Originally Posted By: bbadger This post was automatically imported from our archived forum.
I kind of gave this a quick glance but I will through out an opinion none the less.
Using a 30 amp breaker when the label indicates a 25 is required is an NEC violation. For me when I am out installing equipment that is the end of the story.
Now we have to remember the label indicates the breaker or fuse size that was used when the unit was tested by UL under all kinds of conditions.
If we use a larger breaker and if a component short circuits there is no telling if the units interior wiring will be large enough to open this larger breaker before the interior wiring burns.
Do I think 5 amps larger is a big deal?
No, but that is not the issue 5 amps is larger than indicated and now we can not know if the unit will function as it did during testing.
For breakers 100 amps and below the instantaneous trip is almost identical from one breaker to another.
Instantaneous trip is how the breaker reacts to a short circuit. (hundreds to thousands of amps for a very short time)
Breakers also have thermal trip and that is how they react to overloads, say 30 amps of load on a 15 amp breaker should open the breaker within a minute or so. 18 amps of load on a 15 amp breaker may never trip.
Thermal trips are not that acute but regardless we need to follow the rules.
As far as old breakers vs new breakers that is out of my scope. 
-- Bob Badger
Electrical Construction & Maintenance
Moderator at ECN
Originally Posted By: mrose This post was automatically imported from our archived forum.
Hey Robert,
Your call is correct. Even an engineer knows that a 30a breaker is not he same as a 25a breaker. Just because the electrician may find it difficult to locate the proper sized breaker does not make the substitution acceptable.
Another solution is to install a larger condensing unit that can use a 30a breaker. (just kidding)
While this may not be an extreme oversight it still is incorrect.
I would do the same and report that the breaker is the wrong size.
-- Mike Rose
Cornerstone Home Inspection Co. LLC
Lawrenceville, GA
Originally Posted By: dandersen This post was automatically imported from our archived forum.
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it is helpful to have some idea where that is headed. Dan seemed to miss that intent too.
It was my intent, to head off those "Just wright it up as wrong" posts!
I'm just trying to get some discussion going.
You posted a lot of good info that everyone should look close at.
The "rule of 6" is not just an electrical thing. It is a discipline of thinking where we come up with six possibilities for the issue and weigh those possibilities against the facts. It forces us to look at and consider even the most bizarre possibilities.
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Using a 30 amp breaker when the label indicates a 25 is required is an NEC violation. For me when I am out installing equipment that is the end of the story.
It is a violation, but HI is not equipment installation, and most HI's do not enforce "Code" or do New Construction Inspections. But we do need to look at that it "is" there, why it is there, what could happen because it is there, don't we?
Understanding why they do what they do is good!
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So a slightly oversized breaker might not be a real issue after all, as long as the feeder wires are not being overloaded ... but nothing definitive ...
Correct!
The breaker is there to protect the wire.
Once you get the wire with the appropriate capacity to the unit, it should be broken down again to deal with the wire TO and IN the unit.
Undersizing a circuit (too large a load for the wire) causes the wire to become a heater. If just the breaker is small, it will trip all the time due to the high inductive load of motors in the unit. Voltage drops from inadequate circuits will cause equipment damage. Low voltage supply causes high amperage draw = excessive heat. Heat damages HVAC equipment. HVAC deals with heat transfer, so excessive heat puts the unit outside it's design capacity and it fails early.
Next, what do we do with this information?
How are we going to report it?
Remember, there may not be any supporting indications that can be measured at the time of inspection. Such as, testing a cooling system in the fall.
