adidas532
Member
- Location
- Augusta, ME
I was working on an outdoor A/C unit today, and the nameplate read "16.6 Max current 25A max fuse" does the 125% rule apply to the conductor, or just the brk?
-Eric
-Eric
A/C unit
- Thread starter adidas532
- Start date
- Status
- Location
- Massachusetts
Re: A/C unit
Neither.
Neither.
adidas532
Member
- Location
- Augusta, ME
Re: A/C unit
Nice... looks like I'll be hitting the books this weekend.
Nice... looks like I'll be hitting the books this weekend.
adidas532
Member
- Location
- Augusta, ME
Re: A/C unit
Nevermind, don't mind me I'm retarded.
Nevermind, don't mind me I'm retarded.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Re: A/C unit
Are you sure that it said 16.6 max and not 16.6 minimum circuit ampacity?
Are you sure that it said 16.6 max and not 16.6 minimum circuit ampacity?
jimwalker
Senior Member
- Location
- TAMPA FLORIDA
Re: A/C unit
16.6 sounds like the running amps.That in turn would call for 125% so 25 amp breaker.Most show min and max breaker size
[ July 23, 2005, 12:38 AM: Message edited by: jimwalker ]
16.6 sounds like the running amps.That in turn would call for 125% so 25 amp breaker.Most show min and max breaker size
[ July 23, 2005, 12:38 AM: Message edited by: jimwalker ]
adidas532
Member
- Location
- Augusta, ME
Re: A/C unit
yeah it read 16.6 max max brk 25A that was both the fan and the compressor. it's a small unit.
Eric
yeah it read 16.6 max max brk 25A that was both the fan and the compressor. it's a small unit.
Eric
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Re: A/C unit
Eric,
AC units are usually rated with the
Minimum Circuit Ampacity and Maximum Fuse or Circuit Breaker size. The minimum ampacity is for sizing the branch circuit conductors to the unit, the maximum is for your short circuit and ground fault protection device.
The Minimum Circuit Ampacity does not require you to add 25%. It is already computed by the manufacturer and part of the nameplate listing. If the minimum circuit ampacity on the nameplate were 16.6 amps then #14 THHN would be sufficient with a 25 amp fuse.
Eric,
AC units are usually rated with the
Minimum Circuit Ampacity and Maximum Fuse or Circuit Breaker size. The minimum ampacity is for sizing the branch circuit conductors to the unit, the maximum is for your short circuit and ground fault protection device.
The Minimum Circuit Ampacity does not require you to add 25%. It is already computed by the manufacturer and part of the nameplate listing. If the minimum circuit ampacity on the nameplate were 16.6 amps then #14 THHN would be sufficient with a 25 amp fuse.
adidas532
Member
- Location
- Augusta, ME
Re: A/C unit
I thank you all for your inuput. When I hit the books and did my calculations I came up with a #12 and a 25A fuse. I haven't wired one before, so I though it wise to ask for advice.
Thanks guys
I thank you all for your inuput. When I hit the books and did my calculations I came up with a #12 and a 25A fuse. I haven't wired one before, so I though it wise to ask for advice.
Thanks guys
Mark Beckinger
Member
Re: A/C unit
The a/c load may not exceed 80% of the branch circuit amps if no other loads are served. If other loads are in the circuit than its 50%.
The a/c load may not exceed 80% of the branch circuit amps if no other loads are served. If other loads are in the circuit than its 50%.
Mark Beckinger
Member
Re: A/C unit
I always use a 10-2 wire and a 30 amp breaker.I use 10-2 because there's only a ground terminal anyway.
I always use a 10-2 wire and a 30 amp breaker.I use 10-2 because there's only a ground terminal anyway.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Re: A/C unit
I do not believe that this is correct when applied to an AC unit. Do you have a code reference for this?
[ July 29, 2005, 07:41 PM: Message edited by: infinity ]
Mark,
I do not believe that this is correct when applied to an AC unit. Do you have a code reference for this?
[ July 29, 2005, 07:41 PM: Message edited by: infinity ]
peter d
Senior Member
- Location
- New England
Re: A/C unit
Bottom line, the "general wiring rules of thumb" don't apply to air conditioning units and motor loads.
Bottom line, the "general wiring rules of thumb" don't apply to air conditioning units and motor loads.
macmikeman
Senior Member
- Location
- Planet macmikeman
Re: A/C unit
When I do one with a similar nameplate I use #10 wire for the branch circuit supply. I put a 30 amp breaker at the panel. I install a non metalic Cutler Hammer 60 amp load center at the unit with a 20 amp breaker. (unless I am using emt in which case I switch to a metal 3r 60 amp load center. (rusts eventually unless well maintained.) My theory is that the unit costs somebody big bucks and the breaker at the unit instead of just a disco might save the motor at some point. Of course if the unit is marked for fuse only then there is a fused disconect at the unit.
When I do one with a similar nameplate I use #10 wire for the branch circuit supply. I put a 30 amp breaker at the panel. I install a non metalic Cutler Hammer 60 amp load center at the unit with a 20 amp breaker. (unless I am using emt in which case I switch to a metal 3r 60 amp load center. (rusts eventually unless well maintained.) My theory is that the unit costs somebody big bucks and the breaker at the unit instead of just a disco might save the motor at some point. Of course if the unit is marked for fuse only then there is a fused disconect at the unit.
peter d
Senior Member
- Location
- New England
Re: A/C unit
In the case of your 20 amp breaker at the unit, all it's doing is protecting the wires in the whip, nothing more.
Circuit breakers provide ground fault and short circuit protection for the conductors only. The OCPD does nothing to protect the motor. The thermal protection in the motor is what protects it.
In the case of your 20 amp breaker at the unit, all it's doing is protecting the wires in the whip, nothing more.
macmikeman
Senior Member
- Location
- Planet macmikeman
Re: A/C unit
ocpd- , OVERCURRENT PROTECTION device. I realize that the branch circuit OCPD, is protection enough per 440.52 (A) 3 is enough if there is a disconnecting means provided at the ac unit. Mitsubishi ac compressors have no overloads built in the moter, as far as I can tell. The contactor is just a relay, no heaters. The solid state board might have solid state overload protectors in it but I can see no indications of that in the schematics for these units. There are no clics on devices at the compressor. These machines range in price from about $1500.00 to around $6,000.00 dollars. My load center at the unit adds less than $40.00 to the job. If it trips the circuit on overload due to lock rotor or other problem, I might be saving the owner of the unit quite a bit of dough. I am going to keep using them for the reasons I stated earlier unless you guy's are able to explain better to me why I shouldn't bother.
peter d
Senior Member
- Location
- New England
Re: A/C unit
I gave you the reason why it's unnecessary; I don't understand why that's not good enough for you.
I highly doubt that these Mitsubishi motors inside of a condensing unit are without thermal protection. See if you can get a better look at the label on the motor itself. It will probably say "thermally protected."
Locked rotor current would be more than high enough to open the OCPD.
I gave you the reason why it's unnecessary; I don't understand why that's not good enough for you.
I highly doubt that these Mitsubishi motors inside of a condensing unit are without thermal protection. See if you can get a better look at the label on the motor itself. It will probably say "thermally protected."
Locked rotor current would be more than high enough to open the OCPD.
- Location
- Massachusetts
Re: A/C unit
macmikeman, I certainly respect a guy who is trying to do the right thing.
I can give you some insurance that all AC units have overload protection.
The reason the NEC allows 10 AWG to feed a unit marked with an MCA of 24 amps and use a 50 amp breaker is only because the NEC knows the AC unit will have overloads that prevent the motor and the conductors from overload.
On many (most) Hermetic Refrigerant Motor-Compressors that overload protection is sealed inside the unit itself.
If the unit has trouble starting (say it short cycles) you want the breaker to hold until the self reseting overload protection opens. Than when it cools the compressor can restart.
While using a small breaker will not hurt anything I do not see it adding any protection that is not already in place.
All that said I am sure you know what is working for you.
macmikeman, I certainly respect a guy who is trying to do the right thing.
I can give you some insurance that all AC units have overload protection.
The reason the NEC allows 10 AWG to feed a unit marked with an MCA of 24 amps and use a 50 amp breaker is only because the NEC knows the AC unit will have overloads that prevent the motor and the conductors from overload.
On many (most) Hermetic Refrigerant Motor-Compressors that overload protection is sealed inside the unit itself.
If the unit has trouble starting (say it short cycles) you want the breaker to hold until the self reseting overload protection opens. Than when it cools the compressor can restart.
While using a small breaker will not hurt anything I do not see it adding any protection that is not already in place.
All that said I am sure you know what is working for you.
macmikeman
Senior Member
- Location
- Planet macmikeman
Re: A/C unit
Thanks both Peter and Bob, I have 7 of these ac units at my house, so what was throwing me off is that I looked at the compressors to see if they had ol's or clics on's on them. Of course I did not crack open the sealed part so I could not find anything there. The other part that threw me off on this for quite awhile ( been wiring these for one ac contractor for at least 12 years), is the schematics for the units don't show the ol's. But if Bob say's they are there then I know allready they must be. Next reason I like the breaker units and this may seem dumb to some also is that many of the locations I put in the breaker box for a disconect have small kids (read boys) living there. The pull out disconects are just too easy for little guy's to try sticking something in like fingers. I know I would have messed with anything in the yard I didn't know about when I was little. My breaker boxes for the bigger units, or hp rated switches in a wp box for smaller units need a screw driver to get into.
Thanks both Peter and Bob, I have 7 of these ac units at my house, so what was throwing me off is that I looked at the compressors to see if they had ol's or clics on's on them. Of course I did not crack open the sealed part so I could not find anything there. The other part that threw me off on this for quite awhile ( been wiring these for one ac contractor for at least 12 years), is the schematics for the units don't show the ol's. But if Bob say's they are there then I know allready they must be. Next reason I like the breaker units and this may seem dumb to some also is that many of the locations I put in the breaker box for a disconect have small kids (read boys) living there. The pull out disconects are just too easy for little guy's to try sticking something in like fingers. I know I would have messed with anything in the yard I didn't know about when I was little. My breaker boxes for the bigger units, or hp rated switches in a wp box for smaller units need a screw driver to get into.
Mark Beckinger
Member
Re: A/C unit
- Status