Standard for utility avaliable fault currents as service entrance
Standard for utility avaliable fault currents as service entrance
(OP)
I was wondering if there is a standard in the U.S. that governs the maximum avaliable fault current that the utility can supply for a given utility service entrance.
For many of the utility fault letters I recieve for studies providing such utility fault currents I tyically see numbers between 30kA - 40kA at 480V and between 5kA and 8kA at higher distribution voltages such as 12.47kV and 13.2kV. The 480V values are usually at the secondary of the service entrance transformer if it is utility owned and the distribution votlages are usually at the primary of the transformer if customer owned transformer or at the incoming voltage if facilty distribution voltage is at same voltage as utiltiy distribution voltage.
Is there any standard that governs these values are there typical guidelines?
For many of the utility fault letters I recieve for studies providing such utility fault currents I tyically see numbers between 30kA - 40kA at 480V and between 5kA and 8kA at higher distribution voltages such as 12.47kV and 13.2kV. The 480V values are usually at the secondary of the service entrance transformer if it is utility owned and the distribution votlages are usually at the primary of the transformer if customer owned transformer or at the incoming voltage if facilty distribution voltage is at same voltage as utiltiy distribution voltage.
Is there any standard that governs these values are there typical guidelines?






RE: Standard for utility avaliable fault currents as service entrance
RE: Standard for utility avaliable fault currents as service entrance
I guess maybe the values I am used to seeing at 480V a primarily determined by the transformer let-thru fault current when looking at transofmers in the 1500kVA-2000kVA range. Like I said most values I recieve for 480V are between 30kA-40kA but I guess this is primarily determined by the transofmer let-through.
So is it a coinicdene that I usually see fault values betwen 5-8kA for MV service entracnes at 12.47 and 13.8kV? Are there cases where fault current at this level can be 30kA or so at these voltages? Or is it just the typical nature of the utilites transmission and distribution systems that limit fault current at these voltages to these values?
RE: Standard for utility avaliable fault currents as service entrance
RE: Standard for utility avaliable fault currents as service entrance
RE: Standard for utility avaliable fault currents as service entrance
Davidbeach
Are most of the commonly used subtation transofmers somewhere in the 10-15MVA range (estimated based of 10ka max secondary)? I'm curious what the typical range of commonly used substation distribution transfoemrs are?
I'm also curious what the typical impedances of these commonly used substation distribution transofemrs are? Are they somwhere well below 5%?
RE: Standard for utility avaliable fault currents as service entrance
RE: Standard for utility avaliable fault currents as service entrance
Thanks for providing this insight. You answered my next question related to more actual currents for use with Arc Flash analysis. I'm curious what are some of the lowest fault currents you have seen at customer service entrances? I was curious realistically how low some of these fault currents could go for locations remote from the substation? Would you ever expect to see values below 1000A for locations far from the sub?
When you discuss the switching and paralleling of transformer is this typically 2 units in the same substation or can they be two units in remote substations?
I'm curious what the typical impedances and X/R ratios of these transfomers tend to be.
RE: Standard for utility avaliable fault currents as service entrance
Paralleling can be two in a station or between two stations, just depends on the nature of the switching.
Impedance generally in the 8.5% ish region and X/R generally in the 20 ish region. Lots of variation over the years. A 50+ year old transformer and a new one won't be nearly as close as two new units.