 Fan Tray WS-C6509-E-FAN 188.00 642.00 Total 2,941.83 10,059.96 Power based on AC Power Supplies Figure 33-7. Power and BTU values for a 6509E Documentation Remember that there will be limitations regarding how many devices can be placed in a rack. To illustrate this point, I've built a rack full of 1-RU servers. The specs and rack layout are shown in Figure 33-8. Assume that in the collocation facility where this rack is located, we have been given a limitation of 20,000 BTU per rack. The AC power per rack is limited to two 20-amp PDUs, each containing eight outlets. Ethernet patch panel - 48 port Ethernet patch panel - 48 port Each server Draw: 460W (100VAC@4.6A) Max BTU/hr: 1,570 12 servers - 1U each 12 servers Draw: 5,550W (100VAC@4.6A) Max BTU/hr: 18,840 (1570x12) Power - two circuits (A+B), two 20A PDUs on each Figure 33-8. Figuring out how many servers can fit in a rack When you first look into 1-RU servers, they may seem very appealing because in the- ory you can cram 30 or 40 of them into a rack. However, that's not usually the case in the real world. In my example, each server produces a maximum of 1,570 BTU per hour. (You should always plan based on the maximum numbers provided.) Some sim- ple math shows that 12 servers will potentially produce 18,840 BTU/hr. Based on heat output alone, therefore, we cannot have more than 12 of these servers in a single rack. Looking at power, more limitations become apparent. Because each server draws 460 watts (100 VAC @ 4.6 amps), we should really only plug four or five servers in each PDU (not six). The easy way to figure out power limits is to use the amperage num- bers. If each PDU can only support 20 amps, and each server will use 4.6 amps, you can only fit 4.34 servers in each PDU. Look carefully, though-the specs indicate that the server draws 460 watts when using 100-volt AC power at 4.6 amps. In the U.S., our power is usually 120 VAC, not 100 VAC, SO these numbers will change a bit. What happens if we change the voltage and keep the watts the same? If W = A 