What is a Voltage Drop?
Introduction
What is a voltage drop and how does it play a crucial role in the electrical design process of a building? Throughout this blog, we will provide answers to these questions and discuss why it is important for engineers to follow National Electrical Code (NEC) guidelines.
A voltage drop is defined as the decrease of electrical potential along the path of a current flowing in an electrical circuit. The voltage drop of a circuit is in direct correlation to the impedance of the conductor and the magnitude of the current. For example, an increase in the length of the conductor will increase the impedance which will ultimately increase the voltage drop (decrease the electrical potential). An increase in the current will also result with an increase in the voltage drop. Long run wires often result in voltage drops that exceed NEC recommendations. It is best practice to make sure all circuits are 5% or less in voltage drop from the utility to the device served. This is achieved by either limiting the load on an individual circuit or increasing the size of the wire. Voltage drop can also be mitigated by reducing the overall length of the installed conductor, but this is oftentimes dependent upon field conditions and installation practices.
NEC Recommendations:
NEC recommends that engineers adjust for voltage drop when sizing conductors and that the maximum combined voltage drop for both the feeder and the branch circuit should not exceed 5%, and the maximum on the branch circuit should not exceed 3%. You can learn more about these recommendations in sections NEC 210. 19(A), 215.2(A). Please note that these recommendations are in place for performance issues and not for safety reason and therefore are not typically enforceable as code violation unless in specific jurisdictions. However, it is necessary to install in full compliance with the equipment manufacturer’s requirements as it pertains to delivered voltage. Should a project pursue LEED certification, voltage drop mitigation does become a prerequisite for certain credits.
Why Do We Follow These Guidelines?
- System efficiency – Power losses throughout the electrical distribution system will result in increased electric bills for power that is not practically utilized. Imaginary power is actually very real and is the difference between kilowatts and kilo volt-amperes.
- System performance – Resistive loads such as heaters will not provide the listed power output which could result in longer operating times or reduced water temperature in water heaters.
- Troubleshooting- Due to accommodating the voltage drop during the design, you will know whether your field measurements indicate a problem or if the voltage is low.
- Load Protection- Undervoltage for inductive loads (motors) can cause overheating, inefficiency, and shorter life span of the equipment. Motor controllers such as variable frequency drives (VFDs) can suffer premature failures. Additionally, sensitive electronic equipment can power down or experience data loss.
SUMMARY:
Voltage drop is simply defined as the decrease of electrical potential along the path of a current flowing in an electrical circuit. Voltage drop plays a crucial role when designing electrical projects. You must always consider the current, the conductor, the amount of impedance, and the length of the run in order to stay under the 5% or less voltage drop from the utility. The NEC guidelines are in place to make your project more efficient, have a better performance, and allow your equipment to have a longer lifespan. If you see long wire runs on a project, make sure voltage drop was considered for the wire sizes.
CONTACT US:
For every project that requires new service equipment Proficient does a short circuit analysis as well as voltage drop calculations. Typically, this level of calculation is not done by most MEP consultants unless it is an industrial or government project. Due to this, Proficient utilizes programs such as Easy Power and ETAP to efficiently make these calculations and to ensure that our designs stay under the 5% maximum that the NEC recommends. For any additional information on voltage drops or our engineering services, feel free to contact us or check out our website.
