How the two differ in kind
Choosing a conductor answers two separate questions, and they do not return the same answer. The first is whether the wire can carry the current without getting too hot — an AMPACITY question, decided by the conductor material and size, its insulation's temperature rating, how many conductors share the raceway, and the ambient temperature around it. The second is whether enough voltage survives the journey — a VOLTAGE DROP question, decided by the resistance of the run, which rises with length and falls with size.
For a short circuit the ampacity answer governs and the drop is negligible. As the run gets longer, drop grows in proportion to length while ampacity does not change at all, and at some distance the voltage-drop answer overtakes it. From that point on, a conductor sized only on ampacity is legal, passes inspection, and under-performs for its whole life.
The asymmetry worth understanding is what protects each one. A conductor sized correctly for ampacity is guarded by the overcurrent device: exceed the current it was sized for and something trips. A conductor undersized for voltage drop has no such guardian. Nothing trips, nothing warns, and the symptoms are diffuse — a motor that runs hot and lives a short life, lights that dim when a large load starts, heating elements that take longer than they should, electronics that behave oddly at the end of a long run.
In most codes the split is explicit: ampacity is a requirement and the tables are mandatory, while the voltage-drop figure is a recommendation — commonly expressed as a small percentage on a branch circuit and a slightly larger total including the feeder. Being a recommendation does not make it optional in engineering terms; it makes it the part nobody is compelled to check.
The factors that actually differ
Show
| Sized for ampacity | Sized for voltage drop | |
|---|---|---|
| What the rule protects | The conductor and its insulation. This is a fire-safety limit. | The load at the far end. This is a performance limit. |
| Status in the code | Mandatory, from the tables, with corrections applied for temperature and for conductors bundled together. | Usually a recommendation rather than a requirement — which is precisely why it gets skipped. |
| Sensitivity to run length | None. A conductor's ampacity is the same at two metres and at two hundred. | Directly proportional. Double the run and you double the drop for the same conductor. |
| What happens when it is wrong | The conductor overheats and its insulation degrades — and the overcurrent device is there to stop it. | Nothing trips, ever. The equipment simply receives less voltage than it was designed for, permanently. |
| Which governs | Short runs, and it sets the floor below which you may never go. | Long runs, where it typically demands one or more sizes larger than ampacity alone. |
| Effect of derating | Substantial. High ambient temperature and several current-carrying conductors in one raceway both reduce the usable ampacity. | Unaffected by either. Resistance is a property of the conductor and the run. |
| Effect of the load type | Continuous loads are sized with an uplift, and motors have their own rules. | Motors and other high-inrush loads are the most sensitive: starting current is a multiple of running current, so the momentary drop is a multiple of the steady one. |
| Material | Aluminium has lower ampacity than copper of the same size, so it needs to be larger. | Aluminium also has higher resistance, so it drops more voltage — the penalty compounds on a long run. |
| Relationship to the breaker | Directly coupled. The overcurrent device is chosen to protect the conductor. | Unrelated. Increasing the wire size for voltage drop does not change the breaker. |
| How to resolve the two | Take the larger of the two sizes. Never the smaller, and never an average. | Take the larger of the two sizes. Going up in size costs material once and pays back in performance every day. |
Which one, and when
Choose sized for ampacity when…
- The run is short, where drop is negligible and ampacity is plainly the binding constraint.
- The immediate question is safety and compliance — what is the minimum legal conductor for this load?
- Conditions are adverse: a hot location, or several current-carrying conductors sharing a raceway, where derating matters more than distance.
- Selecting or verifying the overcurrent device, which is coupled to ampacity and not to drop.
Choose sized for voltage drop when…
- The run is long — a detached garage, a workshop, a well pump, a gate, a barn, landscape lighting.
- The load is a motor or anything with high inrush, where starting drop is several times running drop.
- The load is sensitive to voltage: electronics, dimmable lighting, precision equipment.
- The circuit is already at its ampacity limit and the distance is significant — the case where the two answers diverge most.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Which one wins if they disagree?
- The larger conductor, always — they are not competing rules but two independent minimums, and the wire has to satisfy both. Ampacity sets a floor you may never go below because it is a safety limit protecting the conductor and its insulation from overheating. Voltage drop sets a second floor, for a different reason, protecting the equipment at the far end from running below its design voltage. When the run is short, ampacity is the higher of the two and decides. When the run is long, voltage drop overtakes it and decides, often demanding a conductor one or two sizes larger than the load alone would suggest. Averaging them, or picking the ampacity answer because it is the mandatory one, gives a circuit that is legal and under-performing.
- Why does nothing trip when voltage drop is too high?
- Because excessive voltage drop is not an overcurrent condition, and overcurrent is the only thing a breaker or fuse detects. A conductor that is too small for the distance still carries a current within its rating — the current is, if anything, exactly what the load draws — so the protective device sees nothing unusual and never operates. What changes is the voltage arriving at the far end, and nothing in a standard installation measures that. The result is that the fault is invisible at the panel and diagnosed at the load, usually as a symptom rather than a cause: a motor that runs hotter than it should and fails early, lights that dim when a compressor starts, a heater that takes longer to reach temperature. The only way to find it is to measure voltage at the load while it is running.
- What voltage drop is acceptable?
- Most codes express it as a small percentage on the branch circuit with a slightly larger allowance for the branch and feeder combined, and the figures in your local code are the ones to use — they differ between regimes, and a figure quoted from somewhere else is not a defence. Two points matter more than the exact number. It is normally a recommendation rather than a requirement, so nobody will stop you exceeding it; and it is a budget shared along the whole path, so a feeder that has already used most of the allowance leaves very little for the branch circuit beyond it. For sensitive or motor loads it is worth designing well inside the figure rather than up to it, because the recommendation is about acceptable performance rather than good performance.
- Why are motors so sensitive to voltage drop?
- Because starting current is several times running current, so the momentary drop during starting is several times the steady-state drop that was calculated. A run showing an acceptable drop at running load can dip severely for the moment the motor starts, and that has two consequences: the motor may struggle to start or fail to start under load, and everything else on the same feeder sees the dip as well — which is the flicker that makes lights blink when a compressor or pump kicks in. The second issue is continuous rather than momentary: a motor supplied below its design voltage draws more current to produce the same output, which makes it run hotter, and heat is what determines how long its insulation lasts. Motor circuits are therefore worth designing with margin rather than to the limit.
- Does going up a size ever cause a problem?
- Rarely, and the constraints are mechanical and terminal-related rather than electrical. A larger conductor fills more of the raceway, so a conduit sized for the original conductors may no longer comply on fill; it is stiffer and harder to pull and to terminate; and it costs more. The more commonly missed one is terminations: devices and breakers have a range of conductor sizes their terminals accept, and a conductor larger than that range will not land properly — which sometimes means a short pigtail of the smaller size at the device, and that has its own rules. What does NOT change is the overcurrent device: increasing conductor size for voltage drop does not permit or require a larger breaker, because the breaker is sized to the load and to protecting the circuit, not to the wire's capacity.
- Does aluminium change the answer?
- It penalises both calculations, and the penalties compound on exactly the runs where it is most attractive. Aluminium carries less current than copper of the same size, so it needs to be larger on ampacity grounds; and it has higher resistance, so it drops more voltage over the same distance, needing to be larger again. Against that it is cheaper and lighter, which is why it remains standard for larger feeders and service conductors where the run is long and the size is big. The other consideration is terminations, which have their own requirements — listed connectors, the correct compound, and torque to specification — because aluminium's behaviour at a joint is the historical source of its poor reputation rather than anything about the conductor itself.
- How is the run length measured?
- As the one-way distance from the source to the load, with the calculation accounting for the fact that current travels out and back. Two mistakes are common. The first is measuring the straight-line distance rather than the route the cable actually takes: a cable that goes up a wall, along a ceiling, around an obstruction and back down can be substantially longer than the distance across the floor, and the extra length is real resistance. The second is forgetting the feeder: the drop that matters at a piece of equipment is the total from the service, so a long feeder to a sub-panel consumes part of the allowance before the branch circuit has started. Where a sub-panel serves a distant building, the feeder is usually where the size increase belongs, because it benefits every circuit beyond it.
- Is this something to do myself?
- The calculation is worth doing yourself; the installation is electrician's work and in most places is legally required to be. Understanding which of the two constraints governs your run is genuinely useful before you commission anything — it tells you whether a quote that specifies the minimum conductor for the load has considered the distance, and it is a reasonable question to ask. What the calculation does not cover is everything else that decides a safe circuit: the overcurrent device, the protective earthing arrangement, derating for the actual installation conditions, terminations, and the inspection regime. Use the figures to inform the conversation and to sanity-check a proposal; leave the conductor selection of record, and the work, to the person who will certify it.