Variable input current

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Electriman

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Greetings,

I am dealing with a load that draw 900A at 480 VAC and 1050A at 450 VAC. 450 VAC is in the range of line voltage variation 480+/-10%, so my question is to size OCPD do I have to consider 1050A or 900A?

Thanks in advance.
 
Seems a little suspicious that it would vary >15% in current based on 6% voltage change. What's causing the massive change in load?

In a normal situation you size at the nominal. If this is legit and you could regularly see dips going doing to 450V and causing your load to shoot up that high, I would say it falls into the design issue realm. Do you want it to trip at that point or not?
 
bwat said:
Seems a little suspicious that it would vary >15% in current based on 6% voltage change. What's causing the massive change in load?

In a normal situation you size at the nominal. If this is legit and you could regularly see dips going doing to 450V and causing your load to shoot up that high, I would say it falls into the design issue realm. Do you want it to trip at that point or not?
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I may exaggerate about the numbers but my point is since it is ok for the utility voltage to change +/-10%, consequently the input current also varies i.e. VFDs,... Now my question is do we size the OCPD per worse case or per nominal input current which is the input current at 480 VAC?
 
Electriman said:
Greetings,

I am dealing with a load that draw 900A at 480 VAC and 1050A at 450 VAC. 450 VAC is in the range of line voltage variation 480+/-10%, so my question is to size OCPD do I have to consider 1050A or 900A?

Thanks in advance.
Click to expand...
You said 450 VAC is within the range what's the actual voltage?
 
The devil may be in the details in selecting the correct OCP. What type load ? motors involved ? continuous or not ?
nameplate data ? all those play a part.
 
augie47 said:
The devil may be in the details in selecting the correct OCP. What type load ? motors involved ? continuous or not ?
nameplate data ? all those play a part.
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It is a power electronic load (power supply), it is continuous, no motor is involved and since it is a custom power supply the manufacturer said 900A at 480 VAC and 1050 @450VAC.
 
Electriman said:
It is a power electronic load (power supply), it is continuous, no motor is involved and since it is a custom power supply the manufacturer said 900A at 480 VAC and 1050 @450VAC.
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Are the currents that you are quoting the maximum that the power supply is capable of handling, or are these currents specified with a load on the output of the power supply which the manufacturer is providing?. The overcurrent device rating should be based on the total load current and not on the maximum current capability of the power supply.
 
synchro said:
Are the currents that you are quoting the maximum that the power supply is capable of handling, or are these currents specified with a load on the output of the power supply which the manufacturer is providing?. The overcurrent device rating should be based on the total load current and not on the maximum current capability of the power supply.
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The load can be as high as the maximum current. Also when you size a OCPD for VFD we consider the input current f the VFD and not the load current.
 
As far as the NEC is concerned, nominal voltage is normally used for load calculations. 220.5.

For motors and appliances, it may be the utilization voltage that is used for nameplate info (480V motors are figured at 460 volts for example.) But that would already be included in the MCA, or in the NEC tables for full load current for motors.
 
Electriman said:
The load can be as high as the maximum current. Also when you size a OCPD for VFD we consider the input current f the VFD and not the load current.
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The listing of VFDs often specifies appropriate overcurrent protection devices . Manufacturer's instructions regarding OCPDs would need to be followed.
 
The question in post 3 is important ( about a 15% current change with a 6% voltage change).

You state:
Electriman said:
I may exaggerate about the numbers but my point is since it is ok for the utility voltage to change +/-10%, consequently the input current also varies i.e. VFDs,... Now my question is do we size the OCPD per worse case or per nominal input current which is the input current at 480 VAC?
Click to expand...

If you have a constant power load, then current change should mirror voltage change, eg. voltage down by 10%, current up by 10%.

Since you are generally required to size OCPD at 125% of a continuous load, I would have no problem sizing the circuit for the _nominal_ voltage and current. If the utility runs at 10% low, the current will be 10% high, but you will still be below the ampacity of the conductors and the OCPD.

Others will have more information, and I wouldn't trust free internet advice to size a 750 kW feeder!

-Jon
 
NEC rules are all based on nominal voltages and rules for things like OCPD sizing and conductor sizing are purposely conservative in order to allow for expected voltage variations.

If this is for Power Conversion Equipment, normally a responsible mfr would tell you their recommended OCPD, or if it is UL listed, that information would be part of their UL listing requirement. Often times they will give you a "not to exceed" value for the different OCPD options and if so, then it becomes up to you to assess your level o risk of nuisance tripping vs sizing and protecting the conductors feeding it. If they gave you nothing, I'd venture to guess this was not a UL listed power supply and deciding what to use is going to be tough.

But Hv&Lv is right, their input TOLERANCE is different from what the utility is allowed to deliver.
 
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