Why does the difference matter at tender?
For an estimator the difference decides what each drawing can be priced from. The schematic sets the device count and the logic a panel builder has to wire. The wiring diagram sets the terminal and wire count. The cable schedule sets the cable quantities, which on a substation or a large switchroom are often one of the largest items in the estimate.
What does a control schematic show?
The function of a circuit, drawn by logic rather than by location. A motor starter schematic shows the supply, the contactor coil, the overload contact, the start and stop pushbuttons, the run lamp and the interlocks, arranged so a reader can follow the current path from top to bottom. Each device carries a reference designation, usually following IEC 81346, so the same contactor can be found again on the wiring diagram and in the panel.
What the schematic does not tell you is where anything sits. Two contacts drawn side by side may be in different cubicles, and the drawing gives no lengths.
What does a wiring diagram add?
The physical connections. A wiring diagram, sometimes called a connection or interconnection diagram, shows each terminal block, each terminal number and each wire number, so an electrician can land conductor 214 on terminal X2:14 without reading the logic. Panel builders price terminations and labour from it, and test sheets are written against it.
It is usually the last of the three drawings to be finished. At tender it is often missing altogether, and the count has to be estimated from the schematic.
What does the cable schedule settle?
Every cable between two items of equipment, one row each: cable number, from, to, cable type, number of cores, conductor size, route and length. Cable sizes are selected under AS/NZS 3008.1.1 in Australia, or the local equivalent, against the load, the route and the installation method, and the schedule is where that selection is recorded.
On a tender the schedule is the quantity take-off for cable, glands, lugs and installation labour. When the pack includes a schedule without lengths, or lengths without a route, the quantities rest on an assumption that belongs in the qualifications.
Which drawing answers which question
| The question | Drawing that answers it | What it will not tell you |
|---|---|---|
| How is the system connected? | Single line diagram | Dimensions, wiring, cable lengths |
| Where does the equipment go, and does it fit? | General arrangement | How the circuits work |
| How does this circuit work? | Control schematic | Where the devices sit, cable lengths |
| Where does each conductor land? | Wiring diagram | Why the circuit is wired that way |
| How much cable, of what size, from where to where? | Cable schedule | The circuit logic or the panel layout |