Keep in mind a lot of the code is in the interpretation. The city inspector has final say in all decisions. He may be reading the code completely wrong, but it doesn't matter if you are talking about getting things done.
We have an inspector in a nearby city that does not include any AV in the inspection. We have done complete installs (dimmers, LEDs, pulling new circuits) and needed no permit and no inspection. The inspector inspected to the
breaker for the
dimmer rack and no further.
In another city I work in the inspector requires a
system rated to 25% more than the maximum labeled draw of the cabinet. It doesn't matter what you say, put in a
breaker/
wire that only meets the requirements, and he will red tag you.
Same thing with electricians. For example, as I said earlier, my electrician will not install a derated cabinet. He believes it is not safe and will not do it. I am sure I could find an electrician that would, but honestly, I like my guy. I know he doesn't cut corners. Everything is done in the safest way possible, to spec, and beyond code.
Keep in mind also that code is meant to be the MINIMUM guideline. It is the absolute least you should do.
Finally, court is the same way. The law, no matter how much we may think it is, is not black and white (especially in civil cases like we are talking about here). Like Bill said in another thread. You might be able to produce enough evidence to get out of a $125m lawsuit, but it is going to cost enough in attorney costs that it can bankrupt a small/medium sized company.
According to my lawyer, the inspector signing off on it does not end your liability, nor does any paperwork you give to the client, even meeting code does not necessarily end your liability. Everything you do is a risk. The question is how much of a risk do you want to take. I will even readily admit that
derating a cabinet if done properly is probably a 1% chance of producing a result that will lead to a lawsuit. But is that a chance you want to take? Some people will say that lessening your probability from 1% to .5% is not worth it. To me, it is. Especially when I am not going beyond what I believe to be necessary.
Maybe it is because my guys aren't entertainment guys. Maybe it is because I learned electrics and engineering from outside the entertainment world. I dunno, but I have gotten legal advice about this and that is how it came out. I have never been in a situation where the savings is worth the risk.
I'm coming late to this party, because the
power discussion was masquerading as an EIA-485 loading question!
I have a few thoughts:
1. Virtually every
circuit breaker panel installed in the US applies
derating between the main feed and the total of the pole-spaces in the panel. There is no functional difference between a listed
dimmer rack and a
breaker panel, for the purposes of this discussion.
2. Section 520.27(C) was created because inspectors around the country were making up a bogus rule about feeding the
dimmer rack at full
nameplate rating. It had no basis in
safety or even in the
NEC prior to the appearance of 520.27(C). Initially, Code Panel 15 told us "You don't need this section, the Code already allows such
derating in chapters 1 through 4." We produced an "expert opinion" from the then chief electrical inspector for the state of Washington stating that
dimmer racks in article 520 venues had to be
fed at the full
nameplate rating. Only then did CMP15 realize that there was a big problem of inspectors "making up" code on this issue and that 520.27(C) was sorely needed.
3. If you want to spend your customer's money on fully rated feeds where they are not needed, fine. But please don't use the argument that the
breaker might fail and thus set the building on fire. That is simply a specious argument, since the
safety of every code-compliant electrical
system depends on functional overcurrent protective devices. And, to require 125% of the
nameplate rating has no basis in engineering fact or common sense, UNLESS the likely inventory plugged into the
system will load it to 100% and be operated at full for more than three hours. This might happen on a very small
system in a specialized application, but is so unlikely on a normal dimmer-per-circuit performance lighting
system that it is rarely considered.
4. Section 520.27(C) actually contemplates the possibility of overloading the service and
tripping the
overcurrent protective device. That is why
egress lighting must not be
fed from a derated service.
5. Even if 10A Sensor modules were available, you would actually be doing the end user a disservice by using them. Why? You are forcing
system derating at the dimmer level. the user can no longer deploy the available
power where needed, you have forced them into your
derating scheme, rather than theirs.
6. Any
system derating should allow some reasonable overhead and room for expansion, but that is very unlikely to be 100% of the
nameplate rating, except on very,very small systems. It certainly will never be 125% of the
nameplate rating, except as in (3) above.
7. If you have an electrician who refuses to install derated feeds per 520.27(C) and an attorney who supports that view due to "liability", all I can say is that such thinking represents a tiny minority, not supported by facts.
8. Most people who support fully rated feeders to a performance lighting
system fail to realize the impact on the cooling
system for the building. For instance, if there is 50 kW of
fixture inventory in a
system, but 230 kW of
feeder and
dimmer capacity, that can mean 230 kW of air conditioning capacity for the
stage and
auditorium as well. The cost of such an increase in the cooling
system makes the cost of the larger feeders seem like chump change. And these are "empty dollars"--they don't add to the usability of the performance space in any way.
ST
