Circuit for Replacement HVAC System

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Npstewart

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Im working on a project for a local grocery store where they are replacing an existing RTU with a specialized RTU. The existing RTU is wired with 2/0 conductors and is equipped with a 175A circuit breaker. The NEW RTU is 177 MCA and the MOCP is listed as a 200A circuit breaker (FLA listed as 142).

The existing feeder between the electric room and RTU is pretty long and routed through exposed bar joists about 22' in the air. So basically, its going to be very expensive to replace in both material and labor even if the existing 2"conduit can be re-used.

Any suggestions to re-use the existing 2/0 conductors while still replacing the 200A circuit breaker to satisfy the manufacturer?

Screenshot of new RTU electric info is attached.
 

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GoldDigger

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Npstewart said:
Yep, my thoughts exactly. I think the electricians price was about $16k. Thats a lot of money to spend for 2 amps..
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See whether step up/step down transformer pair (maybe even buck-boost configured) would be cheaper. You only need to get the current on the branch wiring down a little while keeping the same power going over it.

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Npstewart

Senior Member
GoldDigger said:
See whether step up/step down transformer pair (maybe even buck-boost configured) would be cheaper. You only need to get the current on the branch wiring down a little while keeping the same power going over it.

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That may be an idea. Im expecting 1 other quote to come in from an electrician. If its unreasonable then ill try that route.
 
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Npstewart

Senior Member
jumper said:
Might work from a physics point of view, but the NEC is gonna say nope.
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I dont think its possible to use a transformer to feed the equipment directly but I do think it may be possible to use a transformer to step up the voltage and provide another transformer to step it back down to keep the existing conductors in place. Question is, would it be worth it. That would be thousands in equipment.
 
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GoldDigger

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jumper said:
Might work from a physics point of view, but the NEC is gonna say nope.
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Are you sure of that? You probably have to add OCPD on both transformer secondaries, but other than that, why not? 200A source breaker, circuit feeder to second transformer is sized for the calculated load, branch from second transformer to load sized for calculated load.
Or are you saying that the feeder would have to go all the way up to the next nominal voltage to justify the current recalculation?

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jumper

jumper

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GoldDigger said:
Are you sure of that? You probably have to add OCPD on both transformer secondaries, but other than that, why not? 200A source breaker, circuit feeder to second transformer is sized for the calculated load, branch from second transformer to load sized for calculated load.
Or are you saying that the feeder would have to go all the way up to the next nominal voltage to justify the current recalculation?

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I am saying I completely misread your post.:slaphead:

What you wrote is not what I read.:ashamed1:

Major brain fart here.
 
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Npstewart

Senior Member
Npstewart said:
I dont think its possible to use a transformer to feed the equipment directly but I do think it may be possible to use a transformer to step up the voltage and provide another transformer to step it back down to keep the existing conductors in place. Question is, would it be worth it. That would be thousands in equipment.
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Unless I had them order a 480v rooftop unit... then I only need 1 transformer.... hmmmmm
 
infinity

infinity

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Jumper has the cheapest solution IMO. :cool:

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steve66

steve66

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Location
Illinois
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Engineer
What am I doing wrong?

Since the fans are 1 phase, 7 amps each, worst case would be 2 fans on the same phase. So:

51.3 amps x 2 + 7 amps * 2 + 16.7 amps = 133.3 amps.

133.3 * 1.25 is only 167 amps?

Even with all 3 fans on the same phase (or if they are really 3 phase), I get an MCA of 175.375 amps?
 
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Ingenieur

Senior Member
Location
Earth
You have to use the nameplate data

if you calc the load based on va
(208 x ((2 x 1.732 x 51.3) + (1 x 1.732 x 16.7) + (3 x 7))) / (208 x 1.732) = 132
125% 165 A

but the nameplate prevails
 
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MyCleveland

Senior Member
Location
Cleveland, Ohio
I think the nameplate is wrong on the MCA calculation also.

Is it not Largest x 1.25 + balance = MCA 145amps

Get the factory involved and the MC and get a new plate, is this possible.
 
T

topgone

Senior Member
steve66 said:
What am I doing wrong?

Since the fans are 1 phase, 7 amps each, worst case would be 2 fans on the same phase. So:

51.3 amps x 2 + 7 amps * 2 + 16.7 amps = 133.3 amps.

133.3 * 1.25 is only 167 amps?

Even with all 3 fans on the same phase (or if they are really 3 phase), I get an MCA of 175.375 amps?
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MCA is computed as 1.25 x (motor rated current + heater current). Somethimes, "motor rated current" is referred to as the equipment FLA (in this case = 142A). So: 142 X 1.25 = 177.5A. That, I think, is where you got your calcs wrong.:)
 
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