Is 12 gauge wire adequate for load with 30A peak current during startup and <10A continuous current during operation?

Question

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?

Answer 1

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.

Answer 2

As noted already in a comment, you need to check the instructions for the cooler. A high surge at compressor startup is quite common. The determining factor for the wire size is normally the breaker size. If it requires a 30A breaker then you (with very rare exceptions) need 10 AWG or larger wire. But it might require a 20A breaker with the 30A startup being noted but not actually requiring a larger breaker. If the manual is not clear (sometimes covers multiple models without including specific breaker and wire sizes for each model) then check with the manufacturer for the requirements for your specific model.

But an incorrect statement that is quite relevant to understanding the problem is:

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 an off as it is a wine cooling unit that must run 24/7.

You turn the unit off only for service, and otherwise it is running 24/7 except for power outages. However, the compressor will typically cycle several times a day, just as it does on almost any refrigerator, freezer or air conditioner. Each time it cycles on there will be a motor/compressor startup surge. Normal operation is via a thermostat. While the specifics will vary, typical is that a system will run once the temperature reaches 1 or 2 degrees above the setpoint and then run until the temperature is 1 or 2 degrees below the setpoint. The time will vary depending on many factors, but with a modern well-insulated unit it might run for 15 minutes every couple of hours if the door is not opened.

Consider that if it actually ran continuously at 8A (80% of 10A) that would be: 240V x 8A = 1,920W x 24 hours x 365 days = 16,819 kWh per year. At an average base price of $0.15/kWh, that would be over $2,500 per year. Just to run a wine cooler. I sure hope it doesn't run like that 24/7.

Answer 3

Most appliances are not breaker protected for startup current. You need to follow the installation directions for breaker size. As mentiond in the comments by @crip659, if the instructions do call for a 30 amp breaker, then #10AWG wire will be required, no "and, buts or ifs" allowed. The coolers I've installed all were either 15 or 20 amp protected and I know their starting currents were a lot more.

Answer 4

Generally speaking, for residential devices like HVAC AC/heatpumps/minisplits, refrigerators/freezers, or pumps/motors like well pumps/pool pumps,garage door openers, ..... you size the breaker on the continuous current, not the starting instantaneous current.

Breakers have a trip curve, where the higher the overload current, the shorter the time until it trips. And the lower the overload current the longer time until it trips. This trip curve is what allows higher starting currents without tripping the breaker.

This link explains the different breaker trip curves. Note, for residential, you will be mostly looking at the B curve. https://www.electricaltechnology.org/2021/07/tripping-curves-circuit-breaker.html

So, for a 20 amp breaker, if you go by the B curve, it should allows 3 to 5 times its rated continuous current for up to 10 seconds.

That's more than enough time for the startup current to settle down to continuous current.

And of course, for an actual short-circuit, where your amps are in the hundreds or thousands, it would trip in a second or under.

Here's a real world example.

I installed a Raypak 8450 Pool Heatpump.

It's rated specs in the manual are: Min Circuit Ampacity: 42 amps, Min Breaker Size: 50amps, Max Breaker Size: 60amps, Recommended Wire length: up to 100 feet: 8AWG, up to 175 feet: 6AWG,

I ran 6AWG wire from the house to a pool area subpanel (running 60 amp breaker in the main panel) The Wire is THWN and @ 75°C (167°F) it allows for up to 65amps.

Then from the subpanel, I ran short length of 6AWG to the Pool Heatpump, on a 50amp breaker.

I also run a 240 volt 1HP pool pump from the same subpanel, which take about 5 amps.

And some other outlets and lights come off the subpanel. The heatpump controls the pool pump, so sometimes both turn on at the same time, with the HP cycling on and off as needed. Haven't had ANY issue with the 60amp breaker in the house tripping. I'm sure the startup current for both added together is well above 60amps.

I will be adding an automatic pool cover this summer, running off that same 60amp panel. I don't expect the 60amp breaker to trip even if I started the cover motor, HP and pool pump all at once. Of course, that's a worst case scenario, and likely wouldn't happen in real life.

Of course, most breakers are rated for 80% continuous current. So, if I would exceed 48 amps continuous draw for say over an hour or two, the breaker would trip. But if it did, the solution is a fully rated breaker that allow 60amps all day long.