What is the best book to buy for (basic) electrical equations, needed for an electrician?

I need a book that has the main electrical equations that electrians use
Book must include volt drop, diversity, and all other equations.
Thanks

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia…).