Control/Dimming Thoughts on the 80% "Rule"

[Rick Brooks]

Just thought I'd throw out a question that I get asked... What are your thoughts on the "80% Rule" where you only load a dimmer up to 80% capacity? My feeling is if the dimmer is rated for 2.4kw then you can load it with 2.4kw. The difference between 4 575w lights and 3 575w lights can be a big deal in some smaller places. But I've heard it both ways... Some people swear by the 80% and others (like me) feel that it has a rating for a reason. Thoughts?

 

[MNicolai]

Commercial-grade dimmers generally are manufactured to be loaded to 100% for a sustained amount of time without tripping. Consumer-grade dimmers generally aren't. Whether you should follow this policy or not depends on which dimmers you're using.

Most of the people who tout 80% are thinking about it as a normal electrician would. Most circuit breakers used in residential aren't built for 100% loading so it's considered best practice not to load a 15A breaker past 12A. It can go above 12A for short periods of time but the closer you get to 15A, the less time it will take before the circuit breaker does inevitably trip.

This is quickly becoming a moot point though. LED's and all. An entire generation is about to be brought up without any idea what it's like to not be able to hang any more lights because you don't have enough power or enough 12AWG power cables and two-fers.

 

[express]

Typically, I'll try to not load a dimmer at 100%- most of the time this isn't a problem. But I'm also not religious about it, and if I need to put four 575s on a 2.4kw, I will.

 

[Les]

While ETC dimmers are rated for 100% continuous loading, I find it quite hard to truly load a 2.4k dimmer to 100%. Most lamps will put you right under. I do tread more lightly on mid-level and lower-tier manufacturers. Shoeboxes as well, since they usually have much lower limits and the branch circuits they are fed by normally fall in to the 80% loading rule (plus having who knows what else sharing the same circuit).

 

[lwinters630]

A lot depends on the venue and situation. Long cable runs, older fixtures. My concerns are not tripping the breaker, but tripping it during a show.

 

[BillConnerFASTC]

For plug and cord connected quartz stage and studio fixtures, no reservations at 100% assuming a good quality dimmer and no wiring shortcomings. LED on non-dimmed circuits - sort of 50%.

 

[Will Elphingstone]

I have no issue loading one to 100% if its an ETC or other well known brand, built to support a full load for more than a few minutes, and is in good condition. If the dimmer does not meet the previously listed items or has another issues, I will figure out an alternate solution. I begin having concerns about loading "healthy" dimmers to 100% when it will be on for extended periods of time, such as supplying hard power to movers/leds. I would rather pull wall power for those, but if I must power from a dimmer, I will evenly divide the power among the dimmers available. Hope that made sense, if not let me know and I will attempt to re-explain my thought process

 

[RickR]

80% is a nice rule of thumb, but not a hard rule. If you don't know it's a great place to start. My preference is to KNOW every piece in the path.

 

[garyvp]

The 80% electrician's rule is for 'continuous duty' - if the load is on more than three hours the breaker must be 125% of the load - NEC Section 210.20(A). If all your loads are non-continuous, you don’t have to worry about the 125% requirement so you can just size your breakers for 100% of your load. In that case, standard, 80%-rated breakers will be more economical..

Certainly not an issue in our theater. We know we can run our 2.4s at 100% with no problem, but we do not allow it ( we allow up to 4x500W on our 20 year old 2.4 kw dimmers). We have 84 circuits, and 24 dimmers(hard patched) so balancing is real important.

However, we de-rate the load on our main dimmer circuit breakers 20% as a matter of choice - cooler running.

 

[SteveB]

80% is a nice rule of thumb, but not a hard rule. If you don't know it's a great place to start. My preference is to KNOW every piece in the path.

Well actually it is a hard rule. Loads must be de-rated to 80% if run more then 3 consecutive hours, unless the OCPD is continuous load rated.

Note that the ETC Sensor D20 dimmers have breakers rated for continuous loading and can safely and legally run 4x575 watt lamps.

 

[JD]

Good quality dimmers can be loaded at 100% without problems. My heyday was in the 1980's and at that time common loads were 500w and 1kw. If you had a 2.4kw dimmer, you generally called it a 2k dimmer and would slap two 1kw loads on it. As such, unofficial derating was happening without conscious attempt. Currently, loads are often 575, so you would slap four on a 2.4kw dimmer. Still, there is something to be said about running a dimmer below capacity if it is not inconvenient. The heat produced by the SSR is directly proportional to the load, so if you run them cooler, they will last longer.
About the only time loading a 2.4 with 2.4 was an issue was when using sets of 600 watt ACL lamps. (GE-4559) These lamps had a prolonged inrush and would sometime trip the breakers. Most times, I ran those off 3.6kw dimmers. (A size that has fallen out of favor in modern times.) The older magnetic breakers used on dimmers were a little touchy. I would suspect an ACL set running on a modern ETC dimmer would not be an issue, although I never tried.

 

[BillConnerFASTC]

Well actually it is a hard rule. Loads must be de-rated to 80% if run more then 3 consecutive hours, unless the OCPD is continuous load rated.

Note that the ETC Sensor D20 dimmers have breakers rated for continuous loading and can safely and legally run 4x575 watt lamps.

Not disagreeing it's a hard rule but I'll suggest no one has ever been busted for any cord and plug connected load in a stage or studio being over 80% and on - at full - over 3 hours.

B-way standard was 4 - 750s on a 3k - and it had to be fully loaded to not ghost.

I do worry about LED inrush and not sure 50% is low enough.

 

[Rick Brooks]

I have no issue loading one to 100% if its an ETC or other well known brand, built to support a full load for more than a few minutes, and is in good condition. If the dimmer does not meet the previously listed items or has another issues, I will figure out an alternate solution. I begin having concerns about loading "healthy" dimmers to 100% when it will be on for extended periods of time, such as supplying hard power to movers/leds. I would rather pull wall power for those, but if I must power from a dimmer, I will evenly divide the power among the dimmers available. Hope that made sense, if not let me know and I will attempt to re-explain my thought process

Running your movers and LEDs off of a dimmer is a great way to ruin your equipment. You'll burn out your power supply in no time.

 

[Dionysus]

As stated, 80% is based on the circuit breakers and dimmers being rated either for continuous (100%) or intermittent (80%, or such). Pay attention to ratings, two devices rated for 20A are not necessarily the same.

Then again I like to avoid 100% loading dimmers when possible, one reason being balance (try to spread heavier loads per phase) and another being that sometimes when you are close to "the edge" things are more likely to creep up and bite you in the bum. Remember that cabling and other things are included in your load so if you have 2.4kw of lighting there, there is no leeway for losses in cabling (which SHOULD be insignificant, but ARE they?).
There are of course many senerios. But why load right up when you don't have to? Do it when required only.

 

[theatricalmatt]

Another issue with twofering/threefering/fourfering lights together is that it becomes nigh-impossible to balance out differences between the instruments. Ordinarily, if they're on separate dimmers, you can program them at different levels, or profile them to a certain percentage. Even if the instruments are identical in terms of lamp and barrel, there may still be differences in terms of lamp life, bench focus, and throw distance (among other factors) that you can overcome with careful programming. Once they're ganged together, you take that possibility away.

 

[JChenault]

. Remember that cabling and other things are included in your load so if you have 2.4kw of lighting there, there is no leeway for losses in cabling (which SHOULD be insignificant, but ARE they?).

This is the second time cabling has been mentioned, and the statement does not make sense to me. It seems to me that if I have a long cable run, I would be adding the resistance of the viable to the resistance of the lamp. This would result in fewer amps and would put less load on the dimmer. What am I missing?

 

[JohnD]

Running your movers and LEDs off of a dimmer is a great way to ruin your equipment. You'll burn out your power supply in no time.

It should be pointed out that the reference to LED inrush was probably in regard to LED sources which CAN be powered by dimmers.

 

[BillConnerFASTC]

It should be pointed out that the reference to LED inrush was probably in regard to LED sources which CAN be powered by dimmers.

I believe the inrush is in any case - and I plan relay controlled circuits for LED. Ten 120 watt LEDs on a 20 amp relay seems like max.

See https://www.etcconnect.com/Support/Articles/Inrush-Current-and-ETC-LED-Fixtures.aspx bur from that article:

"This can create an issue with circuit breaker protection for a line of fixtures all coming from the same non-dim source. The amount of fixtures that you can use at nominal voltage that will cause your circuit protection to engage is much higher than the amount of fixtures you can use upon inrush. To compound this, most circuit breakers have different inrush trip curves. To find the trip curve of a particular breaker, it is best to find the datasheet of the breaker in question. This makes it impossible for ETC to calculate across the board how many fixtures can be on a circuit as it can vary widely and is reliant upon the breaker installed in the system."

I'll not that I could not find anything on the ETC Echo datasheets about inrush, but on ETC Sensor IQ, several bullets, the most specific of which was • Inrush-pulse tolerance: 25 times rated current for half-cycle
at 60Hz. Might be OK to load that to 80 or 100% with LEDs - but I wouldn't recommend it. Besides, who wants to loos 15 or 20 units on a single fault.

 

[MikeJ]

......Besides, who wants to loos 15 or 20 units on a single fault.

THIS, is what makes a lot of the above a moot point. Often, I still single circuit, or only put a few fixtures on a circuit, even when I could do a dozen or more. If one light has a problem during a show, and for example starts randomly flashing or changing color, I can just kill the breaker for that fixture, and finish the set, and the problem is minimal. If I had to kill an an entire truss of fixtures, someone is not going to be happy.

Also, In my usage, I am almost always using soca cable, so I am de-rating by 50% to begin with. Maxing out 20A breakers is almost never an issue. Now there are definitely times that I daisy chain a lot of fixtures, simply because a situation has limits of available power or whatnot, or because simplicity or practicality are more important, but in general I try to balance avoiding single point failures for many fixtures, especially of a failure could be in an inaccessible location. I might run only one circuit to a truss tower on the stage, but I can get to it if there is an issue. It would be much worse if I have a bad cable on a truss 20' above the drummer. During a show, there is little that can be done to fix it.

 

[BillConnerFASTC]

Lots of variables. Fewer circuits/less copper makes it possible to buy LEDs. I might put just 4 - 20 amp circuits on a cyc electric, and know this means 4 or 5 or more fixtures per circuit. If I put 12 or 20 circuits there, I simply can't afford/justify LEDs.