under ground feeder sizing

[Toros]

Hi
I designed a feeder-sets; (3) 3" pvc, (4) #500 MCM,cu in each based on ambient temp. , derating...
for a client who has 1200A service @120/208V, 3 ph.
And the local buildg & Safety approved the plan.!

Now, The inspector said " Change wires, they are down sized,
minu. 600 MCM required".

Any Comments, thoughts....????????

Thank you
Edward/ Toros

 

[curt swartz]

What is your calculated ampacity for this feeder and how did you come up with it?

 

[augie47]

If it's a 1200 main you will probably have a job on your hands getting it passed. I've seen it attempted a few times but don't recall a situation where the AHJ has been sold on a low enough ambient or a short enough distance to satisfy 310.,15(A)(2) exception and 110.14 with the 75° device always ends up requiring something other than the (3) 500s

 

[petersonra]

I think the inspector has a point unless you can get a new CB that protects the conductors at 1160 A.

 

[jumper]

I think the inspector has a point unless you can get a new CB that protects the conductors at 1160 A.

I agree.

 

[ron]

If you used TABLE B.310.15(B)(2)(7) Ampacities of Three Single Insulated Conductors, Rated 0 Through 2000 Volts, in Underground Electrical Ducts (Three Conductors per Electrical Duct) Based on Ambient Earth
Temperature of 20C (68F), Electrical Duct Arrangement in Accordance with Figure B.310.15(B)(2)(2),
Conductor Temperature 75C (167F)

and the column for 3 Electrical Ducts (Fig. B.310.15(B)(2)(2), Detail 2)

and you had dirt with a rho of 90 (typical value used),

then the 500's would be good for 341A per set. or 1023A. So you would end up with a 1000A service disconnect max.

Unless you calculated the mutual heating of the (3) ducts in another way ...... such as Neher McGrath

 

[310 BLAZE IT]

He's right because 240.4(B) does not apply its over 800a. Just get a plug or trip unit on the breaker..

Or use Annex B like described above

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[Toros]

Hi
I designed a feeder-sets; (3) 3" pvc, (4) #500 MCM,cu in each based on ambient temp. , derating...
for a client who has 1200A service @120/208V, 3 ph.
And the local buildg & Safety approved the plan.!

Now, The inspector said " Change wires, they are down sized,
minu. 600 MCM required".

Any Comments, thoughts....????????

Thank you
Edward/ Toros

Can a trip unit be installed on the existing 1200A main circuit breaker supplied by (3) 500 MCM
and adjact the trip for 1150A ?????
Thank you

 

[infinity]

Since these are in the ground can you adjust the ampacity using the ambient temperature factors in Table 310.15(B)(2)(a)?

 

[ron]

Since these are in the ground can you adjust the ampacity using the ambient temperature factors in Table 310.15(B)(2)(a)?

No because there is mutual heating from one duct to the other since the dirt acts like a blanket.

 

[pcanning87]

Can a trip unit be installed on the existing 1200A main circuit breaker supplied by (3) 500 MCM
and adjact the trip for 1150A ?????
Thank you

Short answer is it depends on the breaker.

My experience with newer 1200A+ breakers is they are usually LSI(G) programmable.


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[Toros]

Hi
I designed a feeder-sets; (3) 3" pvc, (4) #500 MCM,cu in each based on ambient temp. , derating...
for a client who has 1200A service @120/208V, 3 ph.
And the local buildg & Safety approved the plan.!

Now, The inspector said " Change wires, they are down sized,
minu. 600 MCM required".

Any Comments, thoughts....????????

Thank you
Edward/ Toros

Sorry Ron,
I gave you wrong info,
the supply voltage is 480/277V / 600V , not 120/208v
now, can I programm the 1200A GFCI type main breaker for trip-@ 1150A ??????

 

[kwired]

Sorry Ron,
I gave you wrong info,
the supply voltage is 480/277V / 600V , not 120/208v
now, can I programm the 1200A GFCI type main breaker for trip-@ 1150A ??????

Still depends on the breaker and what it is capable of.

Ground fault setting is not the same as the overload setting (which may be fixed), it is the level of ground fault current it will allow before it trips.

 

[infinity]

If you used TABLE B.310.15(B)(2)(7) Ampacities of Three Single Insulated Conductors, Rated 0 Through 2000 Volts, in Underground Electrical Ducts (Three Conductors per Electrical Duct) Based on Ambient Earth
Temperature of 20C (68F), Electrical Duct Arrangement in Accordance with Figure B.310.15(B)(2)(2),
Conductor Temperature 75C (167F)

and the column for 3 Electrical Ducts (Fig. B.310.15(B)(2)(2), Detail 2)

and you had dirt with a rho of 90 (typical value used),

then the 500's would be good for 341A per set. or 1023A. So you would end up with a 1000A service disconnect max.

Unless you calculated the mutual heating of the (3) ducts in another way ...... such as Neher McGrath

So the suggestion to use an adjustable trip to get down to 1160 amps (not sure where 1160 came from) is incorrect also? What about the inspector's suggestion of using 600 kcmil?

 

[ron]

So the suggestion to use an adjustable trip to get down to 1160 amps (not sure where 1160 came from) is incorrect also? What about the inspector's suggestion of using 600 kcmil?

He would have to look at that table in the Appendix to see what 600's are good for in that configuration.

Most adjustable trip units have LPTU adjustments in increments of 5% (of course there are odd ones out there), so instead of 1200A, if it is an adjustable long time pick up, then maybe 1140 is available, but still not good if he was using 500's and the conditions put forth in my earlier example which would be 1020A; using a LTPU of 0.85 if it where available would work. Hopefully the service load calc isn't too close to 1200A:happysad:

 

[infinity]

He would have to look at that table in the Appendix to see what 600's are good for in that configuration.

Most adjustable trip units have LPTU adjustments in increments of 5% (of course there are odd ones out there), so instead of 1200A, if it is an adjustable long time pick up, then maybe 1140 is available, but still not good if he was using 500's and the conditions put forth in my earlier example which would be 1020A; using a LTPU of 0.85 if it where available would work. Hopefully the service load calc isn't too close to 1200A:happysad:

If 500 kcmil only gets you to 1020 amps then I'm not seeing how 600 kcmil will get you there either. Sounds like his 1200 CB is non-adjustable.

Can a trip unit be installed on the existing 1200A main circuit breaker supplied by (3) 500 MCM
and adjact the trip for 1150A ?????

If it's a standard 1200 amp breaker than probably not. You would need to change the CB or re-pull the conductors.

 

[Julius Right]

At first I am sorry for the delay.
ron said:
“Unless you calculated the mutual heating of the (3) ducts in another way ...... such as Neher McGrath”
Ampacity of 341 A for Detail 2 -3 conduit 3 single core cables per conduit[pvc] 4"
-as it seems to me-was calculated for 90 oC.cm/W Earth and 55 for concrete[ 20oC ambient].
If the distance between conduits it could be 250 mm[10"] instead of 190.5[7.5"] then the ampacity could be 405 A[For 90oC conductor temperature].
By the way, Neher recommends 85 for concrete.
In order to comply with art.110.14(C)(b)[75oC] a cable tray according to 392.80(2)(c) or (d) could be employed -in order to reduce the temperature below 75oC.