Breaker Rating ?

#12 Gauge wire in 1/2" metal conduit.
100' to load (three receptacles at outdoor barbecue)
What breaker size should I use?
Thanks!
...Jim Thompson
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| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
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15 or 20A.

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Jim Thompson wrote:

20A. But if you have a spare 15A already, go ahead and use it.
Best regards, Bob
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This is Turtle.
Your amp rating of the #12 wire is only good to 80 feet with a 20 amp breaker. If you reduce the breaker to get a 12 amp ability / 15 amp breaker from the circuit the 15 amp breaker will be good for 100+ feet of the run of # 12 wire. These calculation is on the safe side of the wire / breaker sizing and no body will get wild about the 20 more feet of wire added to get to the the 20 amp breaker but if you want to be on the safe side use the 15 amp breaker.
TURTLE
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Uh, there's no safety downside to using a 20A breaker on it.
This "80' limit" is a _guideline_ for circuit lengths with respect to acceptable voltage drops. Nothing will explode if you pull 20A off a 100' 12ga circuit. It'll just have a bit more than usual voltage drop. Which makes no never mind with most loads. And is in fact advantageous with devices that have startup surges, but are otherwise well below full ampacity of the circuit.
There's _nothing_ in the code that prohibits you from using a 20A breaker on 100' of 12ga, unless your circuit is intended to drive high current devices that will have more than a code-acceptable voltage drop.
For a general purpose outlet string, use 20A and don't worry about it.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.
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breaker
run of

get
15
acceptable
circuit.
mind
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on 100'

that will

This is Turtle.
So you are saying it is ok to run my laptop computor and some other electronic equipment on this circuit with lower than nornal voltage or low voltage supply. If what you say is true, Then I will have to call the People at the NEC thing and tell them that their regulation on wire lengths and voltage drops are not needed in the electrical field because Chris Lewis said it don't make no never mine. When i do call the NEC to have their 2 % drop of voltage change to a higher number. What do you want then to set the % voltage drop limit at ? I know they will want to change their limits by Chris Lewis wanting to set the limits for them, but Do you think 15% or maybe 20% voltage drop would be alright ?
Chris You can run your appliances on low voltage anytime you want but don't tell other people to abuse their appliances on low voltage. NEC codes and regulation are there for a reason and if it did not make no never mine as you say. They would not have a regulation for it. Let me give you one to think about before you start setting the % voltage drops for wire circuits. For every 1 % of voltage drop. The wire becomes a electric heating element by 1 % . As the voltage drops , it becomes a load or electric heating element and will burn up at a certain point of % dropping of the voltage. NEC says 2 % is Ok but where does Chris Lewis say it is OK ?
TURTLE
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Considering that almost every laptop, desktop, and most consumer electronics these days works with switching power supplies, "lower than nominal" voltage is almost irrelevant. Electronics is the _least_ of the concerns here.
These things are designed for far wider ranges than our codes permit. Things like desktop PCs have switches to switch between 90-140V and 200-270V operation. Some don't even need switches for that.
My Dell laptop is perfectly happy with anything from 100V to 240V without having to switch anything.

Unless things have changed radically recently, NEC and CEC limits for permissible drops from panel to circuit end are 4% or 6%. Two to three times what you're quoting. Equipment is designed for at _least_ a 10-12% total drop from nominal (allowing for the permissible 5-6% drop from nominal right at the main panel, plus 4-5% in the branch circuit.)
If the equipment doesn't work properly under those conditions, they don't get UL, ULC or CSA approvals.
Secondly, again unless things have changed, circuit lengths vs. wire gauge are generally guidelines based upon predicted load. Not circuit ampacity.
An inspector ain't going to blink at a 100W coach lamp on the end of 200' of 12ga breakered at 20A. Or even 200' of 14ga breakered at 15A. All they really care is that the breaker trips if there's a short. And they would - having overly long circuits does _not_ make the wiring itself any more likely to start a fire.
But an inspector would make quite an issue if you were expecting to run a 1.5HP motor on the end of a 200' 14ga 120V 15A circuit.
Indeed, on motor circuits, where the breaker size is affected more by startup surge reqts. than continuous current reqts., the wire size is sized for voltage drop under the continuous load, and the breaker is a compromise between wire gauge and startup surge.
So much so in fact that with _some_ motor circuits, the breaker is larger (by one or sometimes even two increments) than you'd normally expect to see on that wire gauge. Ie: 14ga circuits with 20A breakers or 12ga circuits with 30A breakers. [Not generally applicable in residential settings, but it can be with certain flavours of consumer level workshops with large motor loads. Ie: that 5HP Unisaw you were drooling over.]
I should point out what I said in another posting, the OP should go with 15A breakers on the 12ga, _not_ because of circuit length, but because it's enclosed in conduit. [This may be finessable, depending on conduit type (plastic versus metal, direct burial yadda yadda yadda), but he needs to consult an inspector for that.]
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.
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low
electronics
voltage
Things
%
the
see
This is Turtle.
You have some very fine 50 cents words about Ok ing low voltage, but you can have all you want of it but for me. If I have 120 volts in my house, then i want to see close 120 volts in my cook house or any out let I have and even at the end of the 100 feet runs at out door areas.
i understand what your tring to tell me and that is the #12 wire is going to keep close to the 120 volts to the outlet if i keep the load at 12 amps or less. Now if i put a 20 amp breaker and allow 16 amps to be drawn. you will not see the 120 volts to the end of the 100 foot run and will be far less that the NEC code will allow. Sorry but i follow the NEC code to the letter and don't have any trouble , but just don't tell other to not follow the NEC code or regulations because chris says so and likes low voltage.
TURTLE
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metal conduit / outside?????????????? overhead (not underground) I hope.

with GFCI breaker outside

20A with (exterior) GFCI breaker (1st outlet) at barbeque.
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Metal thru the attic and where exposed to UV (Arizona problem). Gray PVC underground. (Green ground wire, so I don't need metal conduit all the way.) Exterior weather-proof enclosed GFCI at the BBQ. I was just puzzling over the confusing ampacity tables.
Thanks everyone!
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
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Be aware that you may need to derate, not because of the length of the wire but because it's in conduit.
Check the conduit derating charts, not the "ordinary" ampacity tables.
I suspect that the conduit will derate you to 15A max.
[I know that contradicts my other posting, but that was independent of the issue of conduit, which I forgot ;-)]
--
Chris Lewis, Una confibula non set est
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