# Calculating Arc Flash by Hand

#### walkrand

##### New member
I would like to learn how to do arc flash calculations by hand after using a program to do it. Looking at Equation D.4.2(a) in NFPA 70E 2018 how is the bolted three phase available short-circuit current supposed to be found?

Thanks

#### steve66

##### Senior Member
Download the Cooper Busman SPD. It should still have the calculations and examples for "Short Circuit Current Calculations".

http://electricalsector.eaton.com/bussmann-2018-SPD

The most recent SPD I have is based on the 2014 NEC, and the calculations are on pages 236-242.

The lastest SPD (based on the 2017 NEC) may be slightly different, but I trust the calcs are still in there. They have been for the last 20 years.

#### wbdvt

##### Senior Member
I would like to learn how to do arc flash calculations by hand after using a program to do it. Looking at Equation D.4.2(a) in NFPA 70E 2018 how is the bolted three phase available short-circuit current supposed to be found?

Thanks
In order to calculate the available bolted fault current in your system, you need to get the available fault current from the utility. When you initially ask, a customer service rep will look at a chart and provide you with the bolted fault current based on your transformer size and voltage. This is not what you want as that is based on an infinite bus. You have to be persistent and tell them that it is for arc flash.

Do not use the equations in NFPA 70E-2018 as they are based on IEEE 1584-2002. IEEE 1584-2002 was superseded in November of 2018 by IEEE 1584-2018. This contains new equations based on more testing and will provide a more accurate incident energy value. The downside is that if you are trying to do this by hand, it is very complicated. For example: IEEE 1584-2002 had 3 pages which contained the equations which was 6 equations. IEEE 1584-2018 had 40 pages for the equations and has factors for enclosure size, electrode configuration, etc.

I have attached a page from IEEE 1584-2018 that shows the first equation to calculate the arcing currents. The math is substantial with logs and powers of 6, 5, etc.

View attachment equation.pdf

#### topgone

##### Senior Member
IMO, the POCO ought to give you the available fault current at your POC based on their most recent sub-system short-circuit study. A call to the engineering section of the utility is in order. From there, get your numbers correct and do your arcflash calculations. Please note that the 2018 edition of 1584 requires a lot more calculations as compared to the old one. Be prepared to be surprised!:lol:

#### ron

##### Senior Member
IMO, the POCO ought to give you the available fault current at your POC based on their most recent sub-system short-circuit study. A call to the engineering section of the utility is in order.
Due to grid switching and the "smart grid", power can come from lots of directions with changes / development making it hard to keep track, so they often just defer to unlimited primary calculations because they don't know at any given time which way power is flowing.

#### wbdvt

##### Senior Member
Due to grid switching and the "smart grid", power can come from lots of directions with changes / development making it hard to keep track, so they often just defer to unlimited primary calculations because they don't know at any given time which way power is flowing.
Depends on the sophistication of the utility system but they should be able to give you a max/min value or an average value as they need to know this information themselves to operate the system.