Technical Panel Load Calculation

[caldesac]

Hello,

I'm trying to calculate loads for a/v equipment at both the circuit level and the panel level. My fundamental question is this --

I have obtained/extrapolated amperage information for all of the equipment in the racks. However this gives me a number that seems to represent the cumulative peak/max amperage level and is unrealistically high. If I allocate circuits and panels according to this I think I end up with way too much headroom, which isn't bad except for...

At the circuit level I would ideally like to consolidate equipment to as few circuits as is reasonable. This is for avoidance of noise issues in spreading related equipment over many circuits.

At the panel level I want to ensure that I consolidate the related equipment into one panel.

Is there a calculation that I can do that will give me a more realistic load calculation that I can plan and allocate with?

Thanks!

Chris

 

[iwire]

Are you allowed in your area to do these load calculations?

Unless equipment will not be operated at the same time you must use the total based on the manufactures ratings.

Your right this will likely result in more 'headroom' than you really need but it's the same for every NEC compliant installation.

My suggestion is to hire a design build electrical contractor or an engineer familiar with A/V system requirements.

 

[RayS]

If this is for mobile, I'd measure a few gigs and see what ya average, then go up from there.

I guess this is a fixed install, so you have to project the total. Don't forget turn on surge- if you can, sequence the starts- if you can't, you may have to use nameplate amps for the draw. Assuming they will not all turn on at once:

I'd take nameplate amps from all the sub/bass cabinets (they may see continued peak draw if the signal is compressed and/or clipped badly) then use about 1/3 for the remaining amps.

To figure average draw, take the req'd spl, use the speaker efficiency (which may be lower when they're hot) to figure watts required, then assume enough amplifiers running full on to supply this. Add at least 50% more.

Remember that the gear will work best when there is little droop, so if the runs are long, upsize!

 

[iwire]

I'd take nameplate amps from all the sub/bass cabinets (they may see continued peak draw if the signal is compressed and/or clipped badly) then use about 1/3 for the remaining amps.

Not saying this may not work but I don't believe you can do this if your planing on following the NEC.

 

[petersonra]

I don't see how the number of circuits affects the noise level at all.

Are these going to be plug in devices? If so, I don't see how there is a code issue in making some kind of educated guess as to what the current draw will really be.

 

[caldesac]

Are you allowed in your area to do these load calculations?

I'm not sure exactly what you mean by "allowed". There will be an electrical engineer on the project. I'm not trying to play that role in the slightest. I'm giving guidance specific to A/V systems based on my facilities experience. I don't have the authority to specify that the engineer that is ultimately involved has had A/V systems experience. In past similar experiences I have been frustrated with the general inclination of the engineers I've worked with to not care much about the A/V aspects. They have generally just wanted to "over"power and call it a day. Maybe I've had bad experiences, but that's been it. Just trying to be diligent and give as much guidance as I can. It is my responsibility to hand over the amperage calculation and I want to make sure that it's a representative one.

So can I take away from this that I'm doing the calculation the correct way?

Thanks,

Chris

 

[iwire]

Are these going to be plug in devices? If so, I don't see how there is a code issue in making some kind of educated guess as to what the current draw will really be.

Outlets for specific appliances or loads must be calculated based on the nameplate or load

In the 2002 it is 220.3(B)(1), I think it has been relocated in the 2005.

 

[caldesac]

I don't see how the number of circuits affects the noise level at all.

I've read this in a couple of places, but my info on this is quoted here, but can be read in full at

http://svconline.com/mag/avinstall_keeping_quiet_2/index.html

"The idea is that differences between two devices puts stress on their connective as well as internal wiring. The differences come from voltage and noise variations. The problems come from gear with poor internal grounding and from wall warts placed too close to unbalanced wiring. Even a small audio system can have problems that start when multiple power strips are haphazardly plugged into different wall outlets. Distribute power from a single outlet, and chances are most of the noises will go away. This test is safe assuming that you know the system will not exceed 75% of a 20 amp breaker (15 amps) and that nothing else is on that circuit."

Are these going to be plug in devices? If so, I don't see how there is a code issue in making some kind of educated guess as to what the current draw will really be.

Yes, they are plug-in devices. I wasn't looking at it from the perspective of code (not that I don't care!), but rather from the purely functional aspect. I don't want to underpower the system either. Therefore if I have a bunch of equipment with v/w/a, how might I arrive at this educated guess that you speak of? That's my main question.

Thanks very much!

-chris

 

[RayS]

hmm, don't think the code requires max nameplate amps to be used for everything in a rack. Think about a server rack for computers, or a wall full of displays in a store. I can't think of a specific rule in the NEC for not allowing the diversity of this type of load.

However, I need to sharpen up my code referencing so I can post with more specifics.

 

[RayS]

actually 640.22 says
...the supply cords or cable...shall have sufficient ampacity to carry the total load connected to the equipment rack...

so, maybe ya gotta size it using total of nameplate ratings like Bob posted.

 

[edamico11]

Caldesac

Caldesac

...Distribute power from a single outlet, and chances are most of the noises will go away. This test is safe assuming that you know the system will not exceed 75% of a 20 amp breaker (15 amps) and that nothing else is on that circuit"...

I do not understand how this is possible in most situations. I deal with mobil video and audio racks, and I need a min.. of (4) 20 amp circuits. The key to no hum or noise is: 1) "clean power" (make sure you are the only load on the phases you are using,, if possible, 2) All equipment is grounded properly and 3) proper Line isolation if needed. I always use the nameplate on the gear to calculate load. Just because it is A/V gear does not make to immune to the NEC.