This voltage drop calculator will provide an approximate value for use in project design and it is based on the following public domain formula:
Single phase

Three phase

Vd = (2 K Q I D)/CM

Vd = (1.732 K Q I D)/CM

Where:


K = 12.9 for Copper & 21.2 for Aluminum


(Q) is the ratio of Rac/Rdc for conductors larger than 2/0 (this
takes into account skin effect at larger conductor sizes)


(I) is the current in amperes


(D) is the oneway circuit distance in feet


(CM) is the conductor cross section in Circular mils.

NEC Rules controlling Voltage Drop:
1 NEC article 210.19(A)(1) FPN No.4 and 215.2(A)(3) FPN No.2 :
These FPN’s (Fine Print Notes) suggest that a design with no more than 3% voltage drop for feeders and no more than a total of 5% voltage drop in branch circuits to the farthest outlet will provide reasonable efficiency at operation.
Note: Even though FPN’s (Fine Print Notes) in this case are not code requirements but recommendations, it is still good engineering practice to closely follow these guidelines
2 NEC article 647.4(D):
For Sensitive Electronic Equipment, The voltage drop on any branch circuit shall not exceed 1.5 percent and the combined voltage drop feeder and branchcircuit conductors shall not exceed 2.5 percent.
3 NEC article 647.4(D) (1):
For Fixed Equipments, The voltage drop on branch circuits supplying equipment connected using wiring methods in Chapter 3 shall not exceed 1.5 percent and the combined voltage drop of feeder and branchcircuit conductors shall not exceed 2.5 percent.
4 NEC article 647.4(D) (2):
For cordconnected equipment, The voltage drop on branch circuits supplying receptacles shall not exceed 1.0 %. For the purposes of making this calculation, the load connected to the receptacle outlet shall be considered to be 50 percent of the branchcircuit rating. The combined voltage drop of feeder and branchcircuit conductors shall not exceed 2.0 %.
5 NEC Article 695.7:
For Fire pumps, the voltage at the controller line terminals shall not drop more than 15 percent below normal (controllerrated voltage) under motor starting conditions. The voltage at the motor terminals shall not drop more than 5 percent below the voltage rating of the motor when the motor is operating at 115 percent of the fullload current rating of the motor.
Exception: This limitation shall not apply for emergency run mechanical starting. [NFPA 20, 6.4]
Siemens Voltage drop spreadsheet application manual:
This spreadsheet includes three different worksheets as follows:
 Voltage Drop Calculator worksheet
 Summary worksheet
 Information worksheet
Now, I will explain each worksheet as follows:
First: Voltage Drop Calculator worksheet (see fig.1)
Voltage Drop Calculator Worksheet 
1 Input Data:
1 Load voltage : select the load Voltage from the drop down box which contains the following configurations:  
a For Single Phase  
120V 1Ø

208V 1Ø

240V 1Ø

277V 1Ø

480V 1Ø


b For Three Phase  
208V 3Ø

240V 3Ø

480V 3Ø


2 Conductor size: select the conductor size from drop down box which contains the following sizes: 

14

12

10

8

6


4

3

2

1

1/0


2/0

3/0

4/0

250

300


350

400

500

600

750


1000


3 Conductor Type : select the conductor type by checking the circle beside it and of course the available types will be:  
Copper

Aluminum


4 Number of Sets: select the number of conductor sets from the drop down box, the available selections are:  
1

2

3

4

5


5 Distance (one way): enter the distance of one way only in feet  
6 Load (A): enter the load current in Amperes.  
2 Output Data
Unity
power factor section

85% Power
Factor section

Voltage drop (V)

Voltage drop (V)

Voltage drop (%)

Voltage drop (%)

Voltage at Load in (V)

Voltage at Load in (V)

Minimum Conductor Size for 3% VD


Minimum Conductor Size for 5% VD

3 Notes for output data sections:
The unity power factor section of this Spreadsheet is based on the approximate voltage drop formulae indicated above which assumes the following:
 A power factor of 1.0
 Conductor temperature of 75°C
 Individual conductors run in steel conduit.
While The 85% power factor section is based on the following:
 NEC chapter 9, tables 8; Conductor Properties.
 NEC chapter 9, table 9; AlternatingCurrent Resistance and Reactance for 600Volt Cables, 3Phase, 60 Hz, 75_C (167_F)—Three Single Conductors in Conduit with effective impedance (Z= R cos Ø+ X sin Ø) computed at 85 % P.F (Ø is the power factor angle).
4 Output data evaluation:
If the output voltage drop exceed the applied NEC rule limits (explained above), so you must increase the size of your conductors to accommodate for voltage drop but you must take care from the following:
Check if the new conductor size is compatible with the circuit breaker lugs to which they will be attached and note that circuit breaker manufacturers make available the acceptable conductor sizes, and in some cases offer optional large lugs for this purpose.
NEC Article 250 requires that if the conductors are upsized, the grounding conductor must also be upsized proportionately.
NEC Chapter (9) Table (1) requires certain fill Percent of Cross Section of Conduit and Tubing for Conductors and if the conductors are upsized, you must check the new fill percent and see if you need to upsize the conduit or not.
Second: Summary worksheet
Summary Worksheet 
1 Insert the project name
2 Insert the designer/engineer name
3 Insert the date of making calculations
4 Fill the following table manually to be as a summary for all the voltage drop calculations made by this spreadsheet.
Third: Information worksheet
Use the information work sheet as a guide for selecting the right NEC rule to be applied as per every case condition.
Also, it will guide you for adjusting of your results and for printing the calculation worksheet or summary worksheet or both.
To download a copy from Siemens – Voltage Drop Calculations spreadsheet, please click on the link.
2 Insert the designer/engineer name
3 Insert the date of making calculations
4 Fill the following table manually to be as a summary for all the voltage drop calculations made by this spreadsheet.
Feeder Designation

Source Panel

Source Circuit

Vd (V)

Vd (%)

Third: Information worksheet
Information Worksheet 
Also, it will guide you for adjusting of your results and for printing the calculation worksheet or summary worksheet or both.
To download a copy from Siemens – Voltage Drop Calculations spreadsheet, please click on the link.
let say my power is from feeder to distribution board(DB) and finally to my pump. For cable sizing of DB to pump, did we need to consider the voltage drop from the feeder until the pump? or just the voltage drop from the DB to the pump?
ReplyDeleteThank you
dear Jie Ng
ReplyDeleteif we need to size the cable from DB to pump we must calculate the voltage drop for this path (from DB to pump) which must be equal to or below 3% ( please note that if it is a fire pump, the NEC Article 695.7 must be applied).
in this case, the voltage drop from the feeder until the pump has no effect on this cable size.
please note that , we consider only the combined voltage drop which must be equal to or less than 5% only for main service feeders from the transformer to the main low tension panel (MLTP).
hoping my reply will clarify your question.
thanks