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#80797 05/22/02 01:25 PM
Joined: Dec 2001
Posts: 201
ayrton Offline OP
I am doing work at a building with two 480v services fed from a padmount xfmr.
One set of conductors runs from the xfmr to an end box with a ct cabinet on either side.
The electrician ran 1000 kcmil wire from the padmount to the box to bug to the service conductors. First service is protected by a 600a main breaker. The electrician ran parallel 500 mcm (which he bugged to the 1000 kcmil) into the first ct cabinet which odly enough has 200a ct's.
The second service is protected by a 400a main breaker which also has a ct cabinet with 200a ct's and one set of 500's.
Now this to me is 1000 amps total which is capable (not probable) of being pulled threw those 1000 kcmil conductors! 1000 kcmil is good according to code for a little over 600a. How did this pass inspection and why would the power co energize this?

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#80798 05/23/02 06:04 AM
Joined: Mar 2001
Posts: 2,056
Are you sure the PoCo. doesn't own the xfmf. & Service Lateral? They are not obligated to follow the NEC. Our Poco. regularly runs undersized conductors to the service point.

#80799 05/23/02 09:20 AM
Joined: Dec 2001
Posts: 201
ayrton Offline OP
The PoCo owns the xfmr but not the conductors. The owner is responcible for pulling and providing conductors. The CT's are only 200a CT's which means the loads on each service will be no more than 200a.
But 1000 kcmil is still only good for 615a
per N.E.C.

#80800 05/23/02 10:43 AM
Joined: Nov 2001
Posts: 11
The AHJ should cite 215.5 and the response will determine the compliance with 230.90.ex3 of the NEC.

the load calculation for the service should determine the ampacity of the service conductors (230.90.ex3)if a single device the conductor ampacity MUST excede the "main" if two to six disconects the ampacity MUST excede the calculated load.

#80801 05/23/02 11:16 AM
Joined: May 2002
Posts: 30
The CTs use a rating factor, which should be stated on the CT. The Current transformer is probably marked 200/5. The rating factor is used to determine how much current the CT can safely carry without exceeding the stated temperature rise. Most new 200/5 cts have a rating factor of 3.5 or 4, meaning they can carry up to 700 or 800amps.

#80802 05/24/02 11:45 PM
Joined: Oct 2000
Posts: 2,722
Broom Pusher and
As stated, the Load Calcs are what determined the Service Feeders' size and also what size of Transformer the PoCo installed.

The Main OCPD DOES NOT protect the Service Feeders, nor does it protect the Transformer. It only protects the Main Service's Bus Kit, and doubles as the Service Disconnect.

Most of the time, the Service Feeders are protected from being Overloaded by limiting the max KVA size of the Transformer.

I just finished up a Bank Branch T.I. where there are Two 1000 Amp services being fed from one 75 KVA [Z=3.0%] 4 wire Wye Transformer [120/208 VAC]. The Service Feeders were 500 MCM al and only one set per service [not paralleled conductors].
Even if the loads tried to reach 1000 amps, the max KVA of the Transformer would limit things by an abrupt Voltage drop when the Transformer became "Saturated".

If the case that the Tenant[s] needed to use 1000 amps continuously, by proper design the Transformer and feeders would need to be upgraded.

My $.02

Scott S.E.T.

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
#80803 05/28/02 07:44 PM
Joined: Dec 2001
Posts: 201
ayrton Offline OP
Your answers were very informant and helpful.
Thank you!

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