voltage drop calculation
- Thread starter fireryan
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iMuse97
Senior Member
- Location
- Chicagoland
Use a "reasonably calculated" maximum load. Key word is maximum, because you want to be able to supply adequate voltage at maximum current draw.
kwired
Electron manager
- Location
- NE Nebraska
Voltage drop will be directly related to how much current is flowing. How much current is flowing is not related to what the setting of the overcurrent device is. It will effect how long until the overcurrent device opens if it is over the setting.
makes sense. Thanks
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
For worst case use the max allowable loading on the breaker, i.e. 80% of the fixed trip setting for non 100% rated breaker.
You can draw 100% of the rating of the breaker as long as it is not continuous, three hours or more.
cdcengineer
Senior Member
What's the consensus on considering power factor when calculating voltage drop? I have a 700+ ft. run from a generator with a PF of 0.8. This dramatically increases the 200A, 208V, 3-phase voltage drop calculation.
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
KW is not equal to KVA (except for purely resistive load), so you have to use KVA (which means pf).
How much does the power factor change as a result of voltage drop over the length of wire? That is, for the sake of comparison, what is the difference in circuit power factor between 10' vs. 700'...?
cdcengineer
Senior Member
It's not a matter of the PF changing with distance. The source is a generator which the mfg tells us has a PF of 0.8. Therefore if I know my load of 200A, 208v, 3-phase I can calculate voltage drop at 700+ ft. I was taking into account the source PF as my load PF is negligible. Perhaps I'm looking at this wrong?
I'll wait for you to come up with a formula that figures that one out, and how it affects voltage drop!!
Power factor does affect voltage drop slightly. For example, 1 amp at unity power factor won't produce the same voltage drop as 1 amp at 80% PF. That's why my Square D slide rule has voltage drop for 95% PF, and 80% PF.
To make it more confusing, for smaller wires, 95% PF causes more voltage drop, while 80% PF causes more voltage drop at 3/0 and above.
But many formula's ignore the power factor, and still give results that are accurate enough for most applications.
It's the load that determines PF, not the generator spec. In most cases, just make sure you use the max. current you might have, and you usually don't have to worry about PF.
cdcengineer
Senior Member
Thanks for the input Steve. I think we will use the max connected load and I'll factor in 95% PF while considering that the nominal output of the gen-set is nearly 217V on the 208V system. At the higher output voltage, the voltage drop will be slightly offset due to the higher source voltage. The net result is like considering unity PF. Most systems and equipment can handle +/- 5% variance in voltage.
I'll advise you not to hold your breath while waiting... :huh:I'll wait for you to come up with a formula that figures that one out, and how it affects voltage drop!!
handy10
Senior Member
- Location
- western suburbs, chicago
Th formula usually involves the current in the wire, the length of the wire and the resistance per unit length for the specific wire (see NEC chapter 9, table 8). The formula will be
voltage drop= (res/km)xlength(in km)x(max current) i.e. V=RI
If you know the max current, the power factor has no bearing on the voltage drop.
Not totally true. See NEC chapter 9, table 9. The wiring method affects reactance and thus impedance. The table even shows impedance for a power factor of 85%.
That said, the difference in voltage drop caused by the wiring method reactance and subsequent change in impedance is usully small and disregarded.
kwired
Electron manager
- Location
- NE Nebraska
It may depend on what the reason you are trying to account for / correct voltage drop may be. I once hooked up x-ray equipment at a local medical facility. If you consider what size of wire would be allowed to be used on almost any other load with the same current at the same duty cycle you may likely have been able to feed this equipment with a 6 or 8 AWG feeder or even smaller without any concern of overheating of conductor. Something like maybe a welder would be a good comparison.
Because the equipment was supposably sensitive to voltage drop, it was specefied to use 1/0 copper conductors for the length of the feeder that we were using. This was to make sure voltage dip during inrush current portion of an x-ray did not drop below their recommended levels.
Bottom line is having a load is what causes the voltage drop, not the size of overcurrent device.
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