Series Circuit Calculator

What This Calculator Does

This tool applies Ohm’s Law to a fixed-resistance, DC-style series circuit. Given a source voltage and up to three series resistors, it returns total resistance, circuit current, and the voltage drop across each resistor. It is intended for concept verification, training worksheets, and preliminary component arithmetic.

This calculator does not perform AC impedance analysis (it ignores reactance, phase angle, and power factor), does not select conductors or overcurrent protection, and does not produce an installation- or code-compliance result. Conductor sizing, ampacity, and voltage-drop compliance under the National Electrical Code (NEC) require separate calculations using the actual load type, conductor material, and installation conditions, as covered below.

Governing Formulas

For resistors connected in series, the following relationships apply:

Total resistance: \(R_{total} = R_1 + R_2 + R_3\)

Circuit current (Ohm’s Law): \(I = \dfrac{V_{source}}{R_{total}}\)

Voltage drop across each resistor: \(V_n = I \times R_n\)

Check: the sum of individual voltage drops must equal the source voltage:

\(V_1 + V_2 + V_3 = V_{source}\)

In a series circuit, current is identical through every element; only voltage divides, proportionally to each resistor’s share of total resistance.

Worked Example

Using the calculator’s default inputs:

Step 1 — Total resistance: \(R_{total} = 10 + 20 + 30 = 60\ \Omega\)

Step 2 — Circuit current: \(I = \dfrac{120}{60} = 2\ A\)

Step 3 — Voltage drop per resistor:

• \(V_1 = 2 \times 10 = 20\ V\)
• \(V_2 = 2 \times 20 = 40\ V\)
• \(V_3 = 2 \times 30 = 60\ V\)

Step 4 — Verification: \(20 + 40 + 60 = 120\ V\), matching the source voltage exactly.

All four outputs are independently recalculable from the two governing formulas above; no assumed or empirical correction factors are involved.

Relevance to Field Voltage-Drop Calculations

Field voltage-drop calculations on branch circuits and feeders are conceptually similar — current flowing through conductor resistance produces a voltage drop — but they are not interchangeable with this tool’s output. Field calculations must additionally account for:

• Conductor resistance per NEC Chapter 9, Table 8 (based on AWG or kcmil size and material — copper vs. aluminum), not an arbitrary resistance value.
• Circuit length (one-way distance doubled for the round-trip conductor path).
• Load current derived from actual equipment nameplate or NEC Article 220 calculated load, not a simplified fixed resistance.
• AC characteristics for larger conductors or longer runs, where conductor reactance becomes significant and pure DC resistance calculations understate voltage drop (NEC Chapter 9, Table 9 provides AC impedance values).

NEC 2023, 210.19(A) Informational Note No. 4, and the corresponding feeder provision at 215.2(A)(1) Informational Note No. 2, recommend voltage drop not exceeding 3% on either the feeder or branch circuit, with a combined total not exceeding 5%, for reasonable efficiency of operation. These are informational notes, not enforceable requirements, unless a local jurisdiction amendment makes them mandatory.

NEC 2026 (published by NFPA on August 20, 2025, effective September 9, 2025) retains this same guidance, though section or informational-note numbering may shift due to renumbering in that edition. As of mid-2026, most U.S. jurisdictions continue enforcing NEC 2023, while a small number of states — including Massachusetts and Texas — have adopted NEC 2026. Verify the edition and any amendments enforced by the local Authority Having Jurisdiction (AHJ) before citing a specific article number on a submittal or estimate.