Arc Flash Studies and Motor Loads

[steve66]

How does everyone treat motor loads when doing arc flash studies? Obviously, in most cases, you can never be sure which motors will be running if an arc flash were to occur.

I normally ignore smaller motors, but for groups of motors that add up to a significant HP (like on MCC's), I'll add the motors horsepowers together, and assume some fraction is running.

And for large motors, I'll add them into the study individually.

Then I'll usually try different combinations of motors on and off, and try to get an idea of what the worst case is going to be. (If a building has a lot of motors, you obviously can't try every single combination of on and off).

The reason I ask is that I recently saw an arc flash study for a building and it didn't include any loads what so ever. No motors or any other loads of any kind. At a minimum, I'm sure this building has some larger HVAC units. Its not a huge building, but it has (2) 1500 KVA transformers that supply it.

 

[wbdvt]

I will incorporate motors that are greater than 50hp per IEEE 1584. Many times the facility does not have a electrical one line so my model is their one line. In that case all the motors off a MCC are shown no matter how small. I consider them all running unless I know otherwise.

Then there are motors supplied by a non-regen VFD that it doesn't matter.

 

[JoeStillman]

I will make a rule of thumb guess at the amount of motor load for certain types of buildings - but only if I see the electric bill. I've designed a lot of central offices and office buildings so I've seen a lot of that kind of infrastructure. For a CO, its roughly 45% HVAC loads. For an office building, more like 60%. This is all in Eastern PA/NJ.