Dimmer Load Calculation Problem

[someguy98457]

So I encountered a confusing sketchy Dimmer hook up/power supply that was blowing 200 amp fuses in a 3 phase distro box. The confusing bit comes from the leg that was blowing was the leg that should have been the least likely to blow. The sketchy bit (extremely SKETCHY) comes from the fact that some genius wired a 3 phase dimmer power into a 3 phase motor power distro box so you were missing the dedicated neutral, they had inside the box wired the grounds and neutral together. Also the power going to the motor distro box was 3 phase 200 amp, so a total of 600 amps. The amount of dimmers wired into this box was was 1 etc 24 dimmer 2.4kw sensor plus rack, and 2 dimmex 24 dimmer 1.8kw packs. So a total of 1200 amps.

The dimmer packs were not all wired to the power distro similarly either.
Leg A of the power distro powered Leg A of the two Dimmex packs, and Leg B of the Sensor Rack
Leg B of the power distro powered Leg B of the two Dimmex packs, and Leg A of the Sensor Rack
Leg C powered all the leg C's

We blew the 200 amp fuse for leg C in the power distro box, despite the fact that with the lights we were turning on we were only pulling 183 amps, Leg A had 203 amps and Leg B had 208 amps going through them.


So after this convoluted explanation I have two questions:
First, completely academic, why would the under loaded leg blow first? I know that most dimmer packs take power for the control module from their Leg A, would affect this some how? Am I missing something? I checked, double checked and triple checked the loads, cause my first thought was PEBKAC.

Second, for hooking up multiple portable dimmer packs in the future, should you stagger where you put Leg A for multiple packs so if you happen to loose Leg A, and thus the control modules, you don't loose all those packs? Is there a reason to not do this? I know for 3 phase motors order of Legs matter but I cant think of a reason this would matter in a dimmer pack.

I know that might be confusing, please let me know if something needs to be cleared up. Thanks!

 

[derekleffew]

... We blew the 200 amp fuse for leg C in the power distro box, despite the fact that with the lights we were turning on we were only pulling 183 amps, ...

Was the 183A calculated, or measured? If measured, with what ammeter? True RMS? (Thread: http://www.controlbooth.com/forums/lighting-electrics/6582-clamp-meters.html .) It's entirely feasible that an old or worn fuse could blow at less than its stated rating, for any number of reasons.

So I encountered a confusing sketchy Dimmer hook up/power supply that was blowing 200 amp fuses in a 3 phase distro box. ...

Did more than the C leg blow, or did the C leg blow multiple times?

-----

You're correct that in a lighting system the order of the phases doesn't matter, but it makes little sense NOT to keep track of them to aid in troubleshooting in case of a problem. After all, that's why Cam-lok connectors (or phase tape) come in colors.

As for distributing the "brains" on different phases, I think few would be willing to continue with only 2/3 of their lights. Never seen it done.

 

[someguy98457]

183 was calculated. We measured similar amperage with a Clamp meter. That was my original thought as well until we were able to blow a second new fuse under the same conditions.

The reason I ask about the packs is because this lovely place also did not have a backup system/ emergency lighting. They have dimming system split into thirds located and powered separately. So losing a a third would be more likely a ninth. And losing partial in theory would be better then losing all.

*Also the crossed power for the brains I believe was an accident, we only discovered that (along with everything else) after we had this problem.

 

[Dover]

The sketchy bit (extremely SKETCHY) comes from the fact that some genius wired a 3 phase dimmer power into a 3 phase motor power distro box so you were missing the dedicated neutral, they had inside the box wired the grounds and neutral together.

Your local fire marshal would be VERY interested to know this!!! It is a fire waiting to happen!

Ok, now that has been said lets get down to why it is dangerous to do such a thing. If I understand what you are saying a general purpose 200A 208V delta disconnect (3 hots, a ground and no neutral) was installed on a 120/208V Y service. Nothing wrong with that, it is done all the time. Now I am assuming based on what you describe that tails were installed in this disconnect to hook up the dimmers and the neutral tail was tied to the ground lug. The problem is that when you get into circuits of this size the ground wire is not required to be the same size as the current carrying conductors. Acording to the NEC tables the phase conductors are likely 250Kcmil and the ground 6AWG. Now assuming that the conduit is not picking up a major portion of the load you are going to get symptoms similar to a high resistance neutral, voltages swinging as the load is changed and the associated spikes in current through the phase conductors. If my math is right with a calculated load of 183A it would only take a 12V increase to put you over 200A.

All that to say that I think if you hook those dimmers up to a properly connected disconnect the problem will go away on it's own. And until that happens you are trying to troubleshoot the system with a wild card in the mix.

Hope this helps
Dover

 

[someguy98457]

Your local fire marshal would be VERY interested to know this!!! It is a fire waiting to happen!

The real problem is this is not surprising, nor the biggest problem/safety concern they have. Steps are being taken thankfully though.

Also thank you that was technical explanation i was looking for.

 

[someguy98457]

Why would the least loaded leg be the one to blow? Would that be a result from the other two legs, having such load on them with a high resistance neutral, cause it to act like a 3 phase motor and put more strain on the lesser loaded leg to act as a neutral?

 

[noderaser]

Would that be a result from the other two legs, having such load on them with a high resistance neutral, cause it to act like a 3 phase motor and put more strain on the lesser loaded leg to act as a neutral?

Even though dimmer racks are connected into 3-phase systems, their loads are only single phase (phase-neutral), so there is no return path through any other phase. While a load that is completely balanced across all three phases should yield a net neutral current of zero, you are never going to experience a perfectly balanced load in a dimming system. In addition, the dimmer electronics distort the voltage waveform, which also increases the neutral current--we're in black magic complex-waveform harmonics territory here. This is why it is common to use a larger neutral for dimming systems, because the neutral current can exceed the current of any single phase.

You didn't specify what size the ground/neutral conductor was; I'm assuming that it's probably smaller than the phase conductors, as that would be acceptable for use with a three-phase motor. When you have high resistance on the neutral due to overloading, you will encounter all kinds of crazy voltages on the legs due to the "drifting neutral effect", which will change depending on the load balancing--the lowest loaded leg will show the highest voltage. A fuse will blow at a lower current rating if it is being used at a higher voltage that it's rated for. Another matter is the type of fuse; UL standard 125V fuses are only rated for continuous operating at 70% of their rated current, and there are different fuses designed to blow faster or slower to handle things like startup inrush current.

It sounds like you're just drawing too much current from a distro that is not properly wired for the application. 72 dimmers is a lot for a 200 A feed, you should be looking at a 400 A service, with a proper neutral and separate ground.

Also, note that having a bond between ground and neutral is by code only allowed at one location in an electrical distribution system, typically at the service entrance.

 

[STEVETERRY]

So I encountered a confusing sketchy Dimmer hook up/power supply that was blowing 200 amp fuses in a 3 phase distro box. The confusing bit comes from the leg that was blowing was the leg that should have been the least likely to blow. The sketchy bit (extremely SKETCHY) comes from the fact that some genius wired a 3 phase dimmer power into a 3 phase motor power distro box so you were missing the dedicated neutral, they had inside the box wired the grounds and neutral together. Also the power going to the motor distro box was 3 phase 200 amp, so a total of 600 amps. The amount of dimmers wired into this box was was 1 etc 24 dimmer 2.4kw sensor plus rack, and 2 dimmex 24 dimmer 1.8kw packs. So a total of 1200 amps.

The dimmer packs were not all wired to the power distro similarly either.
Leg A of the power distro powered Leg A of the two Dimmex packs, and Leg B of the Sensor Rack
Leg B of the power distro powered Leg B of the two Dimmex packs, and Leg A of the Sensor Rack
Leg C powered all the leg C's

We blew the 200 amp fuse for leg C in the power distro box, despite the fact that with the lights we were turning on we were only pulling 183 amps, Leg A had 203 amps and Leg B had 208 amps going through them.


So after this convoluted explanation I have two questions:
First, completely academic, why would the under loaded leg blow first? I know that most dimmer packs take power for the control module from their Leg A, would affect this some how? Am I missing something? I checked, double checked and triple checked the loads, cause my first thought was PEBKAC.

Second, for hooking up multiple portable dimmer packs in the future, should you stagger where you put Leg A for multiple packs so if you happen to loose Leg A, and thus the control modules, you don't loose all those packs? Is there a reason to not do this? I know for 3 phase motors order of Legs matter but I cant think of a reason this would matter in a dimmer pack.

I know that might be confusing, please let me know if something needs to be cleared up. Thanks!

You describe a feeder from a "3 phase motor distro box". Was this a delta feed, without neutral, fed from a 208 or 240V delta secondary transformer? If so, let's go no further in looking for your problem.

In such a case, "substituting" ground for neutral will not work. Such a setup will result in unpredictable and dangerous voltages and currents as your "fake" neutral created by the "voltage dividers" of the connected loads moves around with respect to the phases, as the load settings change.

You absolutely need a wye-secondary transformer and a full sized (or oversized) neutral to feed the equipment you are describing.

ST