Control/Dimming Overloading breaker pack

[kitsune]

So we put together a pair of distros that convert 30amp 3-phase to 3x 20amp circuits in order to power a set of Leprecon dimming packs. The distros aren't complicated: it's basically just a sub-panel with three Square D breakers in it. The problem is, the system keeps blowing the breakers at random intervals. Sometimes weeks will pass with no issues and then they pop. Amp-clamp tests indicate the legs are only drawing about 17.2amps per. The breakers never pop on a cold bump-up, but usually blow a few hours later. We installed a cooling fan on both distros to no avail. Cable runs to the dimmers are 14/3 and the supply tails are 10/5
What else could be setting these breakers off, aside from them being from a faulty batch?

 

[avkid]

Are the distros actually getting hot?

 

[SteveB]

So we put together a pair of distros that convert 30amp 3-phase to 3x 20amp circuits in order to power a set of Leprecon dimming packs. The distros aren't complicated: it's basically just a sub-panel with three Square D breakers in it. The problem is, the system keeps blowing the breakers at random intervals. Sometimes weeks will pass with no issues and then they pop. Amp-clamp tests indicate the legs are only drawing about 17.2amps per. The breakers never pop on a cold bump-up, but usually blow a few hours later. We installed a cooling fan on both distros to no avail. Cable runs to the dimmers are 14/3 and the supply tails are 10/5
What else could be setting these breakers off, aside from them being from a faulty batch?

Are you using a true RMS amp-clamp to make the measurement ? (around the hot leg only ?). If not, don't trust a reading on a circuit with a dimmer, whose sine wave is distorted and where a non true RMS amp-clamp cannot accurately measure the amperage draw.

As well, you probably have excessive voltage drop and a subsequent amperage increase trying to pull 17 plus amps thru 14/3, which is rated to 15 amps. In general, you need to be using 12/3 minimum on a 20 amp breaker, so that's problem one (that's a violation of electrical code, as by-the-way, having a cable downstream of the over current device that's at a rating less then the OCD. The cable essentially becomes the fuse in this instance).

Problem two would be cable lengths aggravating the problem and if over 50-75 ft., maybe size up to #10/3 if planning on full loads.

Then there's the problem of de-rating, with the breakers you mention most likely not designed to hold a 20 amp load for that length of time. De-rating applies and the capacity of the breaker falls to 16 amps if used for more then 3 hrs continually.

 

[shiben]

Are you using a true RMS amp-clamp to make the measurement ? (around the hot leg only ?). If not, don't trust a reading on a circuit with a dimmer, whose sine wave is distorted and where a non true RMS amp-clamp cannot accurately measure the amperage draw.

As well, you probably have excessive voltage drop and a subsequent amperage increase trying to pull 17 plus amps thru 14/3, which is rated to 15 amps. In general, you need to be using 12/3 minimum on a 20 amp breaker, so that's problem one (that's a violation of electrical code, as by-the-way, having a cable downstream of the over current device that's at a rating less then the OCD. The cable essentially becomes the fuse in this instance).

Problem two would be cable lengths aggravating the problem and if over 50-75 ft., maybe size up to #10/3 if planning on full loads.

Then there's the problem of de-rating, with the breakers you mention most likely not designed to hold a 20 amp load for that length of time. De-rating applies and the capacity of the breaker falls to 16 amps if used for more then 3 hrs continually.

Unless the breakers are rated for continuous loading, no?

 

[SteveB]

Unless the breakers are rated for continuous loading, no?

All true, but from the general description of the distro build, I'm guessing that these are not fully rated breakers, but more likely standard off-the-shelf stuff.

I'm often wrong but possibly the OP can enlighten is.

Here's what I'd be using (as well as 12/3)

http://www.lexproducts.com/cs/enter...ertainment&productLineId=21&subCategoryId=114

 

[Footer]

Unless the breakers are rated for continuous loading, no?

Yup. However, try to find one at home depot.

Sent from my Galaxy Nexus using Tapatalk

 

[shiben]

Yup. However, try to find one at home depot.

Sent from my Galaxy Nexus using Tapatalk

To be honest, I have never bought a breaker there, only from Grainger/McMaster Carr or the local industrial electrical supply store so thats probably true...

 

[SteveB]

Yup. However, try to find one at home depot.

Sent from my Galaxy Nexus using Tapatalk

I'm also guessing the OP purchased the parts at an electrical supplier, as I have never seen 3 phase stuff at(my local) HD or Lowes, only 1 phase house stuff.

 

[kitsune]

Hey guys, thanks for all of your fast replies.
These are square-d breakers from granger but I don't think they're rated for continuous use, so that might be part of it. I am using a true-RMA clamp around the specified legs within the panel in order to meter amp draw.
I have to double check those cables. I am 70% sure we used cables that were rated for 20amp draw, but in the event that we aren't, you are absolutely right that would do it. These are only 25ft runs, so hopefully drop-over-distance isn't a concern.
So we might just need thicker cables and cont. rated breakers to handle this load.

 

[derekleffew]

A three-phase breaker is not required in this case; three single-pole breakers are fine.

Voltage drop due to excessively long or under-sized cable does not increase current draw (amperage). [See the wiki entry voltage drop for further explanation.]

 

[Morte615]

A three-phase breaker is not required in this case; three single-pole breakers are fine.

If you use three single-pole breakers make sure they are tied together so if one trips all three trip. (I presume this is the main breaker you were talking about)

 

[SteveB]

Hey guys, thanks for all of your fast replies.
These are square-d breakers from granger but I don't think they're rated for continuous use, so that might be part of it. I am using a true-RMA clamp around the specified legs within the panel in order to meter amp draw.
I have to double check those cables. I am 70% sure we used cables that were rated for 20amp draw, but in the event that we aren't, you are absolutely right that would do it. These are only 25ft runs, so hopefully drop-over-distance isn't a concern.
So we might just need thicker cables and cont. rated breakers to handle this load.

Not for nuttin, but at some point the parts and labor swapping to rated breakers may not be worth your while and the Lex Product box becomes cost effective. They are, if nothing else, very, very durable and worth the expense.

I stopped making my own orchestra music stand light distro's for this reason, finding the Lex E-Strings to be a better built solution for the long run. I see a LOT of these Lex boxes and I know they are heavily used in the film business, where stuff gets kicked around (unlike us civilized theatre folks)

@ Derek. I cannot understand the WiKi. I'm dense sometimes. I KNOW there's a rational reason as to why trying to pull 17 plus amps thru cable rated for 15 will heat the cable, making the breakers pop. I've experienced this. Please put on your "Academcian" cap for a second and learn us (or me).

Thanks in advance

 

[FMEng]

That 20 amp breaker is designed for a continuous load of 80% of the rating, which is 16 amps. No breaker is going to hold forever at 100% and trip at 101%.

Chances are if the distro breakers were not tripping, the over current protection on the dimmer pack eventually would.

 

[SteveB]

If you use three single-pole breakers make sure they are tied together so if one trips all three trip. (I presume this is the main breaker you were talking about)

Not required. The individual breakers may be single pole. Think of a regular building circuit panel fed by a 3 pole service. The main feed breaker is a 3 pole, but the individual branch circuits are not. The Lex distro shows this in a portable application.

Granted that it may be good advice as it will take out the entire pack, which may be a good thing depending upon the reasons for the trip.

 

[SteveB]

That 20 amp breaker is designed for a continuous load of 80% of the rating, which is 16 amps. No breaker is going to hold forever at 100% and trip at 101%.

Chances are if the distro breakers were not tripping, the over current protection on the dimmer pack eventually would.

The Leprecon dimmer use full load rated breakers, so possibly a cable melt down as the alternative , a dead give-away.

 

[derekleffew]

If you use three single-pole breakers make sure they are tied together so if one trips all three trip. (I presume this is the main breaker you were talking about)

Handle ties are not required/desired in a distro of this type, as its output is not a MWBC. An overload of one phase should not cause the other two phases to trip. Whatever is feeding the L21-30 however, DOES need to be a three-pole or three-single pole with common trip. The "(6) 20A, 1-pole branch rated breakers" in the (tested to UL508 standards, I presume) Lex mini-distro linked above are NOT tied together.

...@ Derek. I cannot understand the WiKi. I'm dense sometimes. I KNOW there's a rational reason as to why trying to pull 17 plus amps thru cable rated for 15 will heat the cable, making the breakers pop. I've experienced this. Please put on your "Academcian" cap for a second and learn us (or me).

(I'll probably get this wrong, as STEVETERRY explained it to me only recently.) The breaker trips, not due to excessive amperage, but due to the heating of the long/undersized cable. A 17A load is 17A at 120V. As voltage decreases due to voltage drop, so does the amperage. IF it were an ohmic load (which incandescent lamps aren't, but that's another matter), if voltage drops to (120/2=) 60V, the load becomes (17/2=) 8.5A.

No one has yet mentioned running the 17.2A on a (most likely) Edison 5-15.

See also information on thermal, thermal magnetic, and fully magnetic molded case circuit breakers.

 

[Morte615]

Handle ties are not required/desired in a distro of this type, as its output is not a MWBC. An overload of one phase should not cause the other two phases to trip. Whatever is feeding the L21-30 however, DOES need to be a three-pole or three-single pole with common trip. The "(6) 20A, 1-pole branch rated breakers" in the (tested to UL508 standards, I presume) Lex mini-distro linked above are NOT tied together.

Ahhh see I was under the impression that it was a MWBC (shared neutral from the main's) If that is true then circuits that share a neutral must be tied together. Am I correct in that?

 

[JD]

The only time a long run and line drop will increase current draw is if the load power supply is auto-ranging or self regulating. For example, a switch-mode supply (PWM) feeding a 1000 watt discharge lamp will draw about 4.17 amps at 240 volts. If the voltage is lowered, the supply will compensate, so that at a sag voltage of 200 volts, the supply will now be drawing about 5 amps. By the time the voltage gets down to 120 volts, the supply will be drawing about 8.33 amps. (Figures above are slightly low as all ballasts waste a bit of power, and start-up currents will be higher.)

I would have to add "Manually Ranging" supplies to this list, such as the old Troupers and Super Troupers, where you set the range with a knob and a meter. The longer the cable run, the higher the current tap would be set to make up for loss.

When dealing with a resistance load, current will drop with voltage, not increase.

 

[Dover]

That 20 amp breaker is designed for a continuous load of 80% of the rating, which is 16 amps. No breaker is going to hold forever at 100% and trip at 101%.

Can you provide a source for this information. I am unable to find documentation from Schneider Electric (Square D) that confirms this, the available trip curve shows a continuous loading of between 1 and 1.25 times the rated current at an ambient temp of 104 degrees. I believe some of the confusion may be originating from the NEC design requirement that an OCPD not be loaded to more than 80% of its capacity if it will be subject to that load for more than three hours unless it is listed for continuous duty.

Dover

 

[kitsune]

My distros use 3x 20amp breakers that are not tied with common trip handles because A) each pack is only feeding 2 legs worth of leprecon dimmer, so only two legs are needed, and b) you can cut power to half of these dimmers safely. If the A-leg of the dimmer goes dead, it'll take the DMX processor out with it and the whole pack will go dead, whereas if only B-leg fails just 3 of the 6 dimmers will stop.

I believe I was mistaken with the 14/3 runs to the dimmers. I'm pretty sure we replaced them with 12/3 LEX-built runs. In this case, I am assuming signs point to that the breakers are not rated for 100% load. Which is strange; other documentation I've found suggest that while fuses will blow at 80% rated load, breakers have been found to run at 110% for long periods of time. Given what's happening to these distros, it may be that this statement is false.