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redm18 · 02-15-2012, 05:24 AM

I have a 15 amp breaker that powers the outlets electical in my garage which has a freezer for food 5 amps a mini fridge 1.2 amps the .5 HP garage door opener and 3 40 watt bulbs would adding a second 5 amp freezer (keezer) to this breaker overload it to much? I could and probably will get rid of the mini fridge and I could go to led light bulbs if I need to but I'm just wondering if I would be risking burning my house down in the short term

onthekeg · 02-15-2012, 05:28 AM

You won't burn your house down, you will trip the breaker. Plug it in and see. The worst that will happen is all your food in the freezer thaws out and you pour a warm beer.

Unless the freezers start at the exact same moment, you should be fine though.

**MadViking** · 02-15-2012, 11:15 AM

Doing some quick math in my head and it looks like adding the second freezer will throw you over the top. You only want to go to a max of 80% which would be 12 amps on a 15 amp circuit. The breaker is supposed to trip on an overload, but do you really want to rely on that? If it doesn't trip and continues to overheat, suddenly you have yourself a house fire. Not worth the risk in my book.

**Captainfester** · 02-15-2012, 12:05 PM

one thing you have to be aware of is that the running amperage of a device is nothing compared to the starter current required. fwiw it is 600% of the running current.

its the same thing when you turn your vacuum on and the lights dim for a second...

pvtschultz · 02-15-2012, 01:29 PM

I wish people would stop spreading the lie about the 80% rule on this forum, or at least use it appropriately. If you have a 15 amp breaker on a branch, you can use 15 AMPS CONTINUOUS, 24/7/365.25. 14 ga wire is rated for 20 amps minimum in residential settings per the NEC. But, the NEC also requires overload protection of 15 amps for 14 ga wiring. If you do the math, 15/20 = 75%. YOU WILL NOT EXCEED 80% CAPACITY IN YOUR HOME! I hate to yell like that, but internet electricians have been pushing this false fact for as long as I have been coming to this forum.

Source:
http://ecatalog.squared.com/pubs/Circuit%20Protection/Miniature%20Circuit%20Breakers/QO-QOB%20Circuit%20Breakers/0600DB0103.pdf

Now, I am a mechanical engineer, not an electrical one, but I know enough to know how to read and I have taken my share of EE courses. I do encourage someone to disprove me though if I am off base here.

As for wire ratings, I run 25 amps continuous through 14 ga conductors in my control panel. That 15/20 amp rating is only for bundled conductors. The rating is higher for single conductors in open air like in chassis applications. It all comes down to temperature rise and the NEC rules for residential applications are VERY conservative just because you never know.

Source - pg 184
http://std-wire.com/downloads/SWC_Catalog.pdf

shutupjojo · 02-15-2012, 01:37 PM

Stupid question but would it be safe to replace the 15 amp breaker with
a 20 amp ?

pvtschultz · 02-15-2012, 01:59 PM

You can if you have 12 ga wire. You MUST use a 15 amp breaker on a 14 ga branch to keep the NEC and electrical inspector happy.

wildwest450 · 02-15-2012, 02:36 PM

Fwiw, I have 3 freezers(running happily) on the same 20 amp circuit in my garage.

cravej · 02-15-2012, 03:49 PM

Quote:

Originally Posted by pvtschultz

I wish people would stop spreading the lie about the 80% rule on this forum, or at least use it appropriately. If you have a 15 amp breaker on a branch, you can use 15 AMPS CONTINUOUS, 24/7/365.25. 14 ga wire is rated for 20 amps minimum in residential settings per the NEC. But, the NEC also requires overload protection of 15 amps for 14 ga wiring. If you do the math, 15/20 = 75%. YOU WILL NOT EXCEED 80% CAPACITY IN YOUR HOME! I hate to yell like that, but internet electricians have been pushing this false fact for as long as I have been coming to this forum.

Now, I am a mechanical engineer, not an electrical one, but I know enough to know how to read and I have taken my share of EE courses. I do encourage someone to disprove me though if I am off base here.

The 80% rule, IS NOT a ratio of breaker sizing to wire sizing.

Actually the codebook says the breaker 'shall not be less than the noncontinuous load plus 125 percent of the continuous load.' Sec NEC 215.3

When you size a breaker for 125% of the continuous load, then you are at 80% capacity. What this means is you can run a noncontinuous load at the full rating of the breaker, but a continuous load will generate heat and eventually cause the breaker to trip. There are 100% rated breakers available, although you probably have to special order them.

Now, the freezer may not be considered a continuous load with it cycling on and off. But I'm no internet electrician.

I'm also a mechanical engineer, but I work for a switchgear manufacturer.

pvtschultz · 02-15-2012, 04:09 PM

So it comes down to breaker sizing and not conductor sizing. I hate to admit that I am ever wrong, but I stand at least partially corrected. I still do not hesitate to run breakers at capacity, but then again my applications rarely require more than 10-30 minutes of continuous load.

And, I'm sure that our public school system educates people on the "80%" rule of thumb so they never plug a 1,500 Watt heater into an outlet that is on the same branch as their 50" TV and floor lamps because that would surely burn down a house.

Sorry, I had to.

ETA: I'm highly skeptacle that in 90% of applications of multi conductor Romex installation that you are ever going to generate enough heat at NEC ampacity guidlines to impact the integrity of the wire. If breakers trip from internal heating, that is another issue.