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## Calculating Switch Power Requirements for VoIP

I am planning for a VoIP install and I need to make sure we have enough power on the 3850 switch to support the phones. The phones consume 13 watts max, switch is 48 ports, there will be a phone plugged into each port, available watts per switch module in 5 switch stack is 420 watts, and each switch has one 715 watt power supply. How do I calculate how much power I will need to support all of the phones? Is it as simple as multiplying the phone wattage by number of ports and comparing to available watts for the switch module?

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## Re: Calculating Switch Power Requirements for VoIP

Hello Mario3,

you would need at least 13W *48 for the phones and you need also to provide power to make the switch to stay alive and function correctly.

13W*48 = 620 W   you would need this power avaiable for POE

if you power supply is only 715W I am afraid you will not be able to put a phone on each port.

I think you would need a 900W power supply as minimum.

To support also more demanding POE devices 1100W power supplies are used.

see the datasheet

https://www.cisco.com/c/en/us/products/collateral/switches/catalyst-3850-series-switches/datasheet_c78-720918.html

for 48 ports POE+ or UPOE you need a 1100 W PS with 800 W available for POE.

Also what happens in your stack if one power supply fails ?

you should deploy redundant power supplies on each switch

Dual redundant modular power supplies

The Cisco Catalyst 3850 Series Switches support dual redundant power supplies.[6] The switch ships with one power supply by default, and the second power supply can be purchased at the time of ordering the switch or at a later time. If only one power supply is installed, it should always be in power supply bay 1. The switch also ships with three field-replaceable fans

or you should use 1100W PS and stack power cables

>>>Cisco StackPower® technology provides power stacking among stack members for power redundancy

Hope to help

Giuseppe

2 REPLIES 2
Highlighted
Hall of Fame Expert

## Re: Calculating Switch Power Requirements for VoIP

Hello Mario3,

you would need at least 13W *48 for the phones and you need also to provide power to make the switch to stay alive and function correctly.

13W*48 = 620 W   you would need this power avaiable for POE

if you power supply is only 715W I am afraid you will not be able to put a phone on each port.

I think you would need a 900W power supply as minimum.

To support also more demanding POE devices 1100W power supplies are used.

see the datasheet

https://www.cisco.com/c/en/us/products/collateral/switches/catalyst-3850-series-switches/datasheet_c78-720918.html

for 48 ports POE+ or UPOE you need a 1100 W PS with 800 W available for POE.

Also what happens in your stack if one power supply fails ?

you should deploy redundant power supplies on each switch

Dual redundant modular power supplies

The Cisco Catalyst 3850 Series Switches support dual redundant power supplies.[6] The switch ships with one power supply by default, and the second power supply can be purchased at the time of ordering the switch or at a later time. If only one power supply is installed, it should always be in power supply bay 1. The switch also ships with three field-replaceable fans

or you should use 1100W PS and stack power cables

>>>Cisco StackPower® technology provides power stacking among stack members for power redundancy

Hope to help

Giuseppe

Beginner

## Re: Calculating Switch Power Requirements for VoIP

Giuseppe,

Thanks! This is what I was trying to confirm.