Sizing Cables for Replacement AC Rooftop Unit

Status
Not open for further replies.
M

Machiavelli999

Member
We are replacing one AC unit with another.

The existing unit is fed by a 3#6 & #10G on a 60A OCP

The new unit has a listed FLA: 46A / MCA: 52A / MOCP: 70A

Question is can I keep the cables and OCP the same for the replacement unit?

I was going to keep the cables the same since #6AWG has a 55A ampacity which is above the MCA but change the OCP to 70A. However, changing the OCP would require me to upsize the grounding conductor to #8AWG. So now I am wondering if I can keep the existing feeder as is unaltered.
 
I would leave the 60 amp OCPD. In mind I'm not convinced that the 70 amp OCPD would not be code compliant with the cable left as is. :unsure:
 
If the unit name plate says 52 MCA #6 is perfect. 70A MOCP is fine with #6 as that is what the name plate calls for, Over fusing a motor for startup is all that is. 60Amp breaker is likely fine as Its a continuous load 80% of 60=48 and your FLA is 46
 
Eddie702 said:
If the unit name plate says 52 MCA #6 is perfect. 70A MOCP is fine with #6 as that is what the name plate calls for, Over fusing a motor for startup is all that is. 60Amp breaker is likely fine as Its a continuous load 80% of 60=48 and your FLA is 46
Click to expand...
But what size EGC is required for a 70 amp Breaker?
 
IMHO _if_ you change the breaker to 70A, then you have to change the EGC. The circuit conductors would be fine.

I don't have the field experience to know if 60A would be fine for this unit; in your shoes I would make the call based on how difficult/expensive it would be to change the EGC.

Motors draw a large overload startup current; this is why it is common to have a larger breaker than what is required for running the load. If the breaker trips on this startup, I would consider it a 'nuisance trip'; the OCPD tripping on a normal aspect of operation. The existing OCPD has a greater chance of such tripping than a 70A breaker.

But it will likely work just fine.

If your customer would have to pay more for the new EGC and breaker, then you can present the two options to them. If the difference comes out of your pocket then you have to balance the how likely the 60A OCPD will work and the cost of fixing the problem if it doesn't vs the cost of making the change now.

-Jon
 
electrofelon said:
How much difference really is there in the instantaneous trip region for a 70 vs a 60?
Click to expand...

I'd bet there is significant overlap.

I just looked at the trip curves for ITE molded case breakers, and they might actually be the same.

If you go by the curve, instantaneous trip should be between 8x and 12x handle rating. So if the curves apply than a 60 might instant trip anywhere between 480A to 720A, and the 70 560A to 840A. But there is a table in the upper right that says both 60 and 70 instant trip between 575A to 800A. This suggests that 60s and 70s use the same instantaneous trip mechanism.

-Jon
 
winnie said:
I'd bet there is significant overlap.

I just looked at the trip curves for ITE molded case breakers, and they might actually be the same.

If you go by the curve, instantaneous trip should be between 8x and 12x handle rating. So if the curves apply than a 60 might instant trip anywhere between 480A to 720A, and the 70 560A to 840A. But there is a table in the upper right that says both 60 and 70 instant trip between 575A to 800A. This suggests that 60s and 70s use the same instantaneous trip mechanism.

-Jon
Click to expand...
Interesting. I have found with remarkable consistency that actual amp draw is 66% of MCA. So the starting current would be the only potential reason to go max OCPD.
 
Status
Not open for further replies.
Top