Rooftop AC Units - Disconnects, Breaker Size, Conductors and Conduit

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crispysonofa

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Location
New England
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Electrical and Security Contractor
Hello thank you for reading this,

I have 2 Rooftop units that I am wiring on a farm. These are a bit larger than previous ones that I have done and I want to ensure that I do this correctly and also want to make it cost effective. The units have electric heat stages installed and it is accounted for on the nameplate bringing it to 104 amps @ 240V for the heat, Max fuse or breaker size is indicated 150 Amps as this takes into account the sizing for 125% but since there are no breakers between 130 and 150 I understand that the 150 amp rating would be used. The Circulator\Condenser is rated for a max fuse size of 60 Amps.

When I am sizing my conductors 1/0 copper @ 75C is good for 150A , I would need at least a 2 inch conduit to accommodate the conductors for the heat stages alone to each respective unit. The space inside the unit is tight and this seems like its going to be a challenge to get 1/0 Copper into the space provided from the side of the unit. Am I missing something?

There are 3 Existing QO 200 amp panels on a trough located very close to the units on a 600A feeder. With the old units gone the panels are basically empty. Putting a 150A breaker into 2 of the panels to feed the heat on these two units seems to be the best solution. (Although I thought about looking for a 600A Nema 3R panel.) I would then load the 3rd panel with both condensers/circulators on 60A breakers. QO2150 is about $600 a piece which I know is more than double the cost of these existing panels.

I understand that I need to have a service outlet and disconnect readily accessible to the units. The existing panels are NEMA 3R and located just around a corner (6 and 10 feet respectively) from the panels/trough. Would this be an area that is up to AHJ to deem it line of sight? Should I just install a disconnect on the unit or should I bring coffee that day and use the breaker as a means for disconnect?

I very much appreciate the insight from someone with more experience in doing these larger units than myself.

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You are only required to size your conductor based on the MCA, Without considering any derating for ambient, etc. you can use a #1 conductor. (assuming the 130 amp nameplate is the unit in service)
 
I cannot get a 130 Amp breaker so would this be an instance of using the next size breaker and using #1 copper?
 
With the size feeders you mention, fault-current values may be an issue. Do you have any idea what the available short circut amp number is for the service ?
 
800A

These units do not actually draw anywhere near what they say on the nameplate. They are very oversized for the space they are conditioning and generally float around 60 or 70A per unit when running including heat stages and condenser.
 
That brings me to another Question. How much can I reduce the size of my grounding conductor? I would like to save costs where I can and copper sure isn't getting any cheaper these days.

I think I can use a #6 Ground for this situation based off table 250.122
 
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