How to Calculate Voltage Drop in Parallel Circuit?

Voltage drop in parallel circuit is calculated by finding the voltage loss along the shared feeder and each branch's own conductor separately. Because parallel branches connect across the same supply points they receive the same source voltage while the actual voltage drop depends on the current, resistance and length of each cable section.

In a series-parallel circuit, the calculation combines both approaches. You first calculate the voltage drop through the shared series section then add the relevant branch drop to determine the total voltage lost before a specific load.

This guide explains both calculation methods with clear formulas, worked examples and practical guidance so you can determine voltage drop accurately in parallel and series-parallel circuits.

Key Takeaways

Parallel Branches Don't Share Voltage Drop

Unlike a series circuit each branch in a parallel circuit sees the same source voltage with voltage drop occurring independently along each branch's own conductor based on its own current and length.

Shared Feeders Still Create a Common Drop

When multiple parallel branches share a common feeder cable back to the supply that feeder carries the combined current of all branches and produces its own voltage drop that applies to every branch equally.

Series Parallel Circuits Combine Both Methods

A series parallel circuit is calculated by adding the series section drop to the relevant parallel branch drop giving the total voltage lost on the path to any particular load.

What Is Voltage Drop in a Parallel Circuit?

In a parallel circuit each branch is wired directly across the same two supply points so every branch receives the same source voltage before any drop occurs this is the key difference from a series circuit where voltage divides between components.

Voltage drop in a parallel circuit still happens but it occurs independently along each branch's own conductor based on that branch's own current and cable length.

Why Does This Matters for Cable Sizing?

Because each branch carries its own current independently a parallel branch with a heavier load needs its own adequately sized conductor regardless of how the other branches are wired under sizing one branch won't be compensated for by the others.

Where Parallel Wiring Shows Up in Practice

Parallel wiring is the standard configuration for most electrical installations because each load operates independently so if one branch fails the rest keep running.

This makes it the default choice for residential lighting circuits, electrical outlets and commercial power distribution while series parallel arrangements are more common in industrial control panels, automotive systems and battery banks.

How Do You Calculate Voltage Drop in a Parallel Circuit?

Calculating voltage drop in a parallel circuit means looking at two possible sources of loss, the shared feeder cable supplying the parallel branches and each branch's own individual conductor.

The Parallel Circuit Voltage Drop Formula

Feeder Drop: VD(feeder) = 2 × I(total) × R(feeder) × L(feeder)

Branch Drop: VD(branch) = 2 × I(branch) × R(branch) × L(branch)

Total Drop to a Load: VD(total) = VD(feeder) + VD(branch)

Where I(total) is the combined current of all branches flowing through the shared feeder and I(branch) is the current of the specific branch you're checking.

If you need a quick reference for voltage drop formulas and resistance values, see our Voltage Drop Formula and Calculation Table .

Understanding the Variables

The feeder cable carries every branch's current added together so even a small feeder resistance can produce a meaningful drop once multiple branches are combined.

Each branch's own cable only carries that branch's individual current so its contribution to the total drop depends on how long that specific branch run is.

Voltage Drop Calculation Example for a Parallel Circuit

Consider three LED lighting branches sharing a 20m feeder cable with a combined current of 6A flowing through 2.5mm² copper feeder cable (0.0074 Ω/m). Each branch then runs its own 5m cable at 1.5mm² (0.0121 Ω/m) carrying 2A per branch.

Feeder drop, 2 × 6A × 0.0074 × 20m = 1.78V. Branch drop, 2 × 2A × 0.0121 × 5m = 0.24V. Total voltage drop to any one branch load is 1.78V + 0.24V = 2.02V which on a 230V supply works out to about 0.9% comfortably within recommended limits.

What Is a Series Parallel Circuit?

A series parallel circuit combines both wiring styles part of the circuit runs in series then splits into parallel branches or the reverse. This is extremely common in real installations, where a single feeder cable series section runs from the source before splitting to supply multiple parallel loads.

How to Calculate Voltage Drop in a Series Parallel Circuit

To calculate voltage drop in a series parallel circuit treat the series portion using the standard series formula then treat the parallel portion using the branch method described above and add the two results together for the total drop to any specific load.

If you're dealing with a straightforward series circuit, see our guide to Voltage Drop in a Series Circuit for the basic calculation method and worked examples.

Series Parallel Voltage Drop Calculation Example

Take a 24V battery system with a 10m series cable run (0.0046 Ω/m, 4mm²) carrying the full 15A load to a junction which then splits into three parallel branches of 8A, 4A and 3A.

The series section drop is 2 × 15A × 0.0046 × 10m = 1.38V. If the 8A branch runs 6 meters of 2.5mm² cable (0.0074 Ω/m) its branch drop is 2 × 8A × 0.0074 × 6m = 0.71V.

The total voltage drop to that branch's load is 1.38V + 0.71V = 2.09V or about 8.7% of the 24V supply above the recommended 3% limit meaning either the series or branch cable would need upsizing.

Common Mistakes When Calculating Parallel Circuit Voltage Drop

A couple of specific errors show up often when people work with parallel or series parallel circuits.

Forgetting to Add the Feeder Drop

It's easy to calculate only the individual branch drop and forget that the shared feeder cable carrying the combined current of every branch also contributes its own voltage loss to the total.

Using the Wrong Current for Each Section

The feeder section uses the total combined current of all branches while each branch section uses only that branch's individual current mixing these up produces a significantly wrong result.

Series vs Parallel vs Series Parallel at a Glance

• If your circuit combines a shared feeder with multiple branches, working out each section by hand can get complicated fast. Our free Voltage Drop Calculator handles feeder and branch sections separately and shows the total drop to any load instantly.

Conclusion

Calculating voltage drop in a parallel or series parallel circuit takes a bit more care than a simple series run since you need to account for both the shared feeder cable and each branch's own conductor separately. Once you break the circuit down into its series and parallel sections though the same basic formula applies to each part.

For circuits with multiple feeders and branches our free voltage drop calculator can save significant time compared to working out each section by hand.

FAQs

Does a parallel circuit have less voltage drop than a series circuit?

Not necessarily it depends on the cable lengths and currents involved. A parallel circuit can still have significant voltage drop if the shared feeder or individual branches use long or thin cable.

Do all branches in a parallel circuit need the same cable size?

No. Each branch is sized based on its own current and length so a branch carrying a heavier load or running a longer distance may need a larger conductor than the others even though they all share the same feeder.

How do you calculate voltage drop across a resistor in series?

For a resistor sitting within a series circuit its individual voltage drop is simply its resistance multiplied by the shared circuit current see our dedicated resistor voltage drop guide for worked examples specific to resistor calculations.

Can I use a calculator for a series parallel circuit?

Yes, our voltage drop calculator can handle the series section and the relevant parallel branch separately, saving you from manually adding the two results together.

Can voltage drop exceed the supply voltage?

No. The total voltage drop through any circuit can never be greater than the source voltage if a calculation shows a larger figure if there's an error somewhere in the resistance values or current used.