Split Unit Circuit Size

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Daniel S

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Texas, Garland
Hi,

I got a 3 phase split unit that it's spec. says that draws 15.6 Amps. , it's circuit must have a minimum ampacity of 21 Amps and the maximum fuse size is 35.

Is it ok if I use a 20 Amps. Breaker and a cable with an ampacity of more than 21 Amps? If it's not, why?

and generaly why is the maximum fuse size is given in specs ? I mean when one has to use a bigger Amps. circuit breaker?( 35 for example in this case)

Thanks.
 
1. If it says fuse only, ya gotta use a fuse and not a breaker.

2. Using an undersized fuse, or breaker if allowed, is gonna usually mean nuisance tripping. I always use the largest MOCP allowed.
 
First let me say I agree with jumper.

Daniel S said:
Hi,

I got a 3 phase split unit that it's spec. says that draws 15.6 Amps. , it's circuit must have a minimum ampacity of 21 Amps and the maximum fuse size is 35.

Is it ok if I use a 20 Amps. Breaker and a cable with an ampacity of more than 21 Amps? If it's not, why?
Click to expand...

Because 20 amps is less than 21 amps

Because a breaker is not a fuse.

Ultimately because of the units listing and labeling combined with NEC article 110.3(B).

110.3 Examination, Identification, Installation, and Use
of Equipment.

(B) Installation and Use. Listed or labeled equipment
shall be installed and used in accordance with any instructions
included in the listing or labeling.
Click to expand...

Notice it says 'shall be' which means not optional. You must do it.


and generaly why is the maximum fuse size is given in specs ? I mean when one has to use a bigger Amps. circuit breaker?( 35 for example in this case)
Click to expand...

Again it is because of how the unit was tested for the listing. Using a larger fuse or breaker could be unsafe. Wiring inside the unit might burn up during a fault with a larger breaker.

Given the the information you provided it would be NEC compliant and perfectly safe to do the following.

10 AWG NM, 35 amp fuse.

Or if you used MC or pipe and wire you could use 12 AWG with a 35 amp fuse. (Yes, 12 AWG with a 35 amp fuse, its allowed and safe for this application)

You are allowed to use any size fuse or breaker from the minimum to the maximum but good practice is to use the maximum as using anything smaller could lead to tripping as the unit ages and becomes hard to start.
 
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So why can't the op use a 12 awg if the wiring method was at least 75C? 240.4(G) allows that, I believe and you could still use a 35 amp fuse
 
Daniel S said:
Is it ok if I use a 20 Amps. Breaker and a cable with an ampacity of more than 21 Amps? If it's not, why?
Click to expand...
The main performance problem you will likely see is that the 20 amp breaker may not hold during starting. Most units have max OCP of somewhere around two to two and an half times full load current for this reason.
 
iwire said:
(Yes, 12 AWG with a 35 amp fuse, its allowed and safe for this application)
Click to expand...
Just for that reason - any time I have to wire for a unit before it's installed, I always ask the HVAC company for MCA and max OCP

Most HVAC guys would answer "8-gauge wire on a 35 amp breaker", to which I kindly remind them I didn't ask for a wire size. Simply give me the two specs I asked for and let me figure the wire size.

Even a lot of electricians don't read the section on air conditioners right, and waste a lot of money on conductors that are larger than needed



http://www.mikeholt.com/mojonewsarc...ning-and-Refrigerating-Equipment~20040102.htm
 
James L said:
Just for that reason - any time I have to wire for a unit before it's installed, I always ask the HVAC company for MCA and max OCP

Most HVAC guys would answer "8-gauge wire on a 35 amp breaker", to which I kindly remind them I didn't ask for a wire size. Simply give me the two specs I asked for and let me figure the wire size.

Even a lot of electricians don't read the section on air conditioners right, and waste a lot of money on conductors that are larger than needed



http://www.mikeholt.com/mojonewsarc...ning-and-Refrigerating-Equipment~20040102.htm
Click to expand...

And I find even the HVAC guys have no clue until the unit arrives. If they have a specific unit selected you are better off trying to look up the information yourself. Kind of depends on the job and how well planned it is, some places these things need to be submitted and approved before work is done. Around here many projects seem to be design as you go, and you better plan for worst case with things like this.
 
George Stolz said:
Let me throw a wrinkle in. If this run is 500' long and you are using MC Cable or EMT what wire size would you use?
Click to expand...

In that case I am probably more concerned about actual running current so I can figure voltage drop, once VD is taken care of the conductor is probably larger then what the MCA calls for.
 
George Stolz said:
Let me throw a wrinkle in. If this run is 500' long and you are using MC Cable or EMT what wire size would you use?
Click to expand...

In that case it is likely an engineered job and I will follow their directions. :D




If this is 240 I would not be surprised to need 4 or 2 AWG at that length.
 
kwired said:
In that case I am probably more concerned about actual running current .
Click to expand...

My thinking may be wrong but I would be more worried about starting current and the ability to provide enough voltage at the motor windings to get it rolling.

Its going to be a real bummer if you run a 500' long circuit and when the unit tries to run the contactor starts dropping in and out due to low voltage or if it does stay closed the compressor just sits there humming.
 
iwire said:
In that case it is likely an engineered job and I will follow their directions. :D




If this is 240 I would not be surprised to need 4 or 2 AWG at that length.
Click to expand...

That is a difference between your world and mine. Most projects of mine that involve long conductor runs are agriculture related projects and are never PE designed, at least not the electrical parts of the design anyway. It has gotten to where designing for purposes of environmental protection may bring some some required professional engineering to the picture, but I've never seen any electrical design by a PE in any of these projects. I may have to deal with some containment design when routing my raceways or cables at times but is still my decision how I am to do it.
 
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