Educated guess about A/C load from supply

[LarryFine]

On the roof of a building with no access (okay, difficult access) for which I'm drawing plans for permit application, there is an existing A/C unit that I haven't yet seen.

It's 3-phase 240v, apparently cooling only, as I'm sure it's older than how long heat pumps have been available. It is supplied by three #10's protected by 30a FRN-R fuses.

Is there a semi-reliable way to assign to it an estimated load current guesstimate so I don't have to break out a 28' ladder and hope there is even a nameplate on the unit?

Obviously, the fuses, and not the conductors, are the limiting factor here, since they haven't blown recently. So, how large an A/C compressor load can 30a fuses supply?

 

[Minuteman]

Larry, you could go back here where I posted a single phase calculation of a 4 ton unit that resulted 32.54 full load amps. It's late, my head hurts, and I will let you interpolate from there! :grin:

 

[davidr43229]

Larry working backwards here, figure that the FRN-R-30 has been sized at 125%-150% of FLA. I figured this due to the ambient temp within the unit/enclosure in the summertime.
Just my $.02

 

[LarryFine]

I figured this due to the ambient temp within the unit/enclosure in the summertime.

Interestingly, these fuses are in the conditioned space, adjacent to the junction box. I haven't been on the roof yet, so I don't know whether that disconnect is fused, or at what rating.

The only OCPD's in the building are this 3-pole fusible switch and the 2-pole 100a breaker in the temporary panel, and was here before me. I'm in the plans-submission-for-permit stage.

 

[steve66]

I think you would be pretty close by treating it like any motor. From my square d calculation, 25A fuses (dual element time delay) are the right size for a 3 HP motor that draws 17 amps. So you are probably looking at about a 20A load.

Assuming of course, that the fuses were properly sized, and not just something someone had on hand when one blew

Steve

 

[rattus]

Larry, can you measure the actual current at the fusebox?

 

[LarryFine]

I think you would be pretty close by treating it like any motor. From my square d calculation, 25A fuses (dual element time delay) are the right size for a 3 HP motor that draws 17 amps. So you are probably looking at about a 20A load.

Assuming of course, that the fuses were properly sized, and not just something someone had on hand when one blew.

I was guessing 15a for the same reasons.

Larry, can you measure the actual current at the fusebox?

Good idea, if I can find the thermostat wiring.