Continuity Test Resistance Calculator

wirecalc modelUse this calculator to help determine effective thresholds for your wire tests. This calculator will help you estimate the maximum resistance of your Device Under Test (DUT), then add in resistance variations from wire length, temperature, diameter, fixturing, and tester accuracy. If you have any questions give us a call at 1-800-441-9910. We'll be happy to "walk you through" an example. In just a few minutes we can explain how to use this calculator.

   

Step 1: Find the DUT's Maximum Theoretical Wire Resistance

a) Set the units of measurement
                   
All lengths in
All temperatures in
All resistance values in ohms
All wire gauges are AWG
All wire is copper
b) Calculate the Resistance of the Ideal DUT(Theoretical)
                                    
                    
                    
Wire in DUT
Length        Gauge     Note___ ohms
Contact Mating Resistance (DUT to fixture):
At Point A: ohms    Note___ ohms
At Point B: ohms___ ohms
  Total Ideal Resistance of DUT (nominal)___ ohms
c) Add Variation From Ideal for the DUT
                                                                                                 
Account for Resistance Increases from:
Wire Length Tolerance: %     Note___ ohms
Wire Diameters Per Twist Length:     Note___ ohms
Wire Resistance Tolerance: %      Note___ ohms
Temperature Deviation : °      Note___ ohms
Total (In-Spec) Variance From Ideal___ ohms
Estimated Max Resistance of a Good DUT        ___ ohms

Step 2: Allow for Real-World Measurement Issues

Option 1 - Four-Wire
Add Resistance Variation If Using 4-Wire Measurement
                                                                                                       
Variance Due to 4-Wire Tester Measurement Error:
Tester Accuracy for 4-wire Measurement % (of reading)___ ohms
Tester Resolution Error ohms___ ohms
Total Variation Due to Tester (4-Wire Measurement)___ ohms
Resistance From Wear of Mating Contacts ohms___ ohms
  Estimated Max Measured Resistance of a Good DUT (4-Wire)___ ohms
Option 2 - Two-Wire
Add Resistance Variation If Using 2-Wire Measurement
                 
Variance Due to 2-Wire Tester Measurement Issues:
Fixturing wire from tester to:
Point A:  Length Gauge ___ ohms
Point B:  Length Gauge ___ ohms
Resistance of Fixture Wire (nominal)___ ohms
Fixture Wire Variation (diameter/temperature)___ ohms
Estimated Max Fixture Resistance___ ohms
Tester Accuracy for 2-wire Measurement % (of reading)___ ohms
Tester Resolution Error ohms___ ohms
Total Variation Due to Tester (2-Wire Measurement)___ ohms
Resistance From Wear of Mating Contacts ohms      Note ___ ohms
    Estimated Max Measured Resistance of a Good DUT (2-Wire)___ ohms
Option 3 - A-620 Specification
                      
A-620 Specification Class III Default Test Requirement___ ohms

Results: Your Lowest Practical Test Resistance Threshold

We define the "lowest practical test specification" as the lowest resistance setting that passes "in-spec assemblies on in-spec test equipment." If you test at a setting lower than what is practical you will fail assemblies that were manufactured correctly.

A DUT with an ideal wire resistance of ___ ohms can have a resistance as high as ___ ohms and be correctly built and in spec. That increase in resistance comes from variations in wire length, conductor diameter, test temperature, and contact resistance. Taking into account the variations caused by test fixturing and tester accuracy the lowest practical test thresholds for that wire are:

  • When testing 2-wire use a threshold higher than ___ ohms.
  • When testing 4-wire use a threshold higher than ___ ohms.
  • The A-620 Class III Default Threshold is ___ ohms.
    
        

Do these calculations on the wire with the highest resistance (smaller diameter wires and longer wires have higher resistances).

    
    
        

This is the typical resistance of mated contacts. Get this specification from your connector manufacturer's data sheet. (Resistance will increase with increased mating cycles).

    
    
        

This includes both the allowed length tolerance on the DUT's drawing and any pigtail wire length in connectors.

    
    
        

Enter the ratio of the insulated wire diameter to length of the twist. If the wires are not twisted then leave the field blank. For example, a twisted pair conductor with an outside diameter (including insulation) of 0.030 inches and a twist length of 0.5 inches per twist has a ratio of 0.5 / 0.030 = 16.7. You would enter 16.7 in the box.

    
    
        

The maximum spec for stranded wire is typically 8% higher than then nominal spec for solid copper wire. Some variation from nominal comes from the fact that the diameter of copper strands in the wire will vary from spool to spool. Also, the composition of the copper can vary slightly from lot to lot. Use this field to account for these normal wire resistance variations. A good typical value for stranded wire is 8%.

    
    
        

The resistance of the wire is specified at an ambient temperature of 68°F/20°C. If you are testing at a temperature well above that ambient temperature enter the temperature deviation in the box. For example, when testing at 78°F. enter 10°.

    
    
        

If the fixturing connectors exceed thier mating cycle life their contact resistance will go up. The higher you set this value the longer you can use a fixturing connector (or connector adapter) before you must replace it. If this value is set too low it will increase fixturing costs, increase failure rates, and slow your production rate.