Submain voltage drop calculator

Field use cases

Use this calculator when the question is no longer just "what is the drop on this cable run?" but "how much of the project allowance does this run consume before the downstream circuits are checked?" That question comes up on multi-board installations, long runs to detached buildings, tenancy distribution boards, workshops, pump boards, commercial switchboards and consumer mains reviews where the route itself can take a large share of the available voltage-drop budget.

In practice the calculation often starts after another decision has already happened. A maximum-demand worksheet may have produced the current to use. A preliminary cable schedule may already list the proposed route. A drawing revision may have moved a board further away from the main switchboard. A contractor may be comparing whether a switchboard location is still workable after cable routes are measured on site. This page keeps the allocation arithmetic visible so those decisions do not become hidden inside a single voltage-drop number.

The output is most useful when the record travels with its basis: the run reference, demand current, cable data, route length, upstream value already used, downstream value reserved and total project allowance. Without those items the percentage can look precise while still being hard to review. The calculator deliberately does not decide the correct split. It shows the consequence of the split entered by the user.

Minimum export record

Keep the export concise. Longer project records for demand current, cable data, DNSP context, route measurement, AS/NZS 3008 review and final-circuit checks should remain in the project file rather than being folded into the calculator result.

Allocation data to confirm

Confirm the demand current before using the result. For a submain, the current may come from a maximum-demand worksheet, a load schedule, equipment information, a measured review, or a project value issued by the designer. For a consumer mains calculation, the basis can be more sensitive because network and supply conditions may also matter. The calculator accepts the demand value; it does not prove the demand basis is suitable.

Confirm the route length as the installed one-way cable path. Long routes often include vertical rises, switchroom offsets, tray changes, penetrations, service loops and termination allowances. A straight drawing measurement can understate the segment drop and leave too little allowance for final circuits.

Confirm the voltage basis before comparing percentages. A project schedule may use the nominal 230 V or 400 V basis, while a site note may record a measured voltage at a different point in the installation. Keep those as separate records if both are relevant. Do not calculate the voltage drop in volts from one basis and the percentage allocation from another.

Confirm cable data before choosing mV/A/m or impedance mode. The value must match the conductor, circuit arrangement, source document and operating basis being reviewed. A value copied from a different conductor, temperature basis or installation context can make the allocation look better or worse than the project record supports.

Confirm the upstream and downstream values as allocation entries, not as automatic rules. Upstream used is the voltage drop already consumed before this run. Downstream reserved is the allowance held for final circuits or downstream load paths after this run. If downstream circuits have already been calculated, use the reviewed value. If they have not, the reserve should come from the project basis and should not be hidden.

Submain allocation matrix

The status is driven by allocation, not by the segment number alone. A run can have a modest percentage and still cause review if upstream drop and downstream reserve have already consumed the available budget. Conversely, a run can be below the total allowance but above the segment allowance assigned to that part of the installation.

The receiving-end voltage at the end of the submain or consumer mains run is useful for documentation, but it should not be read on its own. A downstream board still feeds final circuits, equipment and sometimes future load. The better field question is whether enough allowance remains after this run for the circuits that follow.

Worked Australian examples

A distribution-board submain is a typical allocation case. The reviewer has a 100 A three-phase demand current, an 80 m one-way route, entered cable data of 1.2 mV/A/m, a 400 V project basis, 0.5% upstream already used, 1.5% downstream reserved for final circuits and a 5.0% total project allowance. The calculator uses the mV/A/m formula:

1. Run drop in volts: 1.2 x 100 x 80 / 1000 = 9.6 V.
2. Run drop as a percentage of 400 V: 9.6 / 400 x 100 = 2.4%.
3. Total allocation used: 0.5% upstream + 2.4% submain + 1.5% downstream = 4.4%.
4. Total margin: 5.0% - 4.4% = 0.6%.
5. Downstream remaining after this run: 5.0% - 0.5% - 2.4% = 2.1%.

That record has margin under the entered project allowance, but only because the downstream reserve has already been stated. It should still travel with the demand-current source, cable data source and final-circuit checks.

A long shed or workshop submain can create a different review condition. With the same 100 A, 80 m, 1.2 mV/A/m and 400 V basis, the run is still 9.6 V or 2.4%. If the project has entered only 1.8% as the submain segment allowance, with 0.2% upstream used and 0.4% downstream reserved, the total allocation is 3.0% and still below a 5.0% total target. The run itself is the problem: 2.4% is 0.6 percentage points above the entered segment allowance. The maximum length for a 1.8% run allowance at these entries is 60 m, so an 80 m route needs review before the result is carried forward.

A tenancy board can show the allocation trap. With 1.4% upstream already used, the same 2.4% submain drop and 1.4% downstream reserved, the total allocation is 5.2%. The submain drop may look ordinary on its own, but the combined record is 0.2 percentage points above the entered 5.0% project target. That is a distribution review problem, not a reason to quietly remove the downstream final-circuit allowance.

For a single-phase consumer mains record, the percentage changes with the voltage basis. A 63 A run over 45 m with entered data of 2.1 mV/A/m gives 5.95 V. On a 230 V project basis that is 2.59%. With 0.4% upstream entered and 1.6% downstream reserved, total allocation is 4.59% against a 5.0% target. If the same 5.95 V is reviewed against a measured 224 V basis, it becomes about 2.66%. The volts have not changed, but the percentage margin has tightened, which is why the voltage basis must be recorded.

Review workflow

1. Establish the demand basis. Use the reviewed maximum-demand or load-schedule value where available, then keep the basis with the record.
2. Confirm the route. Measure or review the installed one-way path from the source board to the downstream board or consumer mains endpoint.
3. Enter cable data from the source that matches the run. Use mV/A/m when that data is available, or R/X values when impedance data is the project basis.
4. Enter the allocation split. Record upstream used, downstream reserved and total allowance as project values rather than hidden assumptions.
5. Keep the voltage basis visible. Use the nominal project basis or a measured basis consistently, and keep separate records if both are being compared.
6. Read the result by margin. The run percentage, total allocated percentage, downstream remaining percentage and receiving-end voltage should be read together.
7. Hand off the next task. Use the one-run voltage drop page for ordinary final-circuit checks, the cable size page for candidate screening, and the maximum-demand worksheet when the demand current itself needs review.

The record should state what changed when a result is reviewed. "Route length increased from 80 m to 110 m" is more useful than "voltage drop exceeded the entered allowance". "Downstream reserve increased after final-circuit review" is more useful than changing the submain target without explanation.

Boundary with voltage drop, cable sizing and maximum demand

This page sits between demand review and cable selection. It uses demand current, route length and cable data to show allocation pressure. It does not create the demand current, choose the cable, reproduce selection tables, check derating or coordinate protection.

Use the voltage drop calculator when the task is a single cable run without upstream and downstream allowance. Use the cable size calculator when a candidate metric cable needs to be screened against entered current-capacity and voltage-drop values. Use the maximum-demand worksheet when the demand current for a switchboard or load schedule needs to be assembled from entered rows.

Stop points

• Stop if the demand current cannot be traced to a suitable project basis.
• Stop if the route length is a straight-line estimate and the installed route is known to be longer.
• Stop if nominal project voltage and measured voltage are being mixed in one percentage comparison.
• Stop if the cable data cannot be matched to the conductor, circuit arrangement and source document.
• Stop if upstream drop has been assumed but not recorded.
• Stop if downstream final circuits are ignored because the submain alone appears within the total allowance.
• Stop if the total allowance is being treated as the calculator's rule rather than a user-entered project value.
• Stop if the result is being used to select a final cable, calculate maximum demand, finalise consumer mains or close protection review.
• Stop if current Australian requirements, DNSP or network requirements, manufacturer information or project documentation set conditions that have not been checked.