Voltage Drop

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kody916

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Per NEC when we have to size a wire (say for a motor), we use the motor horse power table to get the Full load Amps. We then multiply the FLA by 1.25 to get the final amps and select the wire size based on the final amps.
But lets say Voltage drop is an issue here as the motor is at a fair distance from the source.so when we do the voltage drop calculation do we take into account the the FLA of the motor or do we take into account the final amps (which we get after multiplying FLA by 1.25)

Thanks
 
There are a couple of things to consider. I typically just check the voltage drop using the FLA, but if you're concerned about a long feeder/branch circuit, you might also want to check the voltage drop using the LRA (Locked Rotor Amps). The LRA is the maximum current you should see during motor startup. It will have a very short duration, but because it's a relatively high current, it will typically cause a momentary voltage dip. This can lead to things like dimming/flickering lights, etc. So, if that's your concern, check the voltage drop using the LRA also.

To my knowledge, there's no reason to calculate voltage drop using FLA x 125%.

Others here know a lot more about motors than I do, and I'm sure you'll hear from one of them soon.
 
FLA is pretty standard in my book and design for 3%. Just remember if you increase circuit conductor size, be sure to increase EGC size appropriately . If you design for LRA things get expensive and is over kill. If you use LRA use 5% for VD or else you will likely just be throwing money away.
 
I agree with both Dereck and JD, in that I would calculate the VD using FLA. However, to add to their comments, I would say that if you wish to take starting conditions into consideration, I don't think you have to size the wires for a 5% maximum VD. I base that on 695.7, which gives the requirements for VD for fire pump installations. It allows a VD of 15% for the starting transient of a fire pump. So I would think you could use that same 15% as the design criterion for starting VD for other motors as well, if you wished.
 
charlie b said:
I agree with both Dereck and JD, in that I would calculate the VD using FLA. However, to add to their comments, I would say that if you wish to take starting conditions into consideration, I don't think you have to size the wires for a 5% maximum VD. I base that on 695.7, which gives the requirements for VD for fire pump installations. It allows a VD of 15% for the starting transient of a fire pump. So I would think you could use that same 15% as the design criterion for starting VD for other motors as well, if you wished.
Click to expand...

I agree with this approach. Also note, that if you're sizing voltage drop for standby generators, it's not uncommon to use 30% voltage drop under starting conditions, which implies even more tolerance. However, 15% seems like it would be a conservative target.
 
The allowable voltage drop when starting a motor should take into account the sort of load being placed on the motor.

Remember that torque scales as the _square_ of the applied voltage. So if you have a 30% voltage drop you will see a 50% drop in available torque.

However many loads do not require much torque at startup, and the reduced voltage means reduced locked rotor current. Reduced voltage starting is often intentionally used to reduce starting current. 'Wye start - delta run' is a common technique which intentionally reduces the voltage placed across the coils, by about 42%...so clearly a voltage drop of 42% can be tolerated in some applications.

But if the load requires full available starting torque, then you will need to be careful about voltage drop at locked rotor.

-Jon
 
There are two position of voltage drop one has to check: steady state and start.
Steady state overloading has to be for a limited time then in my opinion may be neglected.
Starting current is different. First of all there are-still until now!-induction motor wounded rotor and squirrel-cage rotor.
Squirrel cage may be simple, double, deep bars, copper or aluminum and so on.
Starting method could be: D.O.L. [direct on-line], star-delta, stator resistances, autotransformer, soft start, VSD [VFD] and other.
Different constructions, different start methods: the permissible voltage drop [percent] could be very different.
If high start torque is required -for all other methods without frequency control
-the starting torque is proportional with the voltage square then 30% voltage drop
will reduce the starting torque to 50% of the rated voltage starting torque.
Our practice-in a power station where most of motors are D.O.L. started- based on minimum starting torque at the rated voltage according to IEC 60034-12-and limiting the actual starting torque to the rated torque[at rated rpm] is as follows:
Up to [hp]: 12.5 30%
Up to [hp]: 50 25%
Up to [hp]: 75 20%
Up to [hp]: 200 16.3%
upper: 9.6%
 
From a practical point of view it depends if one is considering a FEEDER that also supplies lighting, or a branch circuit that ONLY supplies the motor.

In the case of a feeder that also supplies significant domestic or commercial lighting, then in my view the voltage drop under motor starting conditions should be limited to about 3%. Any more than that is likely to cause obtrusive lamp flicker.

In the case of a branch circuit that only supplies the motor, then I would aim for about 3% voltage drop under running conditions, subject to the drop when starting not being so great as to impair proper starting.

A little common sense can be applied if the lighting load is small, unimportant, or tolerant of varying voltage.
For example a feeder to a distant pump house might also supply a lamp and a couple of seldom used outlets for maintenance or repair operations. In that case, sizing the feeder for 3% drop under running conditions should be fine. Who cares if a seldom used lamp dims noticeably when the motor starts ?
But for a home or office, no way, even 3% drop during frequent motor starts can be displeasing.
 
You need to read NEMA MG-1, in general, and specifically 12.44.1 and 12.44.2, which is the running and starting variations from rated voltage and frequency.

A running motor is typically capable of +/-10% voltage variation with rated frequency. Keep in mind that is motor rated voltage not system. So, if you have a 480V nominal system and a 460V rated motor than the motor can tolerate 506V to 418V. For the system, that's basically 5% above nominal and almost 15% below nominal.

You can see where the numbers start to come from. So, if you limit the starting voltage drop to 15% of nominal system voltage, you are within the range of the motor (running) but, still need to be aware that the starting torque under load conditions needs to be verified. This could require the voltage drop on starting to be less than the 15%. Special motors can be specified that have greater tolerances, but in general, this is what you are up against.

Size wire based on FLA and limit it to say 5%, of nominal system voltage (not motor rated voltage). Then check to see what the voltage drop will be using the LRA. If it less than 15%, your probably good to go, but would be wise to check the torque speed curves to make sure you can overcome the starting torque at that voltage. If you are greater than 15%, than increase cable size until you get it within the desired starting VD.
 
Actual voltage drop will depend on actual load.

Design voltage drop should be dependent on load level you wish to to design for, this could include some consideration of starting current or temporary overload conditions.

NEC does not tell us how to determine voltage drop, only gives us an informational note that suggests that we take voltage drop into consideration
 
To all who calculate Vd at locked rotor

Q: Do you consider that at locked rotor the load is nearly all reactive? I have not looked at it very hard, but won't the IR drop be at 90 degrees to V.

I don't have a code book in front of me, I think it is in the notes of table 9(?).

ice
 
iceworm said:
To all who calculate Vd at locked rotor

Q: Do you consider that at locked rotor the load is nearly all reactive? I have not looked at it very hard, but won't the IR drop be at 90 degrees to V.

I don't have a code book in front of me, I think it is in the notes of table 9(?).

ice
Click to expand...

That is a good point. Typically, for fire pumps, I will calculate the locked rotor voltage drop assuming a 0.3 power factor to account for the reactive nature of the load at this condition.
 
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