2.0 KiB
2.0 KiB
id, aliases, title, tags, dg-publish
| id | aliases | title | tags | dg-publish | |||||
|---|---|---|---|---|---|---|---|---|---|
| Voltage Drop |
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Voltage Drop
[!info] Ohm's Law
V = I \times R, \quad R = \frac{V}{I}, \quad I = \frac{V}{R}
Step 1: Effective Impedance Z
The formula for effective impedance Z is given by
Z = R \cos(\theta) + X \sin(\theta)
where
R= AC resistanceX= Reactance\theta= Power factor angle =\arccos(\text{PF})
Parallel Runs
The equivalent resistance of parallel resistances is given by
\frac{1}{R_{\text{eq}}} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + \dots + \frac{1}{R_n}
For n parallel resistances of value R
\begin{align*}
\frac{1}{R_{\text{eq}}} &= n \times \left(\frac{1}{R}\right) \\
&= \frac{n}{R} \\
R_{\text{eq}} &= \frac{R}{n}
\end{align*}
Step 2: Voltage Drop
Important
This section assumes a 3-phase 208Y/120V or 480Y/277V voltage system
[!info] 3-Phase Voltage
V_{L} = \sqrt{3} \times V_{P}, \quad V_{P} = \frac{V_{L}}{\sqrt{3}}
3% allowable voltage drop for a 120V line-to-neutral load:
\text{Max}\ \Delta V = 0.03 \times 120 \text{V}_{P} = 3.60 \text{V}_{P}
3% allowable voltage drop for a 208V line-to-line load:
\text{Max}\ \Delta V = 0.03 \times 208 \text{V}_{L} = 6.24 \text{V}_{L}
Line to Neutral Loads
\Delta V_{P} = I \times Z \times 2L
Line to Line Loads
\Delta V_{L} = I \times Z \times 2L
3-Phase Loads
\Delta V_{3\phi} = \sqrt{3} ( I \times Z \times L )
where
\Delta V= Voltage drop in volts (V)I= Current in amperes (A)Z= Effective impedance in ohmsL= Length of wire one way in feet (\text{ft})
Important
"Current" is not the OCPD rating, but the actual load.
It is often more useful to know the maximum length a certain wiring configuration is suitable for.
L = \frac{ \Delta V }{ I \times M } \times \frac{1}{Z}