Using software makes it impossible to show your calculations. So that’s not really a solution. NEC Committee is on track with making SCCR a requirement. The issue is even most engineers don’t know how to do the calculation. When I worked at a huge mine (200 MVA main transformers) it would drive him crazy that I could do arc flash and short circuit calcs faster than he could even boot up the $10,000 software program. I explained I could go faster but I was too cheap to buy a slide rule and just used a calculator. I was always within 10% doing this magic trick.
As suggested as a first step talk to the utility. But there are two sides to this. The first is that most of the time they give you some ridiculous overinflated number that does not reflect anything realistic. All the engineering web sites are full of “my utility gave me an unrealistic number” comments. The fact is that at their end it’s not an easy problem to determine because on any given day they switch things around and change out components and the fact that SCCR changes from one pole to the next. So they do a lot of “worst case” analysis that is often very wrong. All you are after is the transformer size, %Z, and maybe the primary fuse.
As mentioned you can do an infinite bus calculation on a transformer that gives you the SCCR assuming that there is infinite current on the primary side and no major motor loads (over 50 HP). For single phase it’s very easy.
KVA / voltage / (%Z / 100) = kA SCCR
If it’s 3 phase just divide by 1.732.
You can do a little more with the fuse curves but let’s move on.
From there the service drop also drops the SCCR a lot. The calculation is easy to do and does not require a $10,000 computer program or a $500 IEEE book and an engineering degree to understand it. So here is the big secret hidden in plain sight. If you buy the IEEE standard 1584 book for $650 (with your $300 membership!) it comes with a spreadsheet that has an SCCR calculation in it. The calculation refers to the Bussmann point by point method. So what is that and does it require a $650 book, $300 membership fee, $100,000 college degree and a $10,000 computer program? Nope. It’s free. All you need is a calculator and something to write down your calculations.
www1.cooperbussmann.com
Pay attention to the limitations! If you have large motors and transformers nearby they can back feed a lot of current into the fault as their magnetic fields collapse. We use a rule of thumb to ignore anything under 50 HP. Some say 125 HP or less. Either way this more than covers residential situations.
If you want to make the series protection stuff easy get the free Cooper (Bussmann) Selecting Protective Devices Handbook. It has charts of GE, SqD, Siemens, and others series protection all in one place in a few pages of charts.
If you do have to deal with large motors and transformers this is what the ANSI method is all about. That method was invented back when we did the calculations on slide rules. It still works and it is “the” method in North America. It adds a few extra complications to the Bussmann point by point method but it’s not much worse.
Keep in mind that right now SCCR is used for three reasons. First is equipment sizing. As long as we are not undersized we don’t care too much. A high number is OK as long as it isn’t excessive. Taking the earlier statement if infinite bus gets you to 11 kA you might need 22 kA AIC. If the Bussman method gets you under 10 kA you are good to go. We don’t need to use the IEC comprehensive method and a computer program to calculate 7 kA as the most accurate. Second use is for coordination curves. If I know the SCCR I can ignore everything to the right of that point and I can dial in my instantaneous settings to trip instantaneously on short circuits. I can usually set this generously below that point. Third use is arc flash calculations. But if we overestimate short circuit it makes the arc flash calculation go lower. So we can be under protected. This is where accuracy is critical for SCCR and why engineers recently have dropped ANSI and other very simple methods for the more accurate mostly computer driven methods.
But that doesn’t mean a guy working on houses needs to use the IEC comprehensive method. Just that if I already have the software and training it’s easy.
