I have three 12 gauge wires, each connected to a dedicated mini 20 amp breaker (2 20amp breakers per panel slot); each line connects to a single wall outlet. I need to repurpose one (or several of them) to a 30amp breaker to power a wall mounted wine fridge cooling unit.
The unit requires 30amp for start-up, but then settles to less than a 10amp pull for continuous run.
Is it possible/safe to simply put a 30amp circuit on one of the existing 12 gauge wires just to start the unit up? The big problem we have is all three 12 gauge lines run through the walls with no easy way to pull/replace one of them, or run a new line, with a 10 gauge line. So, we want to replace one of the dual 20amp breakers with a single 30amp breaker and use one of the 12 gauge lines already there.
Considering the startup load requires a 30amp pull, and not a continuous pull 30amp pull, could we safely get away with doing this? The AC unit might start up 3 to 4 times a year due to power outages…but outside that, we won't be turning it on and off as it is a wine cooling unit that must run 24/7.
If this is not an option, what are the real drawbacks of knowing all of the above? And last, if the above is not an option and knowing the above are there any other configurations that could be used without having to pull a new 10 gauge wire?
Best Answer
Motor startup surge is already accounted for
in the Motor Rules in NEC Article 230. Refrigeration units are covered in more detail in NEC Article 240, but they simply refer back to Article 230 for startup surge. Don't bother reading it; the takeaway is "the system is already engineered with this transient overload in mind".
For instance, breakers have a liberal "trip curve" which specifically allows for short term overloads - to start motors, to allow for inrush on incandescents and capacitor based power supplies, etc. So the momentary overload is already forgiven y the breaker.
In fact, if Locked Rotor Amperage (aka momentary amperage when power is first applied and the rotor hasn't started spinning yet) is 30A, the last thing you'd want to do is put a 30A breaker on it. That would mean if the rotor did seize up, it would sit there roasting at 30A until it started a fire, and the breaker would be like "kewl... kewl".
So how do we sort out breaker and wire size, then?
We listen to UL. UL sets the standards for how appliances must be designed and certified. Competitors to UL such as CSA, ETL, BSI, TUV can do the actual certification... but UL writes the design standards.
The appliance will have labeling and instructions. Part of that labeling is a "nameplate" which should call out a bunch of data about the motor - including full load amperage, startup (Locked Rotor) amperage, minimum circuit breaker, and maximum circuit breaker. The nameplate, labeling or instructions may also specify the minimum wire size to use. UL approves those instructions and labeling as part of approving the equipment.
NEC 110.3 in turn requires you follow those instructions and labeling.
So that's that. You don't "freestyle" breaker and wire decisions; you read the instructions and labeling.
The circuit breaker is there to catch 3 kinds of fault: short-circuit (hot-neutral short), ground fault (bolted hot-ground fault flowing far too many amps), and overloads. However, motors sometimes have overload protection directly on the motor - which works better because the overload protector can know the motor's temperature. In those cases and with UL approval, overload protection can be off-loaded from the breaker - which means it's no longer necessary for the breaker to precisely match up to the load. The breaker can be enlarged somewhat, which can help avoid nuisance breaker trips. This is covered in the Article 230 motor rules.