Calculating voltage drop for a motor

[sheryf]

One of my apprentices just took his exam and asked me about how to properly calculate voltage drop when a motor is involved. I though I was an ace and quickly realized just how confusing it can be. Now I am lost too. Can you help. Here is how the question read:
What size THHN conductor is required for 3 phase, 5hp, 460V, Design E, PF .20, induction motor when a distance of 405'exists between the motor and MCC Line Voltage is 460V?
I was searching all over for a VD Q&A with a motor involved and had no luck. One website calculated it out utilizing the Locked Rotor Current for the VD calc. I guess we are hung up the most on the 125% FLC rule for the conductor. I can't wait to see how easy one of you makes this for my poor sorry self. Thanks

 

[Ed MacLaren]

Re: Calculating voltage drop for a motor

It is a poorly worded question.

When calculating line voltage drop, the type of load is not a factor.
Only the current flow in the circuit, and the conductor impedance affect the circuit voltage drop.

This is really a question about motor full-load current, and by the way, the power factor given is much too low for an actual (realistic) installation.

Ed

[ April 30, 2003, 07:40 AM: Message edited by: Ed MacLaren ]

 

[ron]

Re: Calculating voltage drop for a motor

The first step is determining the FLA. Then the calculation definitly involves the PF. Consider the code regarding the permitted voltage drop for a fire pump. The tough part of that calc is to remember the starting power factor and include its detrimental effects. PF=.2 is low, but some motors start with a very low PF then recover quickly.
See http://ecmweb.com/ar/electric_determining_submersible_pump/index.htm for an example. It indicates running power factor, but the idea is similar.

[ April 30, 2003, 08:12 AM: Message edited by: ron ]

 

[sheryf]

Re: Calculating voltage drop for a motor

I may have translated the PF incorectly. My apprentice was a little rattled after getting back from the exam and might not have all the info exact. I did research the fire pump example last evening after posting. I saw where the PF comes into play. I agree that the question appears deceiving. I did also locate Mike Holt's VD example # 2 that also gets into the VD properties involved at start-up. The single phase example he takes FLA X 125% to determine his ampacity for conductor sizing, then begins to calculate VD with the origianl FLA to determine if the wire selected for its ampacity also is adequeate to sustain the 3% VD max at the given distance. In the three phase firepump example, things change. This appears to be where Locked Rotor Current is a factor to include.
My apprentice's exam question did not mention anything about a firepump.
Thanks for any more feedback.

 

[maestro_89]

Re: Calculating voltage drop for a motor

Fire pumps are a unique issue. Article 695.7 states that the fire pump shall not have a voltage drop of greater than 15% at starting and 5% when the motor is running at 115% of FLA. So you would have to do two calcs to show compliance: 1 for starting and 1 for running.

 

[sheryf]

Re: Calculating voltage drop for a motor

Okay. ,my apprentice is now settled down and I have the question better understood. Here is the exact question.
What size THHN wire is required for the feeder to a design E 460V 25HP 3phase motor with a power factor of .95, at a distance of 450' with a voltage drop not to exceed 3%. Line voltage is 460V. Use a K factor of 12.0.
Possible Answers:
#8, #6, #4, #2

Here is how my apprentice calculated
FLA X PF X 125% = I
42.5 X .95 X 125% = 40.375 Amps
CM=D x K x I x 1.732 / 13.8 Volts Max
23363.7 CM minimum(this was the answer he chose)
#6 THHN (26240 CM)

If you do not multiply the PF the answer is:
28803.9 CM minimum
#4 THHN
If you use just FLA for your load the answer is
23043.1 CM minimum
#6 THHN
Our shop is in an uproar about this one so if any one can answer us on how PF comes into play it would be much appreciated. The fire pump scenario is another fun debate, but we have concluded that the code test was not asking him that particular type of question. We are however double checking our wire sizing on a recent firepump upgrade. Thanks for all your feedback.

 

[ron]

Re: Calculating voltage drop for a motor

I brought up the fire pump issue because it requires a motor starting calculation, not to divert the question.
The article referenced in my first reply will give you the direction to solve the problem.
You must consider the PF in your calculation, for two reasons. First it effects the voltage drop, and second they give you that information to use.

 

[sheryf]

Re: Calculating voltage drop for a motor

Thanks RON, and also thanks for bringing up the firepump issue because it is relevant to the bigger picture as well. So was the way we calculated load current correct?
I= FLA x 1.25 X PF
The sub pump example uses a 460V 150 hp motor PF of .85. The example states the load draws 191A. The NEC states that motor has an FLA of 180A
180 x 125% x .85 =191.25 Amps. The example doesn't show this method for determining current of the load, but utilizing the equation that my apprentice used it works out great. I just haven't found I=FLA x 125% x PF any where in all of my texts. If it is the correct way in this case to use it, I am in great shape. If not you great folks will have to educate my poor sorry self. Thanks again!!!