Arc Flash Boundary Distance Calculation, NFPA 70E Table D.2.1

Status
Not open for further replies.

cowski

Member
Location
New York
Occupation
Engineer
Table D.2.1 of NEC 70E lists example boundary distances and seems to reference equations D.2.1 d and e.

I can't re-create the calculations in this table....

The equation for calculating arc-flash boundary seems to be Dc = √(53 * MVA * t). They list transformer MVAs of 2.5, 1.5, and 1.0 and clearing times of 0.25, 6.0, and 6.0 cylces (4.17ms, 0.1s, and 0.1s). For the first row of the table (2.5 MVA) it seems like the Dc should be =√(53*2.5*0.25/60) = 0.74ft. For the 1.5 and 1.0 MVA breakers, the distance is 2.1 and 2.3 ft. But the table lists 0.9', 5.4' and 4'.

Has anyone done this calculation? Is there a step that they're not showing? Are they taking more factors into consideration (and using IEEE 1584)?

I tried using equation D.2.1 d as well. I calculated MVAbf by rearranging D.2.1b and c into MVAbf = 1.732*V*Isc. That gives MVAbf = 1.732*V*Isc = 1.732*600*44000 = 45.7. And a boundary distance of Dc = √(2.65 * MVA * t) = √(2.65 * 45.7 * 0.25/60) = 0.71'. The other distances would be 2.67' and 2.16'.

The ISC calculation (D2.1a) does seem to make sense.
 
These are the Lee equations. There are two alternate methods of calculating the arc flash boundary. D.2.3 shows an example of calculations using both equations. Normally, this method is only used for conditions not covered by IEEE 1584.
 
Status
Not open for further replies.
Top