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Voltage Drop

by Zach Brown.

Voltage drop is something that I commonly find when doing home inspections, and it isn’t always easy to understand. Is it dangerous? Will it affect my electrical equipment I have plugged in? Why didn’t my last home inspector test for voltage drop? I usually go over these sorts of questions during or after my inspections, but I’m hoping this can be a more in depth resource to understanding voltage drop and why I test for it.

First, lets explore exactly what voltage drop is.
Voltage drop occurs naturally in every circuit as soon as a load is energized. The amount of the drop is dependent on the specific resistance in the conductor (wire), the distance of conductor between the source and the connected load, and the amount of current the load requires.

Put simply, you are going to lose some voltage in your electrical wiring regardless. How much depends on several factors, like the length of the wiring from your electrical panel to your outlets, or the gauge (size) of that wiring. Or both!

How much voltage drop is safe?
I test for voltage drop to help recognize symptoms of other potentially dangerous electrical defects, but we will get into that in a minute. The NEC (National Electrical Code)  recommends that voltage drop does not exceed 5%. For example, a loss of more than 6V in a 120V outlet exceeds the recommended maximum. That doesn’t necessarily mean anything over 5% is dangerous, and in fact, it is very rare I inspect a house that doesn’t exceed that. Voltage drop is nothing to be immediately alarmed by, but depending on the degree of drop, it can signal other problems.

Let’s discuss a couple examples.
I inspected a home built in 2013 a few months ago, and discovered a 25.8% voltage drop under maximum load in some parts of the house. Since it was a house built in the last four years, it’s safe to assume it passed all of the modern code requirements for electrical. However, it was a very big house–over 5,000 square feet–and the outlets that tested the highest amount of drop were the outlets farthest from the electrical panel. So, I made a note to look more into the drop later, but I wasn’t immediately alarmed because, as I mentioned earlier, the voltage drop naturally occurs as a result of long spans of wiring. Later in the home inspection, I opened the electrical panel and verified that the breakers corresponding with those outlets in the house were not overheating, overworked, or showing signs of defects in the wiring connection to the breaker and panel. In my report, I recommended an electrician review the electrical system as an extra precaution. My clients did hire an electrician, who verified that this was not a significant problem, but gave some suggestions for operating electrical equipment in those rooms.

During an inspection of a home built in 1908 about a month later, I discovered a voltage drop of 15%. When I did more digging, I found aluminum branch wiring and overworked conductors, which can be very dangerous and should be replaced.

Long story short, voltage drop should never be dismissed. More research into the cause is always recommended.

How does this effect things I plug into my outlets?

When I am testing for voltage drop, the percentage is calculated under maximum electrical load for that circuit. Rarely do the occupants of a home maximize the load on a circuit.  Most loads do not respond negatively to minor voltage drops under normal, less than full load applications. Lights may not burn as bright, for example.

The types of loads most likely to experience negative effects are motors. If operated for extended periods at reduced voltage, motors may experience earlier than normal failure.

An example of a household motor operated for extended periods might be a power supply in a computer.

Using the example above let’s look at a technical recommendation: If you have a circuit that tests with 25% voltage drop you would not probably not want to operate anything on regular basis with individual loads exceeding 50% of the usable amperage of the circuit. That would be six amps or 720 watts (1440 watts being a full load). Some more powerful computers are equipped with power supplies that large or larger, and if there is other electrical equipment operating on the same circuit it could damage that computer over time.

So what is a fix if I have high voltage drop and it’s because of a longer run of wire from the panel to the receptacle?
The only option here is to upgrade the gauge of wiring. 14 gauge wire is typically used on 120V receptacles, which meets modern code. 12 gauge or even 10 might eliminate some of the drop, but that would be a wildly expensive undertaking if the drop is not proven to be dangerous. It would benefit a home owner more to just be aware of the electrical equipment they are running on outlets that might have higher than 5% voltage drop.

Why don’t some home inspectors test for voltage drop?
It is not a requirement that a home inspector test for voltage drop and so some inspectors don’t have the appropriate testing equipment. Many avoid testing because it is a difficult concept to communicate to clients, which can extend the amount of time it takes to do an inspection. I think some inspectors find it easier to simply skip it. I’ve even heard stories of real estate agents that pass around the names of inspectors that skip testing for voltage drop because they don’t want to risk losing a sale.

Why does Tabor Northwest test for voltage drop if it isn’t required?
Testing voltage drop is fast, easy to implement into my inspections, and, as I mentioned before, can indicate other problems in the electrical system. It can be important knowledge for a home buyer to have as well. Most likely, they will be living in this new home for a long time, so me taking a little extra time to explain more is not only good for them, but good for my business. I want my clients to be as informed as possible, even if it means few extra minutes in my day as a home inspector.