I have an existing service that will be upgraded by the local utility. The existing service is a 400 amp 600 volt 3 phase 3 wire delta system. It is fed from a pad mount transformer that is leaking oil and needs to be replaced. The current standards here are 600/347 volt 3 phase 4 wire. They are going to install a new pole with three transformers and extend my secondary wiring to the top of the pole and install the 4th (neutral) conductor. They are installing 300 KVA and have indicated that the fault currents on the new system will be slightly higher than the existing system. The new fault current will be around 22k amps at the transformer bushings. The main fusible switch has fuses rated at 300k AIR. These fuses don't concern me. My question is how far down the line do I have to be concerned with the available fault current? The distribution for the building comes from molded case circuit breakers. These feed things like lighting transformers and such. There is also an MCC for the compressor equipment but it has all HRC fusing internally. Do I simply need to check the breakers in the distribution panel to make sure there ratings are higher than the available fault current or do I need to look further than that?
fault currents
- Thread starter jinglis
- Start date
- Status
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
You need to calculate the fault current throughout you system and compare it to the ratings of the equipment.
- Location
- Illinois
- Occupation
- retired electrician
You can go to the Bussmann site and use an online calculator to find the available fault current at the downstream points.
http://www.bussmann.com/apen/software/
Don
http://www.bussmann.com/apen/software/
Don
ramsy
NoFixNoPay Electric
- Location
- LA basin, CA
- Occupation
- Service Electrician 2020 NEC
1) For liability insurance & local codes, doesn't valid SCA's require expert-engineering consultants?
Wouldn't it be more cost effective to hire an EC to document the lowest Z's logged from a load test from within building feeder or panel? Tot.SCA = Vline / [LoggedZ * Sqrt(?)]
2) I'm showing this 3? 300kVA, maxing out at 289A L-to-L. I = 300kva / 600 / 1.732
Check me here, but if accommodated by a #4cu cable impedance, the SCA is under 10kA after 62 feet. Using an existing 400A service conductor, #250cu would raise that SCA to 18.4kA at 62 feet
Wouldn't it be more cost effective to hire an EC to document the lowest Z's logged from a load test from within building feeder or panel? Tot.SCA = Vline / [LoggedZ * Sqrt(?)]
2) I'm showing this 3? 300kVA, maxing out at 289A L-to-L. I = 300kva / 600 / 1.732
Check me here, but if accommodated by a #4cu cable impedance, the SCA is under 10kA after 62 feet. Using an existing 400A service conductor, #250cu would raise that SCA to 18.4kA at 62 feet
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Since this is an existing service, I am comfortable with just doing my homework. My fear is if I don't do my homework and the inspector does find something wrong I am stuck with a large bill to correct this. If that is the case so be it but I would like to budget for these changes before the project starts. The engineers from the utility show the fault current available at the transformer at 22.2KA. Using the calculator from the reply above I get 18.3KA at the main disconnect. From there I will check the rest of the service to ensure all my O/C devices are suitable. I will also call my inspector for his opinion on my findings.
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