Does this select parallel cable sizes?

No. It only estimates current split from entered impedance records. Cable sizing and current capacity remain separate reviews.

Where do impedance values come from?

They must be entered from a project, measurement, manufacturer, engineering or documented source record.

Can this be used for protection settings?

Only as a worksheet input. Protection coordination, fault rating and final settings need their own review.

Why use inverse impedance?

Lower impedance paths carry more current in this simplified sharing model, so each run is weighted by 1 divided by its entered impedance.

Does this prove the installation is acceptable?

No. Thermal effects, grouping, terminations, protection and competent review remain outside this page.

Parallel cable current sharing calculator

Ii = Itotal x (1 / Zi) / sum(1 / Z); Iavg = Itotal / n; spread = Imax - Imin; imbalance_% = spread / Iavg x 100

Run impedances are entered by the user.
The calculator uses inverse impedance sharing only.
The imbalance target is entered by the user.
The result does not select cables, protection or terminations.

Formula variables
Variable	Meaning	Unit	Use
Itotal	Total current	A	Entered current to be shared by the parallel cable runs.
Zi	Run impedance	milliohm	Entered impedance for each parallel run.
Ii	Run current	A	Calculated current carried by each run.
Iavg	Average current	A	Total current divided by populated run count.
spread	Current spread	A	Highest run current minus lowest run current.
imbalance_%	Current imbalance percent	%	Current spread divided by average current.
target	Allowed imbalance	%	User-entered project review threshold.

Parallel cable current sharing calculator technical guide

Estimate Australian parallel cable current sharing from entered total current and run impedance records for project review.

Use this calculator when parallel cable runs have separate impedance records and the reviewer needs to see whether current sharing is balanced enough to carry into cable, protection and installation review. It is a worksheet for entered source values, not a cable-selection tool.

Field use cases

Parallel cable sharing use cases
Work setting	Real question	Useful action from this page
Parallel feeder record	How much current does each entered run carry?	Calculate run currents from inverse impedance sharing.
Unequal route lengths	Does one run take materially more current?	Compare current spread and imbalance percent.
Commissioning review	Do measured or calculated impedances look balanced?	Keep impedance source values beside the current split.
High-current planning	Does the split need engineering review before final records?	Use high-current and imbalance flags as review prompts.
Downstream cable review	Which run needs attention first?	Check the highest calculated run current first.

This page is most useful when the project already has a parallel-run record. It is not useful when the missing data is the cable size itself.

Data checklist

Parallel cable input sources
Value	Where it normally comes from	Stop if
Total current	Load calculation, design record or measured load basis	The current basis differs from the downstream cable review.
Run impedance	Measurement, engineering model, cable data plus route record	The values are guessed or use different reference conditions.
Allowed imbalance	Project, engineering or review target	The target is being treated as a standards rule without source.
Run count	Project parallel cable arrangement	Runs are not actually in parallel on the same basis.

If impedance data is uncertain, export the worksheet as a question record rather than a decision record.

Method comparison matrix

Current sharing method boundary
Method element	What this calculator does	What remains outside
Current split	Weights each run by inverse entered impedance.	Detailed thermal or electromagnetic modelling.
Imbalance percent	Compares current spread with average run current.	Project acceptance criteria.
Target margin	Compares imbalance with user-entered target.	Final engineering or compliance decision.
High-current note	Flags large total current records.	Protection, fault rating and switchboard review.

The method intentionally stays transparent. A useful result shows the inputs and the current split, then moves to a competent review.

Worked records

Parallel cable examples
Situation	Inputs	Result	Record use
Three parallel runs	300 A, 20/21/19 milliohm, 10% target	About 99.83 A, 95.08 A and 105.09 A; 10.01% imbalance	Review impedance source and terminations.
Matched runs	240 A, closely matched impedances, 8% target	Current split remains close to the average	Keep source record with cable review.
High-current review	520 A with different run impedances	High-current review note applies	Carry result into engineering review.

The examples show why small impedance differences can matter when total current is high.

Stop points

Run impedance values are not from the same source basis.
Total current belongs to a different load scenario.
The result is being used to choose cable size or protection settings by itself.
One run carries materially more current and no thermal review has been done.
The record has no source for the entered imbalance target.

The clean export record includes total current, run impedances, calculated run currents, imbalance percent, target margin and reviewer.

Share reference: PARALLEL-RUN-1
Total current: 300 A
Run impedances: 20, 21, 19 milliohm
Allowed imbalance: 10%

Run currents99.83 A, 95.08 A, 105.09 A from inverse impedance sharing.
Current spread10.01 A between highest and lowest run current.
Imbalance10.01% against the entered target.

Current sharing10.01% imbalance

Target margin is -0.01%.

The entered impedances create a current spread just above the entered target, so cable lengths, terminations and source data need review.

Each impedance value is entered by the user from the project record.
Current sharing is estimated from inverse impedance only.
Protection, grouping, installation and thermal checks remain separate.

Share reference: MATCHED-RUNS-1
Total current: 240 A
Run impedances: 18, 18.2, 18.1 milliohm
Allowed imbalance: 8%

Run currents80.44 A, 79.56 A, 80 A from inverse impedance sharing.
Current spread0.88 A between highest and lowest run current.
Imbalance1.1% against the entered target.

Current sharing1.1% imbalance

Target margin is 6.9%.

The current split is inside the entered imbalance target, while the source record should still be retained.

The runs are treated as parallel paths on the same phase basis.
Impedance values include the relevant route and termination basis entered by the reviewer.
No cable table or final selection is embedded.

Share reference: HIGH-CURRENT-PARALLEL
Total current: 520 A
Run impedances: 12, 13.5, 12.8 milliohm
Allowed imbalance: 12%

Run currents183.98 A, 163.54 A, 172.48 A from inverse impedance sharing.
Current spread20.44 A between highest and lowest run current.
Imbalance11.79% against the entered target.

Current sharing11.79% imbalance

Target margin is 0.21%.

The high-current flag keeps the result in review even when the arithmetic is transparent.

The entered total current is a project value.
Run impedance differences can come from length, conductor, temperature or termination assumptions.
Engineering review controls the final project decision.