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Question for you Electrical Engineer types

Westy

the teste
Nov 22, 2002
36,254
2,985
Sleazattle
I need to determine what the power line requirements for a system will be. Our in house EE is a small wire guy and he just shrugged his shoulders when I asked. For a 240V 3 phase system my long term RMS current pull is 5 amps but with a peak draw of 30 amps that may occur for a half second every 30 seconds. Do I tell the facilities guy a power requirement based off of the RMS value, peak or somewhere in between? I was thinking I should calculate the RMS value for a particular duty cycle but wasn't sure what time frame to work with.
 

stoney

Part of the unwashed, middle-American horde
Jul 26, 2006
13,927
1,307
Colorado
Always assume for the max power draw. *not an engineer - just using common sense*
 

Westy

the teste
Nov 22, 2002
36,254
2,985
Sleazattle
Always assume for the max power draw. *not an engineer - just using common sense*
Electrical circuits can handle extraordinarily high currents for short periods of time, especially when an AC signal is charging a DC bus. Current is only drawn when the AC line voltage exceeds the bus voltage. Something that draws an average of say 2 amps will actually draw high currents for short periods of time. Just measured 50 amp pulses on a device that pulls 2 amps continuous.
 

stoney

Part of the unwashed, middle-American horde
Jul 26, 2006
13,927
1,307
Colorado
Electrical circuits can handle extraordinarily high currents for short periods of time, especially when an AC signal is charging a DC bus. Current is only drawn when the AC line voltage exceeds the bus voltage. Something that draws an average of say 2 amps will actually draw high currents for short periods of time. Just measured 50 amp pulses on a device that pulls 2 amps continuous.
I have no idea what that means.
 

heavy metal

Monkey
Mar 31, 2011
193
3
HI
He's pointing out that AC current is in a state of rapid fluctuation. Think of a sine wave. I would think you could work from RMS values? But don't quote me on that.
 

noskcaj

Monkey
Oct 24, 2005
106
0
Northford, CT
I'd shoot for something between the peak and RMS value, leaning more towards RMS. The current rating is based off the thermal limit of the wire at that voltage, etc. So, if you only have a high current burst for a short amount of time, the wire will not heat enough even if its undersized

But, if you size the wire too small you will have a noticable voltage drop across that wire when it tries drawing the 30 amps depending on its length, gauge, material, etc. I'd check the local commercial electrical codes first though, the worst would be installing wire for RMS when code calls for peak.
 
Going with RMS +50% 30KVa.
So you're telling your facilities people that you need a three phase 240VAC feed to power your equipment, correct?

I don't think your estimate's too far off - If you're pulling 30A periodically, you probably want a 50A feed so as to avoid nuisance breaker tripping. N8 would probably give you a better answer than I can - I think his specialty is power transmission.