I would agree with you based on 240.4 (G)

 

Thank you, that's what I thought.

 

why would it be #12 and not #10
you are still using a 40 amp breaker though, right?

 

#12 at 75C is rated 25 amps so that takes care of the 23.8 amps minimum circuit ampacity.

 

Why not just install a 30 amp breaker and #10 conductors?

 

30A cb would not be sufficient...nameplate calls for a 40....30 will trip.

See my last for #10's

 

Well how do you like that...

 

Is an ice machine considered a continuous load, upping the required ampacity to 29.75?

 

No, the MCA already includes the 125% - same as on an a/c unit.

The reason a 40 amp breaker is allowed is because the compressor has overload protection which protects the #12 wire. The breaker is for short circuit or ground fault protection.


Also look at art 440.22. This allows OCPD to be 175% of the branch circuit selection current. 23.8 X 1.75= 41.65 amps so a 40 amp device is compliant.

Of course this is assuming VD is not an issue

 

Why not just install a 30 amp breaker and #10 conductors?

Because it's stupid to waste money?

 

So what was the HVAC's guy POV ? :whistling2:

 

He insisted that I needed to have #8 conductors to the machine. At first I told him, "Well not really, but I would have installed #10s if I knew what the machine was going to be." He was being a really stubborn, insistent prick about it. He almost drew me into an argument but after a minute I just took a mental step back and asked myself, "Why am I arguing with this ****** about electrical s**t?" At that point I just walked off and finished hooking up the machine.

Later I overheard him telling the customer (head maintenance guy), "Yeah the electrician thinks the #12 wires will be okay for this 40 amp unit!" And the maintenance guy said, "Yeah? Well I trust him." :thumbup:

 

On the subject of what happens with the current with smaller wires.

I've always been instructed that motors tend to use an "x" amount of power. So, if the voltage drops a little, the motor will pull more current in order to compensate for the VD so that it will continue to use "x" amount of power.

Assuming I have a correct understanding, with the smaller wires there will be more resistance which will lower the voltage slightly which will cause the motor to pull more current.

So, using #12 wire will cause the motor load to pull a little more current than it would if #10 wire was used.

 

On the subject of what happens with the current with smaller wires.

I've always been instructed that motors tend to use an "x" amount of power. So, if the voltage drops a little, the motor will pull more current in order to compensate for the VD so that it will continue to use "x" amount of power.

That is not as true as often as presented, or at the least it is incomplete. If the mechanical work done by the motor remains constant and the voltage is reduced the current will increase.

However what we have been talking about is not a situation where the work done by the motor remains constant.

During start up you have a locked rotor condition, which in many cases means the current drawn during locked rotor will be 6 to 8 times the FLA on the motor. If the voltage is reduced at this time the current will drop as well. This is exactly why reduced voltage starters can be used to limit current (and torque).

http://constructionmanuals.tpub.com/14026/css/14026_251.htm

 

I starting to understand this better, thank BBQ

 

That is what I have read also. So, more current for the motor circuit would equal lights dimming.

Please see post 29 and the link.

 

Just because the label allows a maximum overcurrent protection of 40 amps doesn't mean you have to install that size. I've done refrigeration for years and would never install a 40 amp breaker on that size of an ice machine. The RLA of the unit probably would be in the neighborhood of 19 amps. If you use a quality breaker it has a built in allowance for the inrush current. The overload protector will protect the compressor, what will protect the conductors ?

 

The overload protector will protect the compressor, what will protect the conductors ?

The overload protector will also protect the conductors. If the overload kicks out then the compressor stops. The breaker is for ground fault and short circuit which a 40 will handle just fine.

 

On the same subject, I understand that #12s are not rated for max 20A in all situations, but lets take an air compressor. Specs say 12A no load, and 23.5A at start up (240V). There is no max fuse size or min circuit ampacity specifically stated. I would see these specs and run #12s on a 25A breaker, but would it be compliant to put these 12s on a 30A breaker?

 

On the same subject, I understand that #12s are not rated for max 20A in all situations, but lets take an air compressor. Specs say 12A no load, and 23.5A at start up (240V). There is no max fuse size or min circuit ampacity specifically stated. I would see these specs and run #12s on a 25A breaker, but would it be compliant to put these 12s on a 30A breaker?

Not in that situation.

 

That is what my gut was telling me as well, but could you specifically tell me why not?

 

That is what my gut was telling me as well, but could you specifically tell me why not?

A hermatic comressor has internal overload protection, and an air compressor does not.

 

You lost me bud.

I do agree about air compressors not having internal OL protection but they are still required to have OL protection and the same rules apply

 

I would point out that Eric asked if his installation met the code minumum.

I think most of us would of bid it with a 40A breaker, #8 MC just because we don't do minumum.

Not saying bare minimum is bad or hazardous, FWIW.

If it meets code, it meets code..

 

I would point out that Eric asked if his installation met the code minumum.

I think most of us would of bid it with a 40A breaker, #8 MC just because we don't do minumum.

I can't tell you what I would have run, really depends on how far, most likely I would have run 10s on a 40.

 

It doesn't surprise me that an HVAC guy doesn't understand code requirements for installing HVAC equipment.

I've had to lead more than a few HVAC guys as well as inspectors through the code for this type of equipment, as well as motors.

#12s on a 40.....that's nothing; according to the NEC, a 1 HP single phase motor operating on a 120 volt circuit can be fed with #14s and can use a 40 amp breaker.

A 1-1/2 HP motor on 120 can be fed with #12s and can have a 50 amp breaker. (Yes, they make 50 amp single pole breakers.)

As noted, the motor overload protects the conductors, the breaker provides only short circuit and ground fault protection.

 

if a 4o amp overcurrent device is used then #8 is minimum 75' cloumn
#14,12 and 10 are restricted to 15,20 and 30 amp overcurrent devices, higher values in tables are just for calculation purposes.
#10-30 amps max >or= 23.8 amps 240.4 (b) 30 amp ocpd
#8 40 amps max, no need to derate either conductor
#10-2 mc minimum or 8-2 mc max

 

#14,12 and 10 are restricted to 15,20 and 30 amp overcurrent devices,

No they are not restricted to those values for many loads such as motors etc.

 

sorry, your absolutely right; where otherwise permitted such as motor loads #10 could be used as high a 60 something amp circut

 

sorry,

No problem and welcome to the forum.