A conductor has been produced according to IEC standard and low voltage electrical facility has been installed according to IEC 60364. So we must comply with international standard for design, construction and inspection. But because of many related and varied standards, it is difficult to apply to design and inspect for electrical equipment. We researched the necessity of design guide which is detailed and systematization from survey. For detailed design guide, we suggest calculation method of protective conductor cross-sectional area at TN-S system according to BS 7671 regulation and KS C IEC 60364 standard.