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Blown circuit breakers?
Dear gentleman,
I'm in the process of wiring a new shop (30 x 40') I built last year. I had an electrician put in the disconnect near the meter and install the buried line and put in a Square D QO 100 amp breaker box. I'm comfortable taking over from there. I've put in approx 20 outlets ( the 20 amp kind ) using 12 guage romex on three 20 amp circuits: No problems.
I then used 12g romex from the breaker box to a switch box and put in two 3 way switches and ran 12/3 romex for the travelers to another switch box. From there I wired 8 light fixture round boxes and began putting in 500 watt halogen light fixtures one at a time and tested by throwing the switch after each one.
I put up 4 fixtures one day and came back another to continue and as I thru the switch the 15 amp circuit breaker tripped. I started taking them down one by one to troubleshoot ... I finally figured out that there was no short, I was overloading the circuit and turning on the switch in colder weather was tripping the circuit. I was surprized. I assumed a 15 amp circuit breaker was plenty for as many lights as I chose to put in.
My goal is six 500 watt halogen fixtures with two 300 watt halogens over a long work bench on one switch. The panel has 12 slots so that's no problem. If I need lights on, I'll always have them all on. Watts = volts x amps right ? So 3600 watts divided by 110 volts = 32.7 amps . Can I just buy a 40 amp circuit breaker and wire it the way I intended ? Some how that seems like a lot of amperage just for a light circuit doesn't it ?
BTW, the other switch was intended for outdoor lighting but can be used to divide the indoor lights. Also, all of the romex was 12 guage.
Thanks in advance,
Leef [img]/forums/images/icons/laugh.gif[/img]
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Re: Blown circuit breakers?
I think I follow along with what you are doing. I get a bit confused when you say you are putting 12ga wire in for lights on a 15A breaker (why not a 20 A breaker?).
Then, you are tripping the 15A breaker with a load of 5 of the 500 W halogens, which seems to calculate (W = V x A) 2500/110 = 22 A (which will trip the 15A breaker - right?). Maybe try these 5 on a 20A breaker, which even then is a bit on the loaded side also.
If you put in a 40A breaker, then you should also increase the wire guage to the correct size to handle the 40A. Is that 8 ga? I don't think you want that.
To switch all the lights on, maybe put in a relay that will control a high amp switch instead of at least three 20A switches to control the 3600 W load.
Make any sense? Maybe I am missing something too.
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Re: Blown circuit breakers?
beenthere,
Yes you get the picture. Sorry for being too wordy and or vague. I knew the 12g wire was overkill, but I had plenty and assume it doesn't hurt to use thicker wire.
I did put in a 20 amp circuit breaker which could power three 500 watt and two 300 watt ( 2100 watts / 110v = 19 a) for two hours then tripped. It also tripped when I flipped the switch on a cold morning.
Good point about needing to change to heavier guage wire if I tried a 40a circuit breaker. I definately don't want to do that.
Thank you for validating my math and formula. With that knowledge, I'm sure I can re-wire the lighting properly ( using three switches and 3 separate circuit breakers ). It still surprizes me that in a shop with 100 amp service, 35 of those amps will be used up on lighting?
Are there some general principals electricians use when wiring homes and garages?
Thanks again
Leef.
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Re: Blown circuit breakers?
There are no purely resitive loads in AC circuitry. Every electrical device has starting current that varies from 2 to 10 times normal running current.
Why not use some flourescent lights for general space lighting and halogens for over the work bench?
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Re: Blown circuit breakers?
Slamfire,
I think that's a good idea too. One of the other threads mentioned cold weather capable fluorescent fixtures. I should look into them tomorrow when I'm out buying more romex and circuit breakers.
I just registered and haven't figured out how to fill out my bio. so I'll tell you I live in central Iowa where regular fluorescent fixtures don't work very well in unheated garages in the Winter.
Leef
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Re: Blown circuit breakers?
I recently bought some "shop lights" that have electronic ballasts. They are supposed to start at -20 degrees F. We'll find out this winter.
They are the metal units that hang on chains and take two tubes. They have a pull chain for a switch and a six foot cord.
Bought them on sale at Lowe's a few months ago.
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Re: Blown circuit breakers?
[img]/forums/images/icons/tongue.gif[/img] I would have expected Inspector 507 to have jumped in here to help out by this time, but perhaps he hasn't checked out the Barns and Buildings threads lately. [img]/forums/images/icons/tongue.gif[/img] You are so far off of good design in your project that I hardly know where to begin. 12Ga wire is a MINIMUM SIZE for what you are doing, and you need to consider making parallel runs. Did you ever wonder why folks have a multi-gang box on the wall with three or four switches to turn on the lights in a big building? It is because the inrush current is too high to use just one switch, one feed, and one breaker. That arrangement is what you will need to change over to. There are plenty of slots in the box to do it, and lighting needs to be on 15A breakers not 20. The reason for that is because although you are using 12 Ga wire in the feeders, the FIXTURES have teeny tiny wires that you connect to, and it is THOSE LITTLE CONDUCTORS that can fry and cause a fire if the breaker doesn't trip. A 40 AMP breaker ....or even a 30 AMP breaker has NO PLACE in any lighting circuit......no way, no how. [img]/forums/images/icons/frown.gif[/img] With just a little drawing board work, I'm sure that you can design a nice parallel system with a nice multigang box with which to operate it. Stay with 15 A breakers for the lighting, and work with NOT MORE THAN 80% of rated trip amps as the permanent load. ...80% of 15 AMPs is 12 AMPs. That will be your individual circuit load for each string of lights. [img]/forums/images/icons/smile.gif[/img]
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Re: Blown circuit breakers?
CJDave is right.....keep the total load down to 80% of the rated ampacity of the breaker. Any load on it for 3 hours or more is considered a continous load.
However....I would disagree just a bit about the 15A circuit max. There should be no problem in a 20A circuit even with the smaller fixture wires. The load on any individual fixture wire will only be 500 watts=4.2A.
Run multiple circuits with multiple switches. You can decide to turn on/off different lights that way when you need them.
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Re: Blown circuit breakers?
[img]/forums/images/icons/tongue.gif[/img] But Inspector, isn't the consideration that a wire fed by a circuit breaker of a given size must be of sufficient wire guage that it can trip that partiicular breaker without melting the wire? The wiring in some of those fixtures is pretty tiny. Heck, if that is not the case, then why not just use 20 AMP breakers for EVERYTHING and not even sell a 15 at all? Are you saying that the small wire CAN trip out the 20A without frying the wire? Wouldn't it sort of depend on how loaded the circuit was at rated load? Suppose he had a couple of fixtures that were kaputzo and only two still working? Wouldn't that be a situation where the leftover trip amps would be more than a fixture wire could take? Just thinking out loud here. [img]/forums/images/icons/smile.gif[/img] I'm glad you saw this thread.......the original poster definitely needs some guide lines, but my problem is that I am horribly conservative when it comes to electrical systems. [img]/forums/images/icons/crazy.gif[/img]
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Re: Blown circuit breakers?
CJDave,
I think you're confusing allowable ampacity of a conductor with the short-circuit current of a particular circuit. The load on a circuit has very little to do with the short-circuit current in a fault to ground condition. Most of the newer breakers for use in residential are either 10,000 Amps or 22,000 Amps Interupting Current.
The load, or excessive load if you will, will trip out the breaker at around 20 Amps. But during a fault to ground condition, you are going to have thousands of amps for a very limited period of time. That "could" melt the wire, but I'd imagine the breaker will trip off first.
I personally use a lot of 15Amp circuits in my house. Fine for lighting and general use receptacles. Besides it's easier to work with...... [img]/forums/images/icons/grin.gif[/img]
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Re: Blown circuit breakers?
Ouch!! [img]/forums/images/icons/crazy.gif[/img] Don't pull any punches guys.
I knew there would be a recommended percentage and 80% makes sense. I did put in 3 gang switch boxes and now I'm glad I did. I intended to use the other two switches for outdoor lighting, but they'll be needed to fix my original problem as you recommend. The walls are all bare studs, so more switches and wiring really is no problem.
I'll plan to use 15 a circuits with parallel runs as suggested. Most of all, I appreciate the lesson on proper design and forethought [img]/forums/images/icons/smile.gif[/img] I certainly underestimated the amperage necessary to power 500 watt fixtures.
I assure you it'll be done safely with your advice.
Thanks again,
Leef
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Re: Blown circuit breakers?
[img]/forums/images/icons/tongue.gif[/img] For the last year-and-a-half I've been involved as a volunteer electrical guy on a church building project. The main lighting; exclusive of the spots and specialty lights; is 15,000 Watts [img]/forums/images/icons/blush.gif[/img] We have a sixteen channel digital dimmer and lots of other good stuff in there. That was my first whack at industrial lighting, and it was a real stretch at times. We have a few channels on the dimmer that are carrying well over 1600 watts per channel. [img]/forums/images/icons/crazy.gif[/img] I just built a shop here at home out of an existing grainery, and since I'm old and need more light to see, my parts rooms have lots of bulbs, but a switch right by the door that is easy to flip off when I'm not in there looking for a gasket or a fitting. [img]/forums/images/icons/tongue.gif[/img]
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Re: Blown circuit breakers?
OK. One more question to add to the confusion. What is the (dis) advantage to running a 20 A circuit when alot of the common switches and outlets are only rated for 15 A?
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Re: Blown circuit breakers?
Don't have much to add about the wiring aspect but I'll echo the fluorescent question. halogens are nice but take a lot of wattage for the lumes you get. 3600 watts at $.12/kilowatt will cost you about $.43 per hour to run as well as use up 30+ amps of you're capacity. Start some welding out there and you have quite the electric draw. I use fluorescent pretty extensively in our barn (unheated) and have not had a problem with starting, even with the balasts not specifically rated as cold weather. (You have to remember that even though it's -10 outside it's still going to be warmer in an enclosed area unless it stays -10 continuosly for several days.)
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Re: Blown circuit breakers?
[img]/forums/images/icons/tongue.gif[/img] I'll try and answer that until Inspector 507 can give you a better explaination...... In lighting circuits you could have considerable inrush when the lights are first flipped on. it is momentary, but considerable. That inrush could cause nuisance tripping. The running current on that light circuit could be fifteen AMPs, but inrush would be much more. The fifteen AMP rating of the wall switch would be adequate because as Insp. 507 pointed out, continuous load is technically a load which exists for three hours or more. Inrush is momentary. [img]/forums/images/icons/tongue.gif[/img]
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Re: Blown circuit breakers?
dummy,
If I had a circuit that was loaded at 14-16 amps, I'd definately spend the few extra bucks on a 20 amp switch. It's going to last longer.
Breakers are designed to withstand "inrush current" for a short period of time. Motor loads for example will spike a test meter at 2-3 times the rating of the breaker, but as soon as it gets going, everythings fine.
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Re: Blown circuit breakers?
The issue with the small wires in the fixture is that they are short. Remember that the power loss in a conductor is I^2R and that R is proportional to the length and 1/(cross sectional area). What causes a wire to burn is that the power dissipated becomes very large and the wire heats up and melts /ignites something.
So when the code calls for a 12 gauge wire that is a assuming you will run it a relatively long ways.
The wires in the fixture are very short and will not burst into flames with a short current spike of 20 amps.
You would not believe the wires we use in power electronics to handle large currents (>50-100 amps) in tiny little wires. The key is they are very short.
With all this said I tend to over do it on the electrical. No inspector will complain that you used to big a wire or under loaded a breaker.
Fred
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Re: Blown circuit breakers?
This thread has been very informative. As I've contemplated different ways to divide my light fixtures, I've come up with a few more questions. Here is my plan:
-Since I used standard 15a switches, I will use 15a circuit breakers. On the first 3way switch I'll feed three 500W and two 300 W fixtures; however, I've only put 300W bulbs in all five fixtures. Therefore 1500W / 110 v = 13.6 amps which is slightly greater than 12 amps ( 80% of the circuit breaker ).
Q. Is it ok to put smaller bulbs in the 500W fixtures?
Q. Should I buy some smaller bulbs to stay at 80% of the cb?
- On the second switch, I'll feed 3 500W fixtures. Once again I'm at 13.6 amps, so same question applies: change bulbs or leave it.
- I have a lot of 12-3 romex. I am planning to use it from the panel to the two 3way switches mentioned above. It'll be a 20 ft length of romex - red wire in one 15a circuit breaker and the black wire in another 15a cb with the neutral and ground wires shared. I know I've seen this done on the PBS Hometime show which is where I got the idea ( so if it's a bad idea, I didn't think of it [img]/forums/images/icons/blush.gif[/img] ).
Q. Is there too much heat or current to put two 15a circuits through one piece of romex for this distance?
Q. Is it alright to share one neutral and ground between two separate circuits?
Thanks again
Leef
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Re: Blown circuit breakers?
Here's another twist. Temporarily, I have a 20a circuit breaker powering all of these light fixtures. I've been repacing the 500 Watt Halogen bulbs (long and skinny type) with 300W bulbs one at a time to decrease the wattage on the circuit. So far I have one 500W bulb in a fixture I bought rated at 500W, two 300W bulbs in two 500W rated fixtures, and two 300W bulbs in two 300W fixtures.
Here's the twist - the 500W fixtures with 300W bulbs LOOK to be as bright as the 500W bulb /500W fixture. The 300W/300W fixtures are noticeably dimmer. Shouldn't ALL of the fixtures with 300W bulbs look dimmer than the 500W bulb regardless of the fixture rating?
I assume it's the same as my table lamp at home - 100W is brighter than a 60W bulb in the same lamp. Is there something different about Halogen versus incandescent? If I put in a smaller wattage bulb, is the fixture still giving it's rated wattage anyway until the bulb burns out?
Or is it all an optical illusion [img]/forums/images/icons/confused.gif[/img] ?
Confused,
Leef
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Re: Blown circuit breakers?
The inspector might not complain, but the equipment on the receiving end can get upset, real quick.
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Re: Blown circuit breakers?
<font color="blue"> Q. Is it alright to share one neutral and ground between two separate circuits?
</font color>
Are the breakers on the same line(120 line to neutral), or are they on each line (220 line to line)?
If they are on the same line the neutral current is the sum of the line currents and you can over rate the neutral line. If they are on each line the neutral current is the difference of the line currents and the neutral line is within rating.
I wouldn't do it run 12-2 it's cheap.
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Re: Blown circuit breakers?
Al Wa,
No these are two separate 110v circuits. You're right 12-2 romex is cheap, but right now it means a drive into Des Moines (35 miles). I have the 12-3 already and could use it up and complete the job without leaving town.
If I did go after hardware again, I could also buy 20a rated switches and stay with 20a circuit breakers. This would allow me to use higher wattage bulbs and stay at the recommended 80%.
Right now I'm in no great hurry b\c it's so cold. I'm not out there for extended amts of time and I don't like working with 12g wire in the cold. This has become a good learning project for me and I appreciate the advice of those more knowledgeable.
Thanks for the advice,
Leef
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Re: Blown circuit breakers?
Inspector 507 would know more about this than me, but,I believe that running 300w bulbs in 500w fixtures to get the amperage within spec would be a "listing" violation if you had it inspected. The reason is that the next guy to come along wouldn't have a clue of why you had de- rated the bulbs and could replace them with 500w bulbs, thereby overloading the circuit.
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Re: Blown circuit breakers?
If you have lots of 12-3 just use it as if it were 12-2 and ignore the red wire. I don't know if this would follow code or pass inspection, but it would get your neutral loads correct.
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Re: Blown circuit breakers?
If you've got the 12/3 and want to use it.....by all means do so. You can run the 2 hots and share a neutral without any problems as long as you put the breakers in the right spots in the panel. Breaker 1 on leg A and breaker 2 on leg B. 2 breakers on leg A or B will overload the neutral.
If the 300 watt bulbs fit, use them. You won't have a problem with the listing of the fixtures.
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Re: Blown circuit breakers?
[img]/forums/images/icons/smile.gif[/img] This has been one heck of an informative thread. [img]/forums/images/icons/smile.gif[/img] I've seen overloaded neutrals with INFRARED on many occasions and was never quite clear as to just what the problem was, other than too much current coming back on one wire. That business of using leg A and leg B on the red and the black is new to me. Makes sense now that I think about it, but still new. [img]/forums/images/icons/crazy.gif[/img] It has been real handy having the in-spec-tor available to post answers to all of this electrical stuff. [img]/forums/images/icons/smile.gif[/img] Thanks Inspector 507!!! [img]/forums/images/icons/smile.gif[/img]
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Re: Blown circuit breakers?
Slamfire,
Sorry for the slow response its been busy lately.
Maybe I did not explain it well. I was not advocating using a larger breaker than is called for, say for example using a 30 amp breaker to run loads on a 15 amp circuit. This is clearly unsafe.
What I am talking about is running multiple loads off of one 15 amp breaker. You are allowed to do this to some degree but many times in new home construction a contractor will string one outlet off another, etc.... Net effect is you have half the outlets in a small kitchen on one 15 amp breaker. Code allows some of this to a point since the idea is if you have three outlets you will not draw 15 amps from all three at one time.
I was pointing out that if you daisy chain fewer outlets than the code allows the inspector will not complain. To many is another story.
Fred