Sizing Electrical Wire for Underground Circuit Cable

Selecting the right size of electrical wire for underground circuit cable is crucial for safety and performance. Proper wire sizing prevents hazardous overheating and voltage drops while allowing desired load currents. This comprehensive guide provides key factors to consider when sizing wire for underground installations.

Understanding Electrical Load and Current Flow

The first step in sizing wire is calculating the electrical load and expected current flow. Total load depends on the power requirements of equipment being served such as lights, appliances, HVAC systems, etc. Additionally, future expansion needs should be considered.

Higher electrical loads require increased current flow. Wire must be large enough to safely carry current levels without overheating per electrical code ampacity requirements. Ampacity rating depends on insulation type, temperature rating, and installation conditions.

For alternating current (AC) circuits, both resistive and inductive loads influence required conductor size. Careful load analysis provides the foundation for proper wire sizing.

Wire Sizing Calculations and Formulas

Common wire sizing calculations include:

  • Voltage Drop – The maximum permissible voltage drop based on wire length and load dictates minimum required wire gauge.
  • Ampacity – Wire must be sized to carry expected load current based on temperature rating and ampacity tables.
  • Derating – Ampacity may be reduced when installed underground, bundled, in high ambient temperature, etc.
  • Conduit Fill – Multiple wires in shared conduit cannot exceed allowable percentages of conduit area.

Accurate wire sizing uses calculations based on:

  • Expected load current (amps)
  • Allowable voltage drop percentage
  • Required ampacity rating
  • Applicable derating factors
  • Maximum conduit fill percentage

Useful formulas for wire sizing:

Load Current (Amps) = Power (Watts) / Voltage (Volts) 

Voltage Drop (V) = 2 * Wire Length (ft) * Load Current (A) / CM (circular mils)

CM = Area Circular Mils for Wire = Diameter^2 * PI/4 

Percent Voltage Drop = Voltage Drop / Source Voltage * 100

Online wire size calculators are also available to perform accurate computations.

Choosing Appropriate Wire Size

Common factors for selecting proper wire gauge include:

Voltage System

  • Match wire insulation rating for applicable system voltage – 120V, 240V, 480V etc. Higher voltages allow reduced current for the same load.

Load Current

  • Size for 125-150% of calculated load as a safety factor.
  • Larger gauge (lower AWG) required for higher amperage.

Maximum Voltage Drop

  • Limit drop to 2-3% for feeders, 3-5% for branch circuits.
  • Lower gauge (larger wire) reduces drop for a given length.

Ampacity Rating

  • Select wire gauge meeting ampacity requirements per NEC tables.
  • Consider all derating factors – underground, conduit fill, ambient temp, etc.

Conduit Fill

  • Limit wire fill to 40% for power cables.
  • More wires require larger conduit size.

Cost vs Performance

  • Wire cost increases exponentially with gauge. Balance cost and longevity.
  • Consider future expansion needs.

Wire Types and Insulation Ratings

Choosing the insulation type and rating is also key for safe underground wiring:

  • THHN – Common insulated wire for wet/dry feeders and cables. Rated 90°C dry, 75°C wet.
  • XHHW – All season underground feeder cable. 90°C rating wet or dry.
  • USE/RHH/RHW – Underground service entrance cable. 75°C wet rating.
  • THWN – Moisture resistant, rated 75°C wet and 90°C dry.
  • MV-90 – 90°C insulated cable for high ambient temperature.
  • UF Cable – Direct burial cable for branch circuits or feeders below 600V.

Ensure insulation rating meets NEC requirements for underground wiring methods. Consider moisture resistance, chemical environments, and continuous ratings for permanent wiring.

Underground Installation Factors

Ampacity derating and other considerations for underground circuits include:

  • Burial Depth – Cables buried over 24″ require derating per NEC.
  • Multiple Cables – More than 3 current carrying conductors requires derating.
  • Ambient Temperature – High ambient temperature may require lower ampacity.
  • Soil Conditions – Thermal resistivity of soil impacts ampacity.
  • Conduit Fill – Limit number of cables per buried conduit.
  • Voltage Drop – Increase wire size if run lengths cause excessive drop.

Adjust wire sizing to account for applicable underground derating factors per NEC guidelines.

Step-by-Step Process for Proper Wire Sizing

The complete process for selecting correctly sized wire is:

  1. Calculate total load – Sum all equipment power ratings, plus expected future loads. For motors and inductive loads, account for inrush currents.
  2. Determine voltage drop limits – Look up max voltage drop percentage allowed for your system. Typically 2-3% for feeders, 3-5% for branch circuits.
  3. Calculate load current – Divide total load watts by system voltage to obtain current in amps.
  4. Select tentative wire size – Consult ampacity tables to choose wire gauge meeting calculated load current for the insulation type.
  5. Run voltage drop calculation – Use voltage drop formula to check if tentative wire size meets drop limit over run length.
  6. Derating adjustments – Reduce ampacity if needed for ambient temperature, number of conductors, burial depth per NEC.
  7. Conduit fill calculation – Calculate total area percentage if multiple wires share a conduit. Adjust conduit size if over 40% fill.
  8. Final wire selection – Choose wire size satisfying all calculations and requirements. Include safety factor.

Following this methodical process ensures electrical wires are sized correctly for each unique underground installation.

Frequently Asked Questions

Below are some common questions about sizing wire for underground circuits:

How much extra capacity should I add as a safety factor when sizing wire?

It is recommended to size your conductors for 125-150% of calculated load current as a safety factor. This accommodates unexpected load additions or motor inrush currents. Over-sizing wire is safer than under-sizing.

Can I run multiple circuits through the same underground conduit?

Yes, you can run multiple circuits in one conduit, but must derate ampacity for more than 3 current carrying conductors. Also observe NEC conduit fill limits, typically 40% maximum fill for power cables.

What is the code for maximum voltage drop in underground feeder runs?

NEC guidelines allow up to 3% voltage drop in feeder runs and up to 5% drop in branch circuit runs. Excessive drop can cause equipment problems, so it is advisable to limit to 2-3% when sizing feeders.

Why does wire ampacity rating decrease for underground installations?

Underground cables have reduced heat dissipation which increases insulation temperature leading to lower ampacity. Deeper burial depth exacerbates this effect. Derating adjusts for installation conditions.

Can I use the same wire sizing techniques for DC circuits?

Sizing principals are similar for DC but maximum voltage drop may be lower – typically 1-2%. Conductor insulation ratings and ampacity tables differ for DC systems. Follow manufacturer derating guidelines for DC wire sizing.

What is the easiest way to avoid excessive voltage drop when sizing long underground runs?

The simplest method is to increase wire gauge (lower AWG size). Larger conductors produce less voltage drop per foot of cable length due to higher cross-sectional area.

Key Takeaways for Sizing Electrical Wire for Underground Circuits

  • Carefully calculate expected electrical load and required current flow. Leave safety margin for future expansion.
  • Use proper wire sizing calculations and NEC guidelines to select correct wire gauge. Consider all derating factors.
  • Choose wire insulation type with appropriate voltage, temperature and moisture ratings for underground use.
  • Limit voltage drop to prevent performance issues and safety hazards. Increase wire size if needed.
  • Leave spare room in conduits for additional wiring. Limit conduit fill percentage.
  • Size wire conservatively for reliability and longevity. Do not undersize to save initial cost.

Properly applying these critical concepts will ensure underground electrical wires are sized correctly for safe operation and long service life. Investing in adequate conductors saves cost in the long run by preventing problems.


Sizing electrical wires correctly for underground circuit and feeder cables involves many important considerations for performance, safety and longevity. Carefully calculated load currents, maximum voltage drop, derating factors, ampacity ratings, conduit fill, and other guidelines from the NEC must be utilized to select adequate wire for the application. Choosing the right size conductor for underground installations prevents issues like overheating failures and voltage drops while providing system capacity for future expansion needs. Following the recommendations in this guide will allow electrical systems designers to specify properly sized wire for any underground circuit or feeder application.