Spare switchboard capacity calculator
Compare Australian switchboard capacity with entered existing demand, proposed load and future allowance records.
PlannedA = ExistingA + ProposedA + FutureA; RemainingA = Cboard - PlannedA; spare_% = RemainingA / Cboard x 100; margin_% = spare_% - target_%- Board capacity is entered by the user.
- Existing demand, proposed load and future allowance are entered on the same basis.
- The minimum spare target is entered by the user.
- The calculator does not decide supply upgrades, protection or connection outcomes.
| Variable | Meaning | Unit | Use |
|---|---|---|---|
| Cboard | Board capacity | A | Entered switchboard capacity comparison value. |
| ExistingA | Existing demand | A | Entered existing demand value. |
| ProposedA | Proposed load | A | Entered proposed additional load. |
| FutureA | Future allowance | A | Entered allowance carried in the review. |
| PlannedA | Planned demand | A | Existing demand plus proposed load plus future allowance. |
| RemainingA | Remaining capacity | A | Board capacity minus planned demand. |
| spare_% | Spare capacity percent | % | Remaining capacity divided by board capacity. |
| target_% | Minimum spare target | % | User-entered spare-capacity target. |
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Spare switchboard capacity calculator technical guide
Compare Australian switchboard capacity with entered existing demand, proposed load and future allowance records.
Use this calculator when a switchboard capacity record needs a transparent spare-margin worksheet. It compares entered capacity with existing demand, proposed load and any future allowance, then shows remaining capacity and spare percentage.
Field Use Cases
| Work setting | Real question | Useful action from this page |
|---|---|---|
| Board alteration | How much entered capacity remains after the new load? | Add existing demand, proposed load and future allowance. |
| Tenancy review | Is the spare percentage above the entered target? | Compare spare capacity with the user-entered target. |
| Future planning | What does a future allowance do to margin? | Keep the allowance visible beside the capacity result. |
| Project handoff | Which source values need review? | Export board capacity and demand assumptions. |
| Demand workflow | Does demand need to be calculated first? | Move to the maximum demand worksheet before this comparison. |
The page is most useful when the board capacity value and demand values already have a source. It should not be used to invent a board rating.
Data checklist
| Value | Where it normally comes from | Stop if |
|---|---|---|
| Board capacity | Board record, drawing, manufacturer data or project review | The value is only guessed. |
| Existing demand | Maximum-demand worksheet, measurement or project record | The demand basis is not recorded. |
| Proposed load | Load schedule or equipment data | The added load has not been converted to amps. |
| Future allowance | Project planning assumption | The allowance is being treated as a fixed rule without source. |
| Minimum spare target | Project or engineering review value | The target is copied without context. |
All current values should be on the same basis. Mixing connected current, demand current and measured current without labels can make the spare percentage misleading.
Review Workflow
| Step | Record to check | Move to |
|---|---|---|
| Confirm board reference | Named switchboard or capacity record | Enter capacity value. |
| Confirm demand basis | Existing demand source and proposed load source | Add future allowance if used. |
| Read planned demand | Existing plus proposed plus allowance | Compare with board capacity. |
| Read spare percentage | Remaining capacity divided by board capacity | Compare with entered target. |
| Choose next task | Demand, phase, cable or protection review | Use the route that owns that task. |
If the spare margin is below the entered target, the result is a review prompt. It does not prove the board is unsuitable, and it does not select an upgrade.
Worked capacity record
An MSB record has an entered capacity of 250 A, existing demand of 145 A, proposed load of 42 A and a future allowance of 20 A. The entered minimum spare target is 15%.
The planned demand is 207 A. Remaining capacity is 43 A. Used capacity is 82.8%, so spare capacity is 17.2%. The spare margin above the entered target is 2.2 percentage points.
| Value | Result |
|---|---|
| Board capacity | 250 A |
| Planned demand | 207 A |
| Remaining capacity | 43 A |
| Used capacity | 82.8% |
| Spare capacity | 17.2% |
| Minimum spare target | 15% |
| Spare margin | +2.2% |
The result is a capacity worksheet record. It still needs source review for the capacity value, the demand basis and any project requirements that can override the comparison.
Method boundary
| Method element | What this page does | What remains outside |
|---|---|---|
| Planned demand | Adds entered existing, proposed and future load values. | Full maximum-demand calculation. |
| Remaining capacity | Subtracts planned demand from entered board capacity. | Switchboard thermal or protection assessment. |
| Spare percentage | Divides remaining capacity by board capacity. | Project target selection. |
| Review status | Compares spare percentage with entered target. | DNSP connection and upgrade decisions. |
The method is intentionally simple because the source values are the sensitive part. If a source value changes, the record can be recalculated without changing the method.
Stop points
- Board capacity is not backed by a source record.
- Existing demand was not calculated or measured on a clear basis.
- Proposed load has not been converted to amps.
- Future allowance is speculative and unlabeled.
- The result is being treated as an upgrade or connection decision.
When any stop point appears, export the worksheet as a review record and resolve the source values before using the margin in design work.
Main switchboard spare capacity
An MSB worksheet compares existing demand, a proposed tenant load and a future allowance against an entered board capacity.
- Board reference
- MSB-1
- Board capacity
- 250 A
- Minimum spare
- 15%
- Planned demand207 A
- Remaining capacity43 A
- Spare capacity17.2%
The status compares only the entered capacity and spare target.
The spare percentage is above the entered minimum target, but the source basis still needs project review.
- Board capacity is entered by the user.
- Existing demand comes from the project record.
- The minimum spare target is a user-entered review value.
Low spare review
A proposed addition leaves little margin after an allowance is carried into the capacity record.
- Board reference
- DB-LOW-SPARE
- Board capacity
- 250 A
- Minimum spare
- 15%
- Planned demand247 A
- Remaining capacity3 A
- Spare capacity1.2%
The status compares only the entered capacity and spare target.
The spare capacity is below the entered target and should stay in review.
- Future allowance is entered by the reviewer.
- No supply upgrade decision is made.
- Capacity values must match the board record.
Small board alteration
A small board review checks whether a minor proposed load leaves the planned spare margin visible.
- Board reference
- DB-SMALL
- Board capacity
- 100 A
- Minimum spare
- 20%
- Planned demand68 A
- Remaining capacity32 A
- Spare capacity32%
The status compares only the entered capacity and spare target.
The worksheet shows the remaining margin for discussion before the alteration is finalised.
- All currents are entered on the same basis.
- The future allowance is a project assumption.
- Protection and enclosure checks remain separate.