Rack Elevations and Math

September 21, 2013 in Tech Talk, The Basics by Sam Davisson


I got sidetracked a little bit with the last post on The Decibel. But now, since we’ve covered Power and Grounding, lets go back to the equipment rack. Before we get into the calculations you need to make and share with the electrical and mechanical contractors lets discuss effective equipment rack layout techniques.

What’s the best way to elevate an equipment rack?

I really wish there was a correct answer to that question. The truth is it’s a very fine balancing act. There are a few rules of thumb but all of them are somewhat situational. But I’ll list as many as I know:

  • Install heavy equipment toward the bottom of the rack.
  • Separate equipment by signal type and/or function
  • Evenly distribute the power/heat load
  • Consider cable lengths
  • Ease if install, in other words keep it simple for the the installers to cable
  • Equipment cooling

Taking these one at a time, I’ll try to explain the logic (or at least my logic) behind them:

Install heavy equipment toward the bottom of the rack
A best practice is to build and test systems in your shop prior to delivering on site. This will help decrease the overall install time. It’s much easier to troubleshoot problems off-site and out of the view of the client. But that means you have to transport the racks. Having the heavy equipment at the top of the rack makes the racks top heavy and increases the possibility of an accident during transport. If your lucky it’s only equipment that gets damaged if something happens and not an employee.

Separate equipment by signal type and/or function
There are a number of things you can do to eliminate the possibility of cross talk and signal noise. This is one of those things. Your much less likely to have problems if you bundle cables by signal type. I practice and suggest bundling your cables in this manner. I also suggest that bundles of different types are kept separate by a minimum of 1/2" except for speaker level cables which should be treated like AC power cables and kept a minimum of 2" from any other signal type. Here are my suggestions for cable type bundles:

  1. Analog, line level, balanced audio
  2. Analog, line level, unbalanced audio
  3. Analog, microphone level audio
  4. Digital audio
  5. Analog Video
  6. Digital Video (HD/SDI)
  7. Digital Video (HDMI)
  8. Digital Video (HDBaseT)
  9. Control Cabling and DC voltages
  10. Network Cabling
  11. Speaker level audio
  12. Other – There always seems to be something that fits in this category. Such as how would you classify a cable with Cobranet information. While it is a CAT5e cable it is neither network or control.

As far as separation by function, I like to keep my video processing equipment separate from my audio processing equipment and source equipment. But here is where exceptions tend to creep in. In order to keep the noise level to a minimum you need to keep any unbalanced audio cables as short as possible, preferably under 15′. This will often dictate that some if not all of the audio processing equipment will need to be in the same rack as the sources.

Evenly distribute the power/heat load
The most important aspect to this is rack cooling. By keeping the loads evenly distributed you keep the heat evenly distributed between the racks. This will simplify cooling requirements and make it less likely that equipment will suffer from heat.

Consider cable lengths
I already touched on one aspect of this. Unbalanced audio cabling needs to be kept as short as possible to maintain a low noise threshold. When we get deeper into things like signal and noise ratios (SNR or S/N) I’ll explain in greater depth as to why this is important.

But there are other cable lengths to consider. The majority of HDMI equipment has cable equalization built into it. That equalization is set for an average HDMI cable of 6′.

HDBaseT suffers when cables are bundled depending on the type of CAT cabling being used and the number of cables in the bundle. With CAT5e UTP cable, a bundle of 7 cables will begin to see catastrophic failures at about 35′. CAT6 UTP bundled in the same manner makes it about 50′. When bundling CAT cable carrying HDBaseT information the best method and the only one that allows for the full 330′ extension is CAT6A.

I know, what about STP cabling. If everything is done correctly, this will work. But a broken shield or a noisy ground floor can actually cause your problems to be greater.

Ease if install, in other words keep it simple for the the installers to cable
This one is pretty much self explanatory, I think. Racking equipment of similar signal types makes building the rack simpler because the installer isn’t jumping from cable type to cable type. It also makes for a cleaner installation.

Equipment cooling
Keeping the equipment cool prevents failure. More importantly it prevents intermittent errors that can be difficult to track down. Your overall rack plan should include looking at where the rack is being installed, how it is being cooled and the method of cooling.

Elevating the Racks
The first thing I do is download all of the equipment specifications for the equipment contained on the Bill of Material (BOM) or estimate. I usually separate them into two folders, field equipment and rack equipment. Then I’ll create a spreadsheet listing equipment, height (in RU’s (rack units = 1.75"), badged current draw, efficiency current, idle current and any other pertinant information I can think of at the time. Badged power is the maximum current draw when a piece of equipment is energized. Efficiency power is the amount of current draw after the equipment is energized and has settled. Idle current is used for some displays and amplifiers and indicates the amount of current drawn when the equipment is not in use.

I also have a couple rules of thumb I use when it comes to "head room" when elevating a rack and calculating the amount of current I’ll need to power the rack. I like to keep my "head room" at 20 – 25% of capacity. So, on a 44RU equipment rack, I try to maintain 9-10 blank spaces for future growth. With my power circuit, I’ll try to keep power at 15 – 16 amp draw on a 20 amp circuit.

Discounting amplifiers, electronic equipment uses power pretty efficiently. I calculate that efficiency at 2.5. So to calculate efficiency current, divide the badged current by 2.5. In other words a piece of equipment that has a 10a fuse will draw 4a after the initial power surge. This is an estimate and not an exact measurement. That’s where your headroom comes into play. It’s your safety factor. Using a power sequencer (a device that delays the initial power rush for equipment) you can effectively exceed the current draw of the equipment over the badged listings to about 20 amps and allow you to maximize the amount of equipment on a single circuit.

Rack Math

I guess it’s time to talk about that math I promised. Everyone who creates rack elevations should be familiar with Ohms Law and how to calculate power usage. Additionally they should be able to use these tools to calculate the heat load in BTU’s for a specific equipment rack. BTU stands for British Thermal unit. Our good friend Wikipedia can explain why it’s named what it’s named. I’m happy to not explain it

Ohms Law – V(voltage)=R(resistance)*I(current)
Power Calculation – W(power expressed in Watts)=V(voltage)*I(current)

Heat Load Calculation =
3.41214 x W(power expressed in Watts) BTU/hr

Once you’ve elevated your racks based on the criteria and exception already noted your ready to calculate power requirements required by the electrical engineer and the heat load needed by the mechanical engineer.

I wish I could give you some hard and fast rules as to when to use a power sequencer and when not. I try to use them whenever there are going to be extended times when the equipment is not going to be in use. This allows you to "power down" the rack until it is needed, saving energy. If you are using a power sequencer then for every 20 amps of power you will need one 20a circuit. If not, then for every 15 amps of power you’ll need one 20a circuit. In either case, with equipment energized and stable your rack should be drawing roughly 15 amps.

Calculate the power
Power Calculation – W(power expressed in Watts)=V(voltage)*I(current) or Power (w) = 110v (US) x 15a. Maximum power draw is 1650 watts.

Calculate the Heat Load.
One would think, looking at my my formula’s above that the heat load calculation would be 3.41214 x 1650w = 5630 BTU/hr. But that doesn’t take into account efficiency. Remember the equipment will only be drawing 1650w for a very short time. So divide the 5630 BTU/hr by equipment efficiency of 2.5 and your estimated heat load should be 2252 BTU/hr.

Please note amplifiers play by different rules and will be covered in their own discussion when we can discuss things like different amplifier classes.

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