Blown circuit breakers?

[Leef]

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]

[beenthere]

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.

[Leef]

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.

[Slamfire]

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?

[Leef]

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

[GaryM]

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.

[CJDave]

[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]

[Inspector507]

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.

[CJDave]

[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]

[Inspector507]

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]