.441503/0.25=8.83.:ashamed1:
In my opinion for non-round cross section cable only 25% of the conduit inner
section could be used. Then 8.83 sqr.inches rigid pvc conduit it is necessary.
4” will be good.
 
Dennis Alwon

Dennis Alwon

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Chapel Hill, NC
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Retired Electrical Contractor
MikeFromBowie said:
I have an application where I want to run 5 microinverter branch circuits (approx. 17 amp peak 240 VAC each) from a ground solar array to a house. These circuits will be terminated in a subpanel with 20 A breakers and then fed (via a disconnect switch) to a 100 A breaker in the main. I am familiar with trench depth requirements (18 inch in conduit, 24 inch without) and with rules concerning no dc or control circuits in the same conduit. However, my questions are these: (1) can I place 5 cables UF 6-3 WG together in a 24 inch trench? (2) Can I put my 5 cables in a single PVC conduit (and if so, what size conduit)? (3) For either of these cases, is any derating required because of the multiple cables? (4) Would it be feasible and more economical to run a single much heavier subpanel feed from the main 100 A breaker to an outdoor subpanel at the solar array end?
Click to expand...


How does this comply with 225.30

225.30 Number of Supplies. A building or other structure
that is served by a branch circuit or feeder on the load side
of a service disconnecting means shall be supplied by only
one feeder or branch circuit unless permitted in 225.30(A)
through (E). For the purpose of this section, a multiwire
branch circuit shall be considered a single circuit.
Where a branch circuit or feeder originates in these additional
buildings or other structures, only one feeder or
branch circuit shall be permitted to supply power back to
the original building or structure, unless permitted in
225.30(A) through (E).
(A) Special Conditions. Additional feeders or branch circuits
shall be permitted to supply the following:
(1) Fire pumps
(2) Emergency systems
(3) Legally required standby systems
(4) Optional standby systems
(5) Parallel power production systems
(6) Systems designed for connection to multiple sources of
supply for the purpose of enhanced reliability
Click to expand...
 
augie47

augie47

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Tennessee
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State Electrical Inspector (Retired)
Smart $ said:
IMO, no derating required for DB in trench; derating required if run in conduit.
Click to expand...

Certainly debatable:
[FONT=&quot] 310.15(B) Adjustment Factors.[/FONT]

[FONT=&quot](a)[/FONT]
[FONT=&quot]More Than Three Current-Carrying Conductors in a Raceway or Cable.[/FONT]
[FONT=&quot]Where the number of current-carrying conductors in a raceway or cable exceeds three, or where[/FONT]
[FONT=&quot]single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.) and are not installed in raceways,[/FONT][FONT=&quot] the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a) Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conduc tor.[/FONT]
 
kwired

kwired

Electron manager
Location
NE Nebraska
augie47 said:
I agree.. I was simply questioning Post #5 as it inferred to me that derating was not ever considered in direct burial.
Click to expand...
Often it isn't, but should be. Also ambient temp underground is low enough to make a raise in ampacity from adjustments - which does even things out if you don't place too many conductors in close proximity in the trench.
 
ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
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EC - retired
kwired said:
Often it isn't, but should be. Also ambient temp underground is low enough to make a raise in ampacity from adjustments - which does even things out if you don't place too many conductors in close proximity in the trench.
Click to expand...
Was that the calculation that could be used under/with engineering supervision, at one time? Made my eyes cross just looking at it.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
augie47 said:
I agree.. I was simply questioning Post #5 as it inferred to me that derating was not ever considered in direct burial.
Click to expand...
FWIW, my reply was specific to OP's (3) which even states "For either of these cases..."

The section you pointed out essentially means there would have to be a section of bundled cables... and I don't know anyone in their right mind that would bundle in a DB trench.
 
Julius Right

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
Burial 3*6 UF-B 5 cables in 24" wide filled trench

Burial 3*6 UF-B 5 cables in 24" wide filled trench

I agree with Smart $. However, in my opinion, if the cables are laid at maximum clearance of 4.47”–in 24” wide trench filled with 250 RHO earth at 30oC-the maximum current –for maximum 60oC copper conductor will be 40-45 A.
So, in my opinion, will be no problem at 20 A.
I forgot to mention the cable [single] row depth 2.5 ft.
 
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