Supply Headroom Worksheet
Add the currents of proposed equipment to the per-phase demand you measured, and see the headroom in amps against the rating you enter. Measured-demand arithmetic only.
Inputs
View resultMeasured-demand arithmetic. This is not the AS/NZS 3000 maximum-demand calculation and not a supply-capacity determination; the network and the licensed designer decide capacity. Maximum demand sits under AS/NZS 3000 and connection capacity under your network's service rules; find them through network authorities.
Supply
Existing demand, per phase
Choose the supply arrangement to enter each phase.
Proposed new loads (0 of 8)
Add each piece of proposed equipment with its datasheet rating.
Live result
Edit inputsStill needed: the supply arrangement; the rating to compare against; where the existing figures come from.
Measured-demand arithmetic. This is not the AS/NZS 3000 maximum-demand calculation and not a supply-capacity determination; the network and the licensed designer decide capacity.
- Measured-demand arithmetic. This is not the AS/NZS 3000 maximum-demand calculation and not a supply-capacity determination; the network and the licensed designer decide capacity.
- Existing demand is per-phase current you measured with a clamp meter, labelled with the window you measured over, or figures you enter and label as yours. A whole-of-site kW figure is not accepted as per-phase demand.
- Each new load is entered in amps, or in kW with the power factor you enter. Voltage, power factor and the rating all start empty; with no power factor a kW load is not converted.
- Each phase reads as headroom in A (arithmetic), or as the amount by which it exceeds the entered rating. On a three-phase site it shows the placement of unallocated single-phase loads that minimises the spread, as arithmetic rather than a design instruction.
How it works
Measured-demand arithmetic. This is not the AS/NZS 3000 maximum-demand calculation and not a supply-capacity determination; the network and the licensed designer decide capacity. The maximum-demand method sits in AS/NZS 3000, and what a connection can carry is set by the distribution network's service and installation rules, which you can find through network authorities. This worksheet does neither job. It adds currents.
- Existing demand. Per-phase current you measured with a clamp meter, with the window you measured over, or figures you enter and label as yours. A whole-of-site kW figure is not accepted, because splitting it across phases would mean assuming how the load is spread.
- New loads. Each proposed load is entered in amps or in kW from its datasheet. A kW figure is converted with the power factor you enter; with no power factor it is not converted. A single-phase load uses
I = P ÷ (V × PF)at the phase voltage you enter. A balanced three-phase load usesI = P ÷ (√3 × V_L × PF)in each phase, at the line voltage you enter. An unbalanced load is entered in amps, phase by phase. - Headroom. For each phase, the rating you enter minus existing plus new current, shown as headroom in A (arithmetic). A negative result reads “exceeds the entered rating by” the difference.
- Phase allocation. On a three-phase site, single-phase loads you have not placed are tried on every phase, and the placement with the smallest spread between the highest and lowest phase is shown, the same spread the three-phase load balance worksheet reports. It is arithmetic, not a design instruction.
Every voltage, power factor, rating and current starts empty and comes from you. If you are not sure of the supply, the single or three-phase check reads it from the main switch or meter. A site survey recorded on this device in My Bench can fill the main switch rating as labelled, only when you tap for it. For battery work, the battery backup architecture guide and the battery charge time planner cover what the battery itself does.
Estimates only: for guidance, not a quote or a compliance certificate. Verify against the current AS/NZS standards and a licensed design. The maths runs in your browser. Nothing is sent to Wattbench unless you sign in and deliberately save the result.
Source ledger1 source
- Standards bodyAS/NZS 3000:2018 — Standards Australia catalogue
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Frequently asked
- Is this a maximum-demand calculation?
- No. It adds the currents you enter to the demand you measured, over the window you measured it. The maximum-demand method is in AS/NZS 3000, and what a connection can carry is decided by the distribution network and the licensed designer. Find the standard on the standards pages and the network rules through network authorities.
- Why is there no default power factor or voltage?
- Because any default would be a figure the worksheet supplied rather than one you measured or read from a datasheet. Enter the power factor from the equipment datasheet, or enter the load in amps, which needs none.
- Can I enter the site's demand as one kW figure?
- No. Headroom is per phase, and a whole-of-site kW figure would have to be split across phases by assumption. Enter the current on each phase from clamp readings, with the window, or enter your own per-phase figures and label them as user-entered.
- Is the suggested phase allocation a design?
- No. It is the placement of unallocated single-phase loads that gives the smallest spread between the highest and lowest phase, found by trying every placement. It applies no imbalance limit and is not a design instruction.
- Is my data stored or sent anywhere?
- The calculation runs in your browser. Nothing is sent to Wattbench unless you sign in and deliberately select Save result.
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