Voltage drop comes up constantly in the field, and it trips a lot of people up on the exam too. The NEC has two numbers you need to know: 3% and 5%. Here is what they mean and where each one applies.
First, what is voltage drop
When current flows through a wire, the resistance of that wire causes some voltage to be lost along the way. The longer the run, the more voltage you lose before it gets to the load.
Think of it like water pressure dropping the further you get from the source. A motor or appliance at the end of a really long run might not be getting the full voltage it needs to run properly.
The NEC numbers: 3% and 5%
The NEC does not make voltage drop a hard requirement in most cases. It shows up as a recommendation in the informational notes, but it is still something inspectors and engineers pay attention to.
Here is how the numbers break down:
- 3% maximum voltage drop for branch circuits or feeders individually
- 5% maximum combined voltage drop for the feeder and branch circuit together
Where each number applies
The 3% number applies to either the branch circuit or the feeder on its own.
So if you have a long feeder run to a subpanel, you want to keep the voltage drop on that feeder under 3%. Same thing for a long branch circuit run out to a receptacle or piece of equipment.
The 5% number is the total you are allowed when you add both together. If your feeder drops 2% and your branch circuit drops 2%, you are at 4% combined and still within the recommendation.
Why it matters in real work
Voltage drop is not just a code thing. It is a performance thing.
If a motor is running on low voltage it works harder, runs hotter, and wears out faster. Lights dim at the end of a long run. Sensitive electronics can act up.
On longer runs, a lot of electricians will bump up the wire size one gauge just to keep the drop in check, even when the breaker size does not require it.
How to fix voltage drop
The main fix is upsizing the conductor. A larger wire has less resistance, which means less voltage lost over the same distance.
Other options include:
- Moving the panel closer to the loads
- Increasing the system voltage where possible
- Breaking one long run into shorter circuits
On the exam, if you get a question about a long run and they ask what you should do, upsizing the conductor is almost always the answer they are looking for.
The quick version to remember
- 3% for a single branch circuit or feeder
- 5% for the feeder and branch circuit combined
- These are NEC recommendations, not hard rules in most cases
- Longer runs mean more drop, upsize the wire to compensate
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