HVAC and SCCR requirements

[jim dungar]

where does the 18 KAIC value come from?

Thank you for noticing.
It was supposed to be a made up number. I must have made a typo during my editing.

It should have read...say you have a main distribution panel source with an available current of 17kA and a total motor contribution of 1.8kA. Your main device and the motor feed would need to be rated 18kAIC, but all of the branch devices would need to be 22kAIC (using typical breaker ratings).

 

[kwired]

There is no control panel here and the units are 480V. I run a power circuit from a 480V, 65kA rated motor control center breaker directly to the HVAC disconnect and the from disconnect to HVAC power terminals. Was wanting to replace disconnect with current limiting fuses.

Does the unit have a motor contactor or electronic drive within to control the motor(s)? That is the control panel portion of it if it does.

 

[kwired]

I was in a bind one time where a big chiller only had 5kA SCCR. I was able to show the manufacturer that if the upstream impedance was high enough to keep the AFC below 5kA, then it would be too high to allow the machine to operate within voltage tolerances.

They upgraded it to 65kA for no charge.

If the upstream impedance is enough to keep the AFC below 5kA there is no need to "upgrade" any rating. The AFC is under 5kA.

 

[kwired]

I am currently working on a Power Distribution Center (PDC); these PDCs are large power houses where utility comes in and is distribution throughout a plant using switchgear and MCC. The ratings of this equipment (power panels, switchgear, MCC, VFDs) are 480V and 65kA interrupting. The HVAC units on this PDC however are rated at only 5kA which I see as a routine rating since HVAC is not normally subject to SCCR testing. We usually put current limiting fuses ahead of the HVAC to satisfy this disparity but I have not seen anything official that allows me to do that. Of course now I have a customer questioning the validity of this practice.

Has anyone else here had this issue and have you been able to satisfy the customers concerns?

Did you calculate the AFC at the HVAC unit? You later mentioned 5 ton units. @ 480 volts that unit could possibly be supplied with 14 AWG, impedance of that conductor likely puts the AFC at the unit well below 65kA and unless immediately adjacent to the main gear likely will still be under 5kA.

Also your 65kA gear means there is less than 65kA available, supposedly. Since next typical lower rating is 42kA it may mean there is at least 42kA available though.

What I am saying is just because the main gear is rated 65kA doesn't mean 65kA is what is available at that gear or even more so at every point in the facility.

 

[JoeStillman]

If the upstream impedance is enough to keep the AFC below 5kA there is no need to "upgrade" any rating. The AFC is under 5kA.

Right, but we tried calculating the size of reactors to put upstream to knock it down to 5k and found that the reactors would also cause too much voltage drop.

 

[jim dungar]

Right, but we tried calculating the size of reactors to put upstream to knock it down to 5k and found that the reactors would also cause too much voltage drop.

Which is why you rarely find reactors in the real world, except for some dedicated loads.