2.4k Dimmer Breaker Trips

[Lotos]

So, there's an electrician in the air focusing a gobo wash... 4 lights on trees DSL and DSR... My laptop is sitting on the edge of the deck, logged into the console remotely, and I'm bringing lights up and down as the designer calls for them... A routinte focus, through and through...

I wander off for thirty seconds, as the LD futzes with a twofered pair of Strand's (One 2209, and one 2212). The other pair on the opposite ladder, I should mention, are all on the same 2.4k dimmer.

As I'm coming back to the laptop, as I sense he is nearing finishing the focus... The lights pop out... "That wasn't me" I yell at the electrician in the lift "Hold tight, I'll see what I can do."

Run the math in my head, just to make sure that 4x 575w does indeed equal less than 2.4k... And then check that they were indeed patched into a 2.4k dimmer (We have a mix of 2.4's and 1.2's)... Nope, everything is kosher... Ah well, send someone up to reset the breaker, finish the focus (running the lights around 80%, just in case it's a weak breaker) and head off to bed a few hours later.

I'm raised from slumber by the LD, who informs me that he's having trouble with this gobo wash at even 50%. We both agree that a 1k or 750w lamp must have slipped in there, and I head in to take a look.

Grab a pair of GLC's from the lamp stock (hey, you never know...) and head up. First fixture, bam, it's an FEL... Swap it out... Something in the back of my head goes "No, there is NO way you are that lucky today..." Sure enough, check the 2nd fixture on that tree... An FEL... Swap that out as well, and run grab two more... Check the tree the other side... Sure enough, two more FEL's...

4 swaps later, with everything lamped at 575w... Lo-and-behold, that breaker isn't tripping anymore... Oi... So let's do the math...
4000watts > 2.4kw... DURR Overloaded? I think so...

Apparently in my haste to hang, I neglected to get the crew to do a lamp change... I don't know what I was thinking... Ho hum... Mystery solved.

Head Lx-FAIL

 

[zac850]

Well, lets see here:
575w x 4 units = 2300 watts / 120 volts = 19.16 Amps
575w x 4 units = 2300 watts / 118 volts = 19.49 Amps
575w x 4 units = 2300 watts / 115 volts = 20 Amps
575w x 4 units = 2300 watts / 113 volts = 20.35 Amps

Remember, your 2.4k dimmer doesn't actually care about the wattage, it will trip at 20 amps or so. When you four-fer 575w units you get very close to that magical 20 amp load, and if your buildings voltage drops a little, you'll pop the breaker. Also, some breakers trip before they hit 20 amps, so sometimes you get lucky and sometimes you don't.


Of course, 4,000 watts is well over no matter how you look at it.

 

[derekleffew]

Well, lets see here:
575w x 4 units = 2300 watts / 120 volts = 19.16 Amps
575w x 4 units = 2300 watts / 118 volts = 19.49 Amps
575w x 4 units = 2300 watts / 115 volts = 20 Amps
575w x 4 units = 2300 watts / 113 volts = 20.35 Amps
...

Um, no. By your logic,
a single 575W lamp at 23V (~20% on a linear dimmer) would draw 25A and trip the breaker. [W=VA; 575W=23V*25A]
See the CA Mathematical Formulas for Lighting for the applicable formula. See also Another current question.

 

[epimetheus]

There's also time-delay to contend with as well. A breaker loaded at 110% wont trip as fast as a breaker loaded at 150%. Generally speaking, a breaker loaded at 101% will trip, but it will take quite a while.

Of course, you should never load a breaker beyond it's rating intentionally.

As to Zac's example, the wattage will vary with the voltage. The impedance (or resistance) is more or less a constant. A GLC is rated for 575W at 115V. That's a resistance of 23 ohms (P=V^2/R) @ 115V. At 113V, a GLC will draw ~555W. At 120V, a GLC will draw ~626W. Interesting to note that, per these calculations, (4) GLC's will overload a 2.4kW dimmer @ 120V.

http://www.controlbooth.com/forums/question-day/9282-acceptable-put-4x-s4s-dimmer.html

 

[SHARYNF]

This is a very common error on folks with basic electricity ohm's law application

The assumption that the resistance of the lamp stays constant is INCORRECT. If fact the resistance changes with lower voltage and lower temperature. If you took the incorrect assumption of ohm's law application in this case to its extreme conclusion, a lamp being supplied with voltage approaching zero would have a amperage draw approaching a crazy high number.
Of course this would mean that dropping the voltage would not dim the lamp.


I have found time and time again that folks make this mistake, your typical physics teacher will tend to get this wrong every time. ;-)))
It is a great way to win a few wagers from techie friends.

Here is a link that you might find interesting

Lamp Data

Sharyn

 

[epimetheus]

Yes, it is true that the resistance is not constant over the full input voltage range. What I was getting at above is that the power draw is not constant. I guess I should've phrased that differently. Within a short range, say 110-120V, the resistance is more or less constant. At least constant enough for the sake of determining whether 3 or 4 575W lamps can be used on a 2.4kW dimmer. The incorrect application of ohm's law above is not in V=I*R, but in P=I*V.


I disagree that typical physics teachers get this wrong though. You can't teach non-linear loads in high school physics. Non-linearity is, generally speaking, a college application. In high school, it should be a tech theater class that should touch on the non-linearity of a lamp filament.

 

[fredthe]

The assumption that the resistance of the lamp stays constant is INCORRECT. If fact the resistance changes with lower voltage and lower temperature. If you took the incorrect assumption of ohm's law application in this case to its extreme conclusion, a lamp being supplied with voltage approaching zero would have a amperage draw approaching a crazy high number.

Um, no. If you assume that RESISTANCE is constant, then Decreasing Voltage will result in Decreasing Current, not Increasing as you state.

The largest error lies in assuming that POWER is constant.

-Fred

 

[SHARYNF]

Um, no. If you assume that RESISTANCE is constant, then Decreasing Voltage will result in Decreasing Current, not Increasing as you state.

The largest error lies in assuming that POWER is constant.

-Fred

take a look at the charts in the link I supplied, that is what I am talking about.
Sharyn

 

[jared555]

I know I am bringing up an old thread, but I have a question. In most cases I hear when running servers in a datacenter fire code states that you are not allowed to put a constant (non surge) load of more than 80% on a breaker. Is this a general fire code or something specific to certain environments?

I don't know how this applies with devices that have their own fuses/breakers though.

 

[cdub260]

I know I am bringing up an old thread, but I have a question. In most cases I hear when running servers in a datacenter fire code states that you are not allowed to put a constant (non surge) load of more than 80% on a breaker. Is this a general fire code or something specific to certain environments?

I don't know how this applies with devices that have their own fuses/breakers though.

The NEC (I don't recall which article off the top of my head.) requires that the Overcurrent Protection Device (Fuse or Breaker) be derated to 80%, so essentially you're correct. It's just not the fire code that places the restriction. There is, however an exception to this rule. If the OCPD is UL Listed for 100% capacity, it may have a load of 100%. Most, though not all, theatrical dimming systems are UL Listed for 100% capacity.

Feel free to jump in with any corrections here, as I do not have my code book with me at the moment.

 

[jared555]

So the big thing is to keep in mind that you don't put more than 80% x 10k watts combined worth the dimmer packs on a circuit/main breaker that is rated for 10k watts (I know the rating is in amps but I figured for this discussion taking voltage/amperage into account isn't as big of a deal)