How to calculate cables and circuit breakers designing electrical system. 
<p>It becames quite often, when you want to connect some new devices to your home's network. How to calculate suitable parameters? 
</p>
<ul><li>First, find out power of your electrical device and amount of phases.</li>
<li>Second, calculate it's <a href="https://en.wikipedia.org/wiki/Electric_current" class="extiw" title="en:Electric current">current</a> by dividing power to voltage.
<ul><li>If you have 3-phase device, also divide it to 3.</li></ul></li>
<li>Third, calculate full wiring resistance (to find out cable parameters):
<ul><li><a href="https://en.wikipedia.org/wiki/Transformer" class="extiw" title="en:Transformer">Transfomer</a> has resistance approximately 0.25...0.3Ω</li>
<li>Cable length between distribution substation and building is usually 1 kilometer</li></ul></li>
<li>Four, calculate <a href="https://en.wikipedia.org/wiki/short_circuit" class="extiw" title="en:short circuit">short circuit</a> current (to find out suitable circuit breaker to avoid overheating or fire).</li>
<li>Last step, divide short circuit current (from step 4) to circuit breaker's charactristic
<ul><li>to 5, if circuit breaker has B characteristic ("B" is put to protect devices without coils)</li>
<li>to 10, if circuit breaker has C characteristic ("C" is put to protect devices with coil cause their high current during start).</li>
<li>try to not put "D"-type, cause any small malfunction of your device will put to switch off main circuit breaker additionaly to protectable group's breaker and leave all building without electricity till you turn on main circuit breaker.</li>
<li>Remember! Circuit breaker's maximum current should be less than cable's short circuit current (to avoid fire) and some more than your devices current (to avoid false deenergizing).</li></ul></li></ul>
<a href="//commons.wikimedia.org/wiki/User:Dmitry_G" title="User:Dmitry G">Dmitry G</a>
<span class="int-own-work" lang="en">Own work</span>