Knowing a machine’s power is the starting point for choosing the cable and the protective device that will supply it. The length of the circuit, the environment in which the cable is installed and the characteristics of the protective device are also part of the assessment. Two devices with the same power may need different supply solutions in different facilities.
Especially in capacity increases, instead of the approach “let’s just take one more outgoing from the existing circuit”, the effect of the new load on the entire supply chain should be examined. The assessment runs from the machine connection all the way to the main distribution point.
First step: defining the load current correctly
For a sinusoidal voltage and a balanced three-phase load, the relationship between active electrical input power and current can be written as:
I = P / (√3 × U × cosφ)
Here P is in watts, U is the line-to-line voltage in volts and I is in amperes. If the power on a motor nameplate is the mechanical output power, efficiency must be taken into account to find the electrical input power. If the input power is already known, applying the efficiency a second time gives a wrong result.
As an example, take 400 V, cosφ = 0.85 and an electrical input power of 15,000 W. The calculated current is about 25.47 A. This value is only the load current under the stated assumptions; it is not a direct conclusion that “this cable size and this breaker are suitable”. Power factor is the ratio of active power to apparent power; with harmonic loads, cosφ and the total power factor may not be the same. Schneider Electric — Definition of power factor (opens in a new tab)
Why is a cable not chosen by amperes alone?
The current-carrying capacity of a cable depends on conditions such as conductor and insulation type, installation method, ambient temperature, the number of loaded conductors and grouping with other cables. Catalogue values should not be used without corrections appropriate to the site conditions. Schneider Electric — General method for cable sizing (opens in a new tab)
This is why a quotation that only states a cable size does not provide enough information. Which cable type will be used, along which route and under which conditions should also be defined. Representing different installation conditions for the same cross-section with a single universal ampere value is misleading.
What does voltage drop change?
The voltage drop along a cable makes the voltage at the load end different from the source voltage. An approximate steady-state calculation for a balanced three-phase circuit is:
ΔU = √3 × I × L × (R cosφ + X sinφ)
If L is the one-way length in kilometres and R and X are in ohms per kilometre, the result is in volts. For a single-phase go-and-return circuit with equal conductor sizes, 2 is used instead of √3. The resistance used should match the conductor’s operating temperature. Schneider Electric — Voltage drop in steady load conditions (opens in a new tab)
For example, on a single-phase circuit with U = 230 V, I = 20 A, L = 0.05 km, R = 4 Ω/km and cosφ = 1, the drop is 8 V, or about 3.48%. The resistance here is an example input; it is not a recommendation for a particular cable size.
The permissible drop is determined by the project conditions, the applicable rules and the requirements of the load. The transient drop during motor starting can be examined separately. A single limit chosen by a calculation tool should not be an automatic compliance decision for every facility. Schneider Electric — Voltage drop limits (opens in a new tab)
The breaker rating and type are assessed together
In the basic coordination check, the load current, the rated or setting current of the protective device and the corrected current-carrying capacity of the cable are considered together: Ib ≤ In ≤ Iz. The tripping characteristic and breaking capacity must also be checked; satisfying this inequality alone does not mean that all design checks are complete. Schneider Electric — Basic conditions of the protective scheme (opens in a new tab)
The letters B, C or D relate to the tripping behaviour of the device. Tolerating a higher starting current does not mean being more suitable for every circuit. The selection should be verified against the actual product’s time–current curve and the fault conditions of the circuit. Schneider Electric — Time–current curves (opens in a new tab)
If you are planning a new machine or a facility extension, you can request a technical assessment by sharing the power data together with circuit lengths, the existing panel and operating conditions.