# How do dual power supplies behave

When a server has two power supplies that are fault tolerant, how does it draw power?  My understanding is that it draws roughly half from each, and if one fails, the other goes to full load.

So if a server is drawing 2 amps on each power supply, when one power supply fails, the other would begin to draw 4 amps.

Is this correct?
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Commented:
Sorry - And yes, if you cut the power to one distribution unit, the other one will be powering everything, so it will need to be rated above 14A!
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Commented:
Not entirely...
A redundant power supply will draw a bit of power to maintain operations of the circuits and fan...
When the primary ps dies or is removed then the secondary will pick up and it will draw almost the same amount of power as the primary.
What you need to do for planning purposes though is have two separate circuits for each power supply, so if one circuit fails the other will pickup...so in that case you need to say that the server needs two separate 30 amp circuits.
Does that make sense?
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Commented:
Depends on the server. Dells for example, at least 2800 and above split the load

Otherwise standard is for redundancy. One fails, the other takes over. Making it hotswappable to replace if one goes down without needing to shut down the system
Redundant power supply theory of operation: Each Redundant Power Package consists of the two power supplies installed in either a Rack Mounting or Wall Mounting assembly. The output voltage of the primary supply is set approximately 0.2 volt higher than that of the backup supply. Under this condition, the backup supply's diode is not forward biased; only the primary supply delivers current to the load. If the output voltage of the primary supply decreases by more than 0.2 volt, the situation is reversed and only the backup supply delivers load current. There is no interruption of output power during the transition.
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Commented:
In the main, the load iwill be balanced equally between the two PSUs to give the best service life for each (rather than running one at full whack, and the other mosly off), but each will be capable of running the server by itself should the other fail.

Hope this is what you're after.

Cheers.
Steve.
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Author Commented:
This is the situation.  We have two 20 amp circuits in a rack.  To meet code, we can only run the circuits up to 16 amps.

Currently, we have 5 servers in the rack, and each PDU is running at about 7 amps.  All of the servers have dual power supplies, and the two power supplies on each server are plugged into separate PDUs.

Now, let's say we cut the power to one of the PDUs.  Does that mean I'll now be drawing about 14 amps on the PDU that's still live?
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Author Commented:
They're Dell 1950, 2850, and 2970 servers.
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Commented:
You will need to make sure that any circuit can support the server running on one PSU, based on the assumption that if.when one fails there won't be any overload of any circuits.
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Commented:
Also remember that your rated amperage is also based on your voltages that you use for those devices...
total wattage divided by voltage will give you your amperage.
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