Parallel Transformers

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Joined: 2016-05-06

I could use some help please.

I am looking at a facility with 3 parallel MV transformers. The electric utility is providing 26.4kV with a fault current of 31.5kA. The building's 480V switchgear is rated at 100kAIC. When questioned about the available short circuit rating the "utility" will only tell us it is 107kA at the line terminals of the switchgear.

The building is over 30 years old with original equipment and we are trying to determine if the "utility" calculation is accurate or just an estimate so we can determine what the most feasible course of action is to properly protect the electrical equipment.

Here is the information I have obtained on the system so far:

• Primary voltage: 26.4kV
• Primary fault current: 31.5 kA
• Secondary voltage: 480V
• Secondary fault current: 107kA  (as per utility)
• Five conductors per phase output

The utility has stated that trade off for secondary side conductor type/length/raceway type/bus bar is not acceptable.

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Joined: 2013-11-07

Depending on the type of utility service, the fault current stated may only be an estimate. If you are being supplied with a primary service and the distribution transformers are owned by the customer, the utility may not have the full information on the transformers, such as the impedance and current limiting protection, which is critical in determining the fault current.

If the utility is feeding your service through a looped radial system, the fault current can vary depending on the distance to the source and the other load on the feed circuit. As a person who works for a utility, I would assume the fault current to be a worst-case scenario and plan likewise.

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Joined: 2016-05-06

@eddiehill
Thank you for the input. The transformers in question belong to the utility. Only one of the three still has an information plate on it. I am trying to get more information from that plate.

Primary voltage: 26,400V
Primary side fault current: 31,500A
kVA: 1,500
Impedance: 6.2%
Secondary voltage: 480V
Five parallel copper conductors
Length of secondary feeders and raceway material unknown
Service equipment kAIC = 100
Load side bus gear kAIC = 85

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