Temperature Control Panel Fault Current Calculation

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RMS

Member
Location
Minden, NE, USA
I field assemble our HVAC digital temperature control panels. The panel typically includes atleast (1) 120:24vac 100va transformer, digital temperature controller(s), 24vac devices, relays (switching 24vac loads), strictly low voltage in the panel except for the 120V branch circuit supply the transformer(s). How do I calculate the fault current available on the branch circuit and downstream of the 120:24vac transformer. My local inspector told me I need to protect the "weakest" component in the panel. I plan on talking to the project electrical engineer but I was hoping to get a little info on calculations and options to reduce the available fault current if necessary.
 

SceneryDriver

Senior Member
Location
NJ
Occupation
Electrical and Automation Designer
Use the Mike Holt fault calculator here for your 120vac supply circuit:

http://www.mikeholt.com/files/other/Fault_Ver_2014.xls

As far as available fault current on the 24vac side of your 100VA transformers, you should have a little over 4A available. It will be limited by the transformer's impedance at worst, and (hopefully) by the proper application of fuses to protect the secondary. 4-5A of fault current is negligible.



SceneryDriver
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Are you really looking to come up with a fault current, or the short circuit current rating for the control panel? In general control panels must be marked with their short circuit current rating.
 

RMS

Member
Location
Minden, NE, USA
Yes I guess the fault current for the panel, however the inspector told me I need to protect for the lowest rated component in the panel. Thanks for the help definitely appreciated.
 

Russs57

Senior Member
Location
Miami, Florida, USA
Occupation
Maintenance Engineer
FWIW

FWIW

The total fault current for the transformer can be measured easily. Basically you are going to wire nut the secondary together (short circuit) and measure current as you slowly ramp up voltage to primary. At the point that you measure rated primary current see what the voltage is.

Example: say you have a 120 volt-amp 120 to 24 VAC transfomer. Connect 24 volt leads together in a dead short, apply rising voltage slowly with variac until primary draws 1 amp. Lets say that is 12 volts. Given that 1/10 of the rated primary voltage allowed full rated amps into a short it follows that ten times that amount of voltage (rated primary voltage) would allow ten times the rated current into a dead short. Therefore any interuppting device must be able to handle a fault current of 10 amps. In the real world one also gets to adds the impedance of stuff between said transformer and utility source.

Somehow I don't think I answered your question at all:)

Always felt circuit breakers protect wires, ground fault protects people, over current relays/devices protect equipment.
 

arham

New User
Location
troy, alabama.
That is what we prefer, I've used the 'it's not listed for that type of installation' when they tried to get us to recess our speaker strobes into the ceiling or install speakers behind panels. That is probably what I need to do again.
 
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