Low voltage circuit
Calculation Power
\[ Pdesign = n · P · ks · ku \]
- Pdesign: Power design, kW
- n: Number of load or utilization points
- P: Unit power of the load, kW
- ku: Utilization factor, average utilization factor of the maximum load power
- ks: Simultaneity factor, ratio of loads connected simultaneously to the total
Intensity
\[ I = \frac{P_{design}}{K · U · cos φ} \]
- I: Intensity, A
- U: Voltage, V
- cos φ: Power factor
- K:
· Three-phase K = √3
· Other cases K = 1
Voltage drop
\[ e = K · L · I · (R · cosφ + X · senφ) \]
\[ R = R_{DC} · (1 + Ys + Yp) \]
\[ R_{DC} = \frac{1}{σ·S} \]
- e: Voltage drop, V
- K:
· Trifásica K = √3
· Otros casos K = 2
- L: Circuit length, m
- X: Conductor reactance, Ω
- Rdc: DC conductor resistance, Ω
- R: AC conductor resistance, Ω
- YS: Increase in resistance due to skin effect, p.u.
- YP: Increase in resistance due to proximity effect, p.u.
- σ: Copper conductivity at the expected temperature, m/Ω·mm²
- S: Conductor cross-section, mm²
Conductor temperature
\[ T = To + (Tmax - To) · \left(\frac{I}{I_\text{Imax}}\right)^2 \]- T: Estimated conductor temperature, ºC
- To: Environment temperature, ºC
- Tmax: Maximum allowable conductor temperature according to insulation type, ºC
- I: Expected current for the conductor, A
- Imax: Maximum allowable current for the conductor according to insulation type, A
Conductor conductivity
\[ σ = \frac{σ_{20}}{1 + α·(θ-20)} \]- σ: Conductor conductivity, m/Ω·mm²
- σ20: Conductor conductivity at 20ºC, m/Ω·mm²
- θ: Conductor temperature, ºC
- α: Coefficient of variation of specific resistance with temperature, 1/ºC
Overload protection
\[ Ib ≤ In ≤ Iz \] \[ I_{2} ≤ 1.45 · Iz \]
- Ib: Circuit design current, A
- In: Rated current of the protection device, A
- Iz: Continuous current-carrying capacity, A
Short-circuit protection
\[ k^2 · S^2 \; [cable] > I^2 · t \; [protection] \] valid for t ≤ 5 s
- k: Factor depending on conductor material and its maximum permissible temperature, A/mm²·√s
- S: Conductor cross-section, mm²
- I: Short-circuit current (RMS value), A
- t: Operating time of the protection device, s
