×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Standard for utility avaliable fault currents as service entrance
2

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?

RE: Standard for utility avaliable fault currents as service entrance

No.  Utilities generally try to limit single family residential fault currents to some amount, such as 10kA for 200A and less.  But for larger services the fault current is just what ever it is.

RE: Standard for utility avaliable fault currents as service entrance

(OP)
Thanks davidbeach.

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

Those values are fairly typical for commonly used substation transformers.  Bigger transformers, or paralleling the low side, can result in substantially higher fault currents.  Most of our system we quote 20kA at 12.47kV for equipment ratings, allowing us to parallel transformers during switching.  We have one network at that voltage level with four larger than usual transformers paralleled and the fault currents are in the 40kA range.

RE: Standard for utility avaliable fault currents as service entrance

Most US utilities will limit MV distribution fault currents to 10 kA because that is the limit for standard fused cutouts and loadbreak elbows.

RE: Standard for utility avaliable fault currents as service entrance

(OP)

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

Our most common distribution (substation) transformer is a 16.8MVA unit with a 28MVA top rating.  But we reserve the right to replace it with a bigger transformer, or build a new substation closer to the customer's site, and we routinely parallel them for short periods of time during switching, so out primary customers get told 20kA for equipment ratings unless they're really big and we know we'll go beyond that.  Once we know you've got the right equipment ratings, we'll talk about real fault currents for coordination studies and arc flash studies.  But those fault numbers are only for one system configuration, today, and may slightly or significantly different tomorrow.

RE: Standard for utility avaliable fault currents as service entrance

(OP)
Davidbeach

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

Less than 1kA; maybe but not often.  But, then again, I don't really do any work with the distribution feeders.  Strange things can happen to feeders when circuits need to be rerouted to get around a problem.

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources