It’s a basic question that I get at least weekly. When estimating cabinets (and PDU), there are physical considerations (space, weight, and design features) and power/cooling considerations.
This is a physical space limitation. Rack Units (RU or just U) are 1.75” measures usually having 3 holes each for mounting equipment. For example, a 45 U cabinet has rails that are 78¾ long with 135 holes for mounting equipment. However, you probably can’t mount 45 1 RU servers in the cabinet because you may need to reserve some space for switches in the cabinet or save an RU at the bottom to tuck excess power cords or an RU elsewhere to pass cables from back to front.
On the plus side, most cabinets have racks (a pair of rails is called a rack) in both front and back so there may be some things that can be mounted behind others to increase capacity; but be careful. The front and back rails are generally 29” apart. Some equipment is more than 29” deep or close enough that there is no space behind it. Some equipment pieces (mostly old stuff) require rails to be closer or further apart. It is very frustrating to find two pieces going in the same cabinet that require the rails to be at different distances.
You can estimate the number of cabinets by adding up the RU sizes of all the physical equipment and dividing by the cabinet height (less a few RU for switches, management, and cable tucking).
Most raised floors have structural limits for the weight they can support. The floors may even vary from point to point. The weight of the cabinet and its contents may not exceed the specified weight per square foot where it is placed. Be careful of the capacity of any flooring that you may have to roll the cabinet across when moving. Watch out for ramps and other uneven floors.
Other than usable RU space, it is important to have space for various cabling and fittings. If some units require cabling in the front of the cabinet, how will it get from front to back? Is their enough cabling that you need horizontal and/or vertical wire managers? Remember to calculate the amount of space that will be needed for patch panels and/or top of rack switches. Do you have other needs to leave certain RUs open?
Remember to calculate the number and type of power cords to assure you have enough PDU outlets. Be sure the PDUs and cabinets you select will match. This is especially important if you have specific requirements for PDU placement. Some cabinets are designed to have dual PDU in certain places that can make cabling difficult. Will the number or placement of PDUs block cable paths?
Power provided must be matched with power required or demand. The demand side is important for calculating what will fit in a cabinet. Running power demand should not exceed 80% of breaker supply. The subject of selecting the right PDU configuration is a whole separate topic.
The demand side is a little more complicated. A server with two 600W power supplies units (PSU) usually means that either can provide up to the 600W the machine may require. Note that to provide 600W of power the supply usually uses 10% to 20% more power than stated. That is not the reason for the 80% factor in the supply computation. Most electronics require several seconds of higher demand during start-up; that is the reason for using 80% of peak for the running load. Some equipment may spike to a very high Wattage for a split second when first powered on; this is usually not long enough duration to trip a breaker; but most equipment needs several seconds of additional start-up to accelerate disks, fans, etc., and to charge capacitors, etc. That is the purpose of the 80% factor; because the several seconds are enough to trip a breaker.
Most facilities provide two power sources, so you probably need to have two PDU in each cabinet. Each PDU should be able to support all the equipment in the cabinet because facilities occasionally need to shut down each of the sources for testing and maintenance.
If you have equipment with single PSU, you may require automatic switching units (ASU) to assure operation when either power source stops. Remember to provide space for them in the cabinets. Colocation facilities usually shut down
If equipment has more than two power supplies, you must refer to its documentation to determine its true running load requirements. Also, the 600W PSU demand is referred to as the “plate” value. Most equipment functions somewhere between 25% and 80% of the plate value; this is called the “derated” value. To safely use derated values, you must be familiar with how the individual pieces of equipment are being utilized.
When selecting PDU it is important to cover the number and type of receptacles you will need in each cabinet. If your PDU does not have any 110V receptacles, you may occasionally need to provide a separate low voltage PDU. You should also note that PDU generally have multiple internal breakers and require high loads to be evenly distributed.
Many data center facilities have a maximum cooling rate either per cabinet or per square foot. Heat dissipation is generally 3,412 BTU/Hr for 1 KW of running power. There are small deviations for POE that dissipates at the device instead of in the data center; but it is usually immaterial.
Check with facility engineering for overall cooling limitations. They can generally give you a BTU/Hr per square foot. Remember to count the square feet under and around your cabinets. For example, a 7KW cabinet that is 48” deep and 30” wide in a datacenter with 4 foot aisles is OK if the data center allows 1200 BTU/Hr / SqFt. (30” is 2.5’. 48” is 4’. Add 2’ for half of each aisle and you get 8’. 8’ times 2.5’ is 20 SqFt. 20 SqFt times 1200 BTU/Hr is 24000 BTU/Hr allowed. 7 KW times 3,412 BTU/Hr = 23,884 BTU/Hr which is under the limit.) A 42” deep and 24” wide cabinet would be limited to 5¼KW in that data center.
Also remember that the facility limit can average across several cabinets. If your average BTU/Hr/SqFt is still too high, you may be required to leave space around your cabinets (to protect other’s equipment). Rather than renting empty space, you may prefer the cabling flexibility larger cabinets give.
Having said that, if your BTU per cabinet is too high, you may have to get cabinets with special cooling (to protect your equipment). Depending on the air circulation in your data center, you may want to put temperature sensitive equipment low in the cabinets and put less sensitive equipment higher. (Many servers require <70°F; many switches can function at 104°F.
Estimating the number and type of the cabinets you need can range from very straight-forward to very complicated. You should be paying attention to RU capacity, weight, features, power, and cooling as you make decisions since different cabinets in a data center may be limited by different factors.