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100 & 200 Amp Service Wire Size Calculator

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Find the NEC reference conductor size for a 100, 125, 150 or 200 amp service, compare copper with aluminium and check whether run length requires a voltage-drop upsizing.

Inputs

1

NEC ampacity basis

2

Run-length check

How to use this calculator: 100 & 200 Amp Service Wire Size Calculator

Find the NEC reference conductor size for a 100, 125, 150 or 200 amp service, compare copper with aluminium and check whether run length requires a voltage-drop upsizing. The example below is calculated by this page's real engine from the displayed inputs.

  1. 1Select 100, 125, 150 or 200 amperes. The actual dwelling or building load must first be calculated under the applicable NEC load-calculation rules; panel rating alone does not prove the load.
  2. 2Use the dwelling option only for a qualifying single-phase dwelling service or feeder that carries the dwelling's entire load. Otherwise use the general Table 310.16 option.
  3. 3Aluminium normally needs a larger gauge and must use conductors, lugs, oxide-control practice and installation instructions suitable for aluminium.
  4. 4The code-minimum conductor can be thermally adequate but still have excessive drop on a long service. Include the complete routed length, not the straight-line property distance.
  5. 5Check terminal temperature rating, ambient and bundling correction, service load calculation, grounded conductor sizing, equipment grounding, local amendments and utility requirements with the AHJ.

Input guide and example values

Use values from the same measurement basis and time period. Conditional fields appear only when the related option is selected.

InputExample valueWhy it matters
Service rating200 ASelect the option that matches the real installation or scenario.
Installation coveredDwelling service / feeder eligible for NEC 310.12Select the option that matches the real installation or scenario.
Conductor materialAluminium or copper-clad aluminiumSelect the option that matches the real installation or scenario.
One-way service length75 ftMeasured or known one-way service length used by the calculation engine.
Service voltage240 VMeasured or known service voltage used by the calculation engine.
Design voltage-drop limit3%Select the option that matches the real installation or scenario.

Formula inputs & variables for 100 & 200 Amp Service Wire Size Calculator

These are the named quantities used by this calculator. When the source formula does not define a mathematical symbol, OneCalcApp keeps the real input label instead of inventing one.

Variable / inputUnitMeaning in this calculation
Service rating—Select the option that matches the real installation or scenario.
Installation covered—Select the option that matches the real installation or scenario.
Conductor material—Select the option that matches the real installation or scenario.
One-way service lengthftMeasured or known one-way service length used by the calculation engine.
Service voltageVMeasured or known service voltage used by the calculation engine.
Design voltage-drop limit—Select the option that matches the real installation or scenario.

Formula, derivation and worked example

A common one-line answer—such as 4 AWG copper for 100 amps or 4/0 aluminium for 200 amps—is only correct when the installation qualifies for the dwelling-service sizing allowance. A general feeder or service uses the normal Table 310.16 ampacity and can require a larger conductor. This calculator keeps those two cases separate, then checks the selected minimum at full service current for voltage drop. The authority having jurisdiction (AHJ), terminal ratings, cable type and locally adopted NEC edition still control the installation.

Dwelling method: permitted conductor from NEC 310.12 for qualifying 100–400 A, 120/240 V single-phase dwelling services/feeders
General method: conductor ampacity ≥ service rating using the 75 °C column of NEC Table 310.16 when permitted by the equipment terminals
Single-phase voltage drop ΔV = 2 × I × R₇₅ × length / 1000

Substitution steps

  1. 1. Ampacity minimum
    NEC 310.12
    = 4/0 AWG
  2. 2. Minimum-size voltage drop
    2 × I × R₇₅ × ft / 1000
    = 3 V (1.25%)
  3. 3. Final selection
    larger of ampacity minimum and voltage-drop size
    = 4/0 AWG

Computed example results

Recommended ungrounded conductors
4/0 AWG aluminium
dwelling ampacity method governs
NEC reference minimum
4/0 AWG aluminium
NEC 310.12 qualifying dwelling
Voltage drop at selected size
3 V
1.25% over 75 ft at 200 A
Drop at code-minimum size
1.25%
3 V
Table 310.16 ampacity of selected size
180 A
75 °C column before corrections
Approximate metric area
107 mm²

Understanding the result

Read the main result together with supporting checks, assumptions, limits and intermediate values.

For a manual check, repeat the first equation, confirm the units and change one input at a time.

Common mistakes when using 100 & 200 Amp Service Wire Size Calculator

  • Do not mix units for One-way service length (ft), Service voltage (V). A unit mismatch changes the input magnitude even when the typed number looks reasonable.
  • Do not leave Service rating on the default choice unless that choice matches the real scenario; the selected option can change the calculation path or factor.
  • Do not replace the displayed Dwelling method: permitted conductor from NEC 310.12 for qualifying 100–400 A, 120/240 V single-phase dwelling services/feeders relationship with a different convention without also changing the underlying assumptions; compare like-for-like methods when checking the result.
  • Do not treat Recommended ungrounded conductors = 4/0 AWG aluminium from the worked example as a universal answer. It belongs to the displayed example inputs and must be recalculated for the actual case.

Next logical calculator

Continue with Voltage Drop Calculator

Useful next check because both tools use Conductor material, while Voltage Drop Calculator answers a different part of the same workflow.

Open Voltage Drop Calculator

100 amp service wire size explained

The familiar 4 AWG copper or 2 AWG aluminium answer comes from the dwelling-service provisions. If those provisions do not apply, the 75 °C ampacity table moves the minimum to 3 AWG copper or 1 AWG aluminium for 100 A.

A long run can need a larger conductor even when the code ampacity is adequate. The calculator compares the reference minimum with a full-load voltage-drop check and reports the larger result.

200 amp service wire size explained

For a qualifying dwelling, 2/0 AWG copper or 4/0 AWG aluminium is the common 200 A service answer. A general 200 A application instead needs 3/0 AWG copper or 250 kcmil aluminium at the 75 °C table values before correction factors.

Utility service rules can specify cable types, raceways, meter equipment or conductor sizes beyond the NEC minimum. Obtain the utility service manual and AHJ approval before purchasing material.

Why service load and panel rating are different

The service rating is the capacity of the service equipment; conductor design still begins with a calculated load. Continuous loads, heating, cooking, dryers, vehicle charging, air conditioning and permitted demand factors are handled by the applicable load-calculation articles.

Do not assume every 200 A panel carries 200 A continuously. Conversely, do not reduce the conductors below the service-rating rules merely because a recent utility bill shows a low average demand.

Formula

  • Dwelling method: permitted conductor from NEC 310.12 for qualifying 100–400 A, 120/240 V single-phase dwelling services/feeders
  • General method: conductor ampacity ≥ service rating using the 75 °C column of NEC Table 310.16 when permitted by the equipment terminals
  • Single-phase voltage drop ΔV = 2 × I × R₇₅ × length / 1000

A common one-line answer—such as 4 AWG copper for 100 amps or 4/0 aluminium for 200 amps—is only correct when the installation qualifies for the dwelling-service sizing allowance. A general feeder or service uses the normal Table 310.16 ampacity and can require a larger conductor. This calculator keeps those two cases separate, then checks the selected minimum at full service current for voltage drop. The authority having jurisdiction (AHJ), terminal ratings, cable type and locally adopted NEC edition still control the installation.

100–200 amp service conductor reference — copper and aluminium

Dwelling sizes apply only when NEC 310.12 conditions are met. General sizes use the 75 °C Table 310.16 ampacity column before any required correction or adjustment. Verify the NEC edition adopted locally.

ServiceDwelling copperDwelling aluminiumGeneral copperGeneral aluminium
100 A4 AWG2 AWG3 AWG1 AWG
125 A2 AWG1/0 AWG1 AWG2/0 AWG
150 A1 AWG2/0 AWG1/0 AWG3/0 AWG
200 A2/0 AWG4/0 AWG3/0 AWG250 kcmil

What size wire for a 100 amp service?

For a qualifying dwelling service, the reference minimum is 4 AWG copper or 2 AWG aluminium. Under the general 75 °C Table 310.16 method it is 3 AWG copper or 1 AWG aluminium, before correction factors and voltage-drop upsizing.

What size wire for a 200 amp service?

For a qualifying dwelling service, the reference minimum is 2/0 AWG copper or 4/0 AWG aluminium. Under the general 75 °C Table 310.16 method it is 3/0 AWG copper or 250 kcmil aluminium, before corrections and voltage-drop upsizing.

NEC 310.12 is conditional

The dwelling allowance is not a general 83% shortcut for every 100A or 200A feeder. Confirm the occupancy, voltage, rating range and whole-dwelling-load conditions in the locally adopted NEC text.

Neutral and grounding conductors are separate checks

This result sizes the ungrounded service conductors. The grounded conductor, grounding electrode conductor and equipment grounding conductor follow different rules and must not be copied automatically from this answer.

Voltage Drop Calculator

Run a more detailed NEC voltage-drop check with ambient, bundling and termination inputs.

Wire Gauge Chart

Compare AWG, mm², conductor diameter and resistance.

Assumptions

  • 75 °C equipment terminations and conductors are assumed for the Table 310.16 reference sizes.
  • Voltage-drop calculation uses NEC Chapter 9 Table 8 conductor resistance at 75 °C and the selected service rating as a conservative current.
  • No ambient-temperature, rooftop, bundling or parallel-conductor correction is applied by this simplified service page.

Warnings

  • Never apply the dwelling-service sizes to a feeder or occupancy that does not meet NEC 310.12 conditions.
  • The result does not size the neutral, grounding electrode conductor, equipment grounding conductor, raceway, meter equipment or service disconnect.
  • Service work is hazardous and normally permit- and utility-controlled; use a licensed electrical professional and obtain AHJ approval.

Standards & references

  • NFPA 70 (NEC) 2023, 310.12
  • NFPA 70 (NEC) 2023, Table 310.16
  • NFPA 70 (NEC) 2023, 110.14(C)
  • NFPA 70 (NEC) 2023, Chapter 9 Table 8

Frequently asked questions

Can I use 4 AWG copper for a 100 amp service?

Yes for a qualifying dwelling service or feeder under NEC 310.12, subject to all listed conditions and local adoption. A general 100 A application normally uses 3 AWG copper from the 75 °C Table 310.16 column.

Can I use 4/0 aluminium for a 200 amp service?

It is the common reference size for a qualifying dwelling service under NEC 310.12. A general 200 A application normally requires 250 kcmil aluminium at 75 °C before corrections.

Is 2/0 copper enough for 200 amps?

It can be permitted for a qualifying dwelling service under NEC 310.12. Under the general Table 310.16 method, 2/0 copper is 175 A at 75 °C, so 3/0 copper is the 200 A reference size.

Does a long service run need larger wire?

Often. Ampacity can pass while voltage drop is excessive. This page checks the reference minimum against the selected run length and voltage-drop target.

Does this calculator size the neutral and ground?

No. Neutral, grounding electrode and equipment grounding conductors use separate NEC rules and require project-specific checks.

What inputs does the 100 & 200 Amp Service Wire Size Calculator use?

It uses Service rating, Installation covered, Conductor material, One-way service length, Service voltage, and Design voltage-drop limit. Follow the unit printed for each field and choose any selectable option to match the real scenario.

What does the 100 & 200 Amp Service Wire Size Calculator calculate?

It calculates Recommended ungrounded conductors, NEC reference minimum, Voltage drop at selected size, Drop at code-minimum size, Table 310.16 ampacity of selected size, and Approximate metric area. With the displayed default inputs, Recommended ungrounded conductors is 4/0 AWG aluminium.

Which formula does the 100 & 200 Amp Service Wire Size Calculator use?

The primary relationship is Dwelling method: permitted conductor from NEC 310.12 for qualifying 100–400 A, 120/240 V single-phase dwelling services/feeders. The page also shows substituted values and calculation steps so the result can be checked independently.

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