Sizing C/T Cabinet Meter Conductor

[electricalintern]

I'm uncertain how they determined the conductor size of 4#250 kcmil with a #2 ground for the C/T cabinet meter, given that the 150-foot run uses 4 x 1 AWG with a #6 ground. [400AS/125AF/3P at 480V]

Another location had the conductor size for 4#600 kcmil with a #350 kcmil ground for the C/T cabinet meter [400AS/400AF/3P at 480V]
The feed to the store was 4#600cmil,#2 ground.

Here’s an example of the incoming service from the electrical room for reference. I'm not sure why the conductor sizes differ between the C/T cabinet meter and the feeder on the landlord's drawings.

 

[don_resqcapt19]

Is this all outside or is some of it inside? The EGC for the tap is sized based on the upstream 2500 amp OCPD, and would normally have to be 350kcmil, but since the circuit conductors are 250 kcmil the EGC is not required to be larger than that.

 

[electricalintern]

Is this all outside or is some of it inside? The EGC for the tap is sized based on the upstream 2500 amp OCPD, and would normally have to be 350kcmil, but since the circuit conductors are 250 kcmil the EGC is not required to be larger than that.

It's inside a electrical closet located in a parking garage.

Not sure why they sized it at 250kcmil if the store is only feeding 4#1.

So why was 350kcmil used for the second store when the feed to the store was 4#600cmil,#2 ground.
Should the feed to the store ground be 1/0 (not #2 ground) based on table 250.66

I get why you matched the 350kcmil now but why was there upsize for the 250kcmil and the ground not matching the 350kcmil now

 

[infinity]

Not sure why they sized it at 250kcmil if the store is only feeding 4#1.

Did you check the 10' tap rule?

 

[electricalintern]

Did you check the 10' tap rule?

I read Section 240.21(B)(1), but I don't understand how it effects it the grounding and feeder sizes.

 

[don_resqcapt19]

I read Section 240.21(B)(1), but I don't understand how it effects it the grounding and feeder sizes.

The EGC for a feeder tap is sized based on the upstream OCPD...in this case 2500 amps and that requires a 350 kcmil copper EGC.
A 10' tap conductor must have an ampacity of not less that 10% of the rating of the upstream OCPD, so in this case at least 250 amps. 250 kcmil copper is the smallest that meets that requirement.
If the tap conductor is over 10' then is must have an ampacity of 1/3 of the upstream OCPD, or 833 amps.
Note that the 10' and 25' rules are based on the actual end to end lengths of the conductors, and not the distance between the two enclosures.

 

[don_resqcapt19]

It's inside a electrical closet located in a parking garage.

Not sure why they sized it at 250kcmil if the store is only feeding 4#1.

So why was 350kcmil used for the second store when the feed to the store was 4#600cmil,#2 ground.
Should the feed to the store ground be 1/0 (not #2 ground) based on table 250.66

I get why you matched the 350kcmil now but why was there upsize for the 250kcmil and the ground not matching the 350kcmil now

These are equipment grounding conductors sized from Table 250.122, not grounding electrode conductors sized from Table 250.66.

 

[electricalintern]

These are equipment grounding conductors sized from Table 250.122, not grounding electrode conductors sized from Table 250.66.

Store #1 [400AS/125AF/3P at 480V]
10' Tap Rule: 4#250 kcmil with a #2 G
Based on Table 250.122, wouldn't a load of 255 AMP require a #4G
How do you get the #2 G based on Table 250.122

 

[don_resqcapt19]

Store #1 [400AS/125AF/3P at 480V]
10' Tap Rule: 4#250 kcmil with a #2 G
Based on Table 250.122, wouldn't a load of 255 AMP require a #4G
How do you get the #2 G based on Table 250.122

The EGC for a tap is based on the upstream OCPD, so you look at the size required for a 2500 amp OCPD. You need a 350 kcmil EGC to the 400 amp disconnect, but since that is larger than the circuit conductors, you can use a 250 kcmil EGC.

 

[don_resqcapt19]

Also you are going to need a very large power distribution block to make the tap. The block will have to fit 14 500 kcmil conductors plus the 4 250 kcmil tap conductors. Taps from a paralleled conductors must connect to all of the paralleled conductors for each phase and neutral.

 

[electricalintern]

The EGC for a tap is based on the upstream OCPD, so you look at the size required for a 2500 amp OCPD. You need a 350 kcmil EGC to the 400 amp disconnect, but since that is larger than the circuit conductors, you can use a 250 kcmil EGC.

The diagram shows a #2 EGC and NOT a 250kcmil EGC

 

[infinity]

The diagram shows a #2 EGC and NOT a 250kcmil EGC

Just because it shows the #2 doesn't mean that it's correct. Don gave you the correct sized based on the tap conductors.

 

[don_resqcapt19]

The diagram shows a #2 EGC and NOT a 250kcmil EGC

It doesn't matter that the engineer or designer does not understand the requirements of the National Electrical Code.

That is a code violation and I would hang a nice big red tag on it if I was inspecting. Hopefully that would be caught in the plan review so extensive rework is not required if it was installed based on those plans.

Would also be interested in how you are actually going to tap the four 250 kcmil conductors to the twenty eight 500 kcmil conductors?

 

[infinity]

Would also be interested in how you are actually going to tap the four 250 kcmil conductors to the twenty eight 500 kcmil conductors?

The drawing post #11 says "ex bussed retail through" (sic).