HVAC conductor and breaker sizing - AGAIN???

kwired

Electron manager
Location
NE Nebraska
?

Since mca is 23.9 amps the conductors AND disconnect would be rated for 30 amp
While the ocp is the circuit breaker rated at 40 amps.

The disconnect is to be sized as the conductors. In this case a 60 amp disconnect is not required.
Disconnect does need ability to carry MCA just like the conductor. But unlike conductors the disconnect also needs to be rated for the motor load, unlike the conductor it may end up making or breaking the load. This is not quite as straightforward with air conditioning units as they are not marked in HP, but is more clear with general purpose motors as they are usually marked and rated in HP and a "safety switch" usually is marked with a rated HP also.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Generally the disconnecting means needs to be 115% of the FLA of a motor. With an AC unit the MCA is already calculated at 125% so I see no reason why the disconnect would need to be larger than the MCA.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
For a unit with multiple motors, 115% of the sum of the rated load currents might be higher than the MCA, or it might be lower than the MCA, but probably not by much either way.
 
Location
San Diego
Everyone saying forget the pullout?

Everyone saying forget the pullout?

If the pullout is fused properly for the unit and the pullout is rated for 60amps but fused 40 to protect everything down to the condenser what does it matter?
I would rather have oversized done right, less affect on lighting during motor startup, less loss, actually more efficient and reliable.
To me it just comes down to the fuses in the pullout being fused for what goes down to condenser, everything sized right can be 60amp to that point.
 

ActionDave

Moderator
Staff member
Location
Durango, CO, 10 h 20 min without traffic from wing
Occupation
wire pulling grunt
If the pullout is fused properly for the unit and the pullout is rated for 60amps but fused 40 to protect everything down to the condenser what does it matter?
More than one of us said that.
I would rather have oversized done right, less affect on lighting during motor startup, less loss, actually more efficient and reliable.....
Size of the utility transformer supplying the house would have more to do with this than anything else.
 

iwire

Moderator
Staff member
Location
Massachusetts
I would rather have oversized done right, less affect on lighting during motor startup, less loss, actually more efficient and reliable.
Truthfully the larger the conductors you use between the panel and the condenser the more condenser start up will affect lighting as the larger conductors allow more current causing more voltage drop on the service conductors and transformer as Dave mentions.
 

spurlockda

Member
Location
Nevada
Micro-managing

Micro-managing

Please don't worry about micro-managing because I am learning and everything so far has been great and I thank everyone for making me smarter. My Chief Petty Officer used to tell me to go get a hammer. For a while I would say, Why do you need a hammer Chief? and he would say, Because it is smarter that you are. :lol: Eventually he stopped but then he switched to 'a box of rocks' reference. :D Finally, he started treating me like an equal - once I made Chief. :cool:

Anyway, I think I am on track now except for one point. The disconnect has a slide switch which I presume could be a form of fuse but I would like to clarify that in my hammer like brain.
I know what the "normal" fuses look like and a "normal looking" circuit breaker looks like. This disconnect doesn't match either of those "normal" looking devices. This one is a sliding switch. I have it in my hand now. It says: "This switch is suitable for use on a circuit capable of delivering not more than 10,000 amperes, RMS symmetrical, 240 volts maximum when protected by --> 60 ampere maximum fuses, or circuit breaker rated 60 amperes bla bla bla.

Please tell me where my thinking gets off track. Since this "switch" says 'when protected by' 60 amp fuse or breaker And the circuit breaker they left with me is a 60 amp breaker And the whip they left with me is #8 hots and THHN 10 gage gnd.

So so with all of that, me thinks they think that the whole 60amp circuit is OK meaning that they hadn't read the spec plate on the compressor unit which says 'max fuse amps or max circuit breaker 40'. Thus they are wrong and I should not let them proceed with that plan?
 

ActionDave

Moderator
Staff member
Location
Durango, CO, 10 h 20 min without traffic from wing
Occupation
wire pulling grunt
So so with all of that, me thinks they think that the whole 60amp circuit is OK meaning that they hadn't read the spec plate on the compressor unit which says 'max fuse amps or max circuit breaker 40'. Thus they are wrong and I should not let them proceed with that plan?
What does Maximum Over current Protection 40 Amps mean to you?
 

spurlockda

Member
Location
Nevada
Clear up

Clear up

Sorry, I think when I was quoting the statement from the spec plate you may have thought that that was what thinking about.

What I was trying to say is that the ID plate specifies 40a bkr max yet the HVAC guys want to install a 60a branch circuit. They left me a 60a bkr for the main pnl; they left a THHN 8awg hots,10awg THHN gnd whip; and a "switch" for a disconnect that specifies that it must be protected by a 60amp bkr.

That's why I think they are wrong.

Sorry for the confusion.
 

spurlockda

Member
Location
Nevada
If the pullout is fused properly for the unit and the pullout is rated for 60amps but fused 40 to protect everything down to the condenser what does it matter?
I would rather have oversized done right, less affect on lighting during motor startup, less loss, actually more efficient and reliable.
To me it just comes down to the fuses in the pullout being fused for what goes down to condenser, everything sized right can be 60amp to that point.

They didn't leave a 40 amp breaker to be installed - they left a 60 amp breaker.
 

GoldDigger

Moderator
Staff member
Everything save the 60A breaker is fine. You need a 40A breaker, max. Installing a larger breaker than what the mfg allows is a violation of 110.3(B), for starters, which i see I wrote a month ago. Deja vu.
The motor must be protected by OCPD at no more than 40A even though the circuit wiring is good for 60A. But using a 60A (max) fused disconnect containing 40A fuses will provide that required protection. Period.
The breaker feeding the circuit does not have to be limited to 40A as long as the required motor protection is somewhere else in the circuit.
The OP stated that an unfused disconnect was provided, but one option is to replace it with an inexpensive fused disconnect.
 

ActionDave

Moderator
Staff member
Location
Durango, CO, 10 h 20 min without traffic from wing
Occupation
wire pulling grunt
Sorry, I think when I was quoting the statement from the spec plate you may have thought that that was what thinking about.

What I was trying to say is that the ID plate specifies 40a bkr max yet the HVAC guys want to install a 60a branch circuit. They left me a 60a bkr for the main pnl; they left a THHN 8awg hots,10awg THHN gnd whip; and a "switch" for a disconnect that specifies that it must be protected by a 60amp bkr.

That's why I think they are wrong.

Sorry for the confusion.
Look at the switch again. Does it really say it Must be protected by a 60A breaker?
 

peter d

Senior Member
Location
New England
Sorry, I think when I was quoting the statement from the spec plate you may have thought that that was what thinking about.

What I was trying to say is that the ID plate specifies 40a bkr max yet the HVAC guys want to install a 60a branch circuit. They left me a 60a bkr for the main pnl; they left a THHN 8awg hots,10awg THHN gnd whip; and a "switch" for a disconnect that specifies that it must be protected by a 60amp bkr.

That's why I think they are wrong.
60 amps is the maximum rating of the disconnect switch, which is common with a/c disconnect. Tt does not have to be protected by a 60 amp breaker. It can be protected by a 15, 25, 50 60 or anything in between.
 

GoldDigger

Moderator
Staff member
It seems that would leave him with #8 feeder conductors protected by a 60A c/b. I don't see how that would be compliant.
In the same way that any unit which specifies both an MCA and n MOCPD can use wiring sized to MCA and a breaker sized to MOCPD? Not really because the overLOAD protection is not directly in the unit since the breaker exceeds MOCPD.
But if the wires are sized to 40A and terminate in a 40A fused disconnect they could also be compliant following a 60A breaker IF tap rules are followed. In the OP's case that requires more information.
If the wires are sized to 60A then there is no issue at all.
 

spurlockda

Member
Location
Nevada
They didn't leave a 40 amp breaker to be installed - they left a 60 amp breaker. The pullout is not fused, it's just a sliding switch. If it made a snapping noise it would just as well be a big snap switch. Speaking of which, that's what they used on the furnace for a disconnect: a regular ol' light switch.
 
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