Voltage Drop

My question pertains to using a SureTest or similar instrument for testing voltage drop.

According to the pamphlet I have it states the NEC recommends no more then a 5% voltage drop.

It also states that a higher voltage drop leads to heat buildup and performance issues.

What percentage would you recommend evaluation by a licensed electrician?

What are the causes for higher voltage drops.

How would you report high voltage drop.


Sorry I am an retired Electrician and I do not see where I as a home inspector should be doing voltage testing so I do not test for voltage drop.

… Cookie

Same here.

You might want to consider a consultation for doing voltage drops. :mrgreen:

I do :mrgreen:

First thing to remember is, the NEC statement is a “Permissive Rule,” and not a “Mandatory Rule.” Simply put, this is not an “enforceable code.”

Generally, excessive VD is an indication of high resistance within a circuit.

I don’t have time this morning to get into this, but I’ll check back later. . .

You have apply a bit of common sense to such findings.

A 10% drop at the end of AWG 14 outlet circuit in the far corner of a McMansion may not be evidence of a wiring “defect”.

The same voltage drop 20’ from the service equipment and half way down a string of kitchen counter outlets suggest there is a problem.

Using #14 for long wire runs is a defect by itself.

I agree Ron.

Reporting on voltage drops can get you into trouble. I would suggest that anyone considering this, educate themselves first.

The first consideration is the line voltage. If the voltage at a given receptacle is 125V, a 10% drop with a 15 amp load is not necessarily a “big deal,” as this will have little effect on appliances because you still have 112.5V at the receptacle. However, if your initial voltage reading is in the 110V range, a 10% drop may cause problems with some appliances.

A large variation in VD at receptacles on the same circuit is more of a concern (IMHO). All receptacles on the same circuit should generally have similar drops when a load is applied. A big difference could indicate a poor connection or worse.

There’s much more to understand before attempting to accurately interpret voltage drop measurements and I’m not the one you should be learning from :wink:

Maybe some of our resident experts will chime in. . .

That might not be a good rule to live by. I have installed lights such as those in the photograph as far as 700 feet away using #14 UF cable.


The door operator that is used on this gate could plug into a receptacle that is on that same circuit and not be hurt due to a voltage drop.

The amount of voltage drop is proportionate to the load it is serving.

Something else that needs to be considered is the voltage that the power company is supplying.

Here at my home, most of the time, I can read as high as 124 volts through out the house. I am the sole user of the transformer supplying my house.

My brother shares his transformer with a couple of others and I have saw his at 114 volts especially on a real hot or cold day.

Not only is it not a good rule to live by, it is totally false.

Someone who is not well versed on the subject may consider it an opinion that is it not good, but there would be NO factual information to support it. At least in the blanket way it was presented.

Replace “is” with “could be” and you are making a more reasonable statement.

I would not run any motor on a 14 wire, especially a 700’ 14 wire. It may appear to be okay, but it will have a premature death. I would not use and high wattage bulbs either. Some times those gate lights only use 15 watt bulbs. I hate to be one but if you’re having to ask these type of questions, you might not want to report on VD, it is not part of a home inspection. JMO

That gate operator is a 12 volt DC system and has a trickle charger that plugs into a general purpose receptacle.

With two 40 watt bulbs and the little charger that is around 75 watts the total draw on the circuit would only be 115 watts and would not be a very large voltage drop at all.

A very big misconception about voltage drop can be hushed with the only mandatory calculation for a Voltage Drop found in the NEC

695.7 Voltage Drop.
The voltage at the controller line terminals shall not drop more than 15 percent below normal (controller-rated voltage) under motor starting conditions. The voltage at the motor terminals shall not drop more than 5 percent below the voltage rating of the motor when the motor is operating at 115 percent of the full-load current rating of the motor.

Proper use of a volt meter is not something that requires a person with a lot of training

Voltage drop percentage is not hard to compute if you did not sleep through high school math 101

Measure the no load voltage

Measure the voltage under load

Do the math

To check the drop to the home

Turn everything off - read the voltage at the panel

Turn everything on - read the voltage at the panel

Yes, this is out side of a home inspection but not beyond the skill level of someone who can use a volt meter

Remember that voltage drop % does not require one to load a circuit to its max rating – just the current of what it is used for

That is why a long run of #14 to the gate lights is not a problem.

If one wants to go the next step - go and compute the % of voltage drop on #14 or #12 wire per 200 feet (or 1000 if you want) at rated current.

Having a feel for what readings should be is a good thing - at least if you see something that does not look good you can get on the NACHI BB and ask the experts

Remember that when the gage of wire is computed for a job all this math is done by someone that knows ohms law E=IR

A good electrician can teach this subject in one day



In a home where the outlets are supposed to be able to deliver 15 amps, when is it OK for #14 to be used for long runs? All the calculators and tables I have seen show 5% drop (at 15 amps) in the wire alone for a 75ft run. Many larger (not mansion) homes may runs much longer than that by the time the wire is routed from the panel on one side to an upper bedroom on the opposite side. I don’t consider 75ft to be a long run. Several cities in my area have banned use of #14 for new construction. Is it a problem when pulling 2 amps for a radio? Nope. But the outlets are rated for 15 amps.

Are we home inspectors or are we trying to do an electricians job.
Where does it end.
Balancing Air handling units .
Size of the feed and return on AC.
Are the air ducts the correct size .
Do they have too many bends in the Dryer vent.
Is 1/2 pipe large enough for a gas demand water heater.
The choice is yours .
How long do you wish to spend on a visual Home Inspection.

… Cookie

I for one, am not trying to do an electricians job but it doesn’t take an electrician to go “wow, it’s a long way from the panel to this bedroom” and then plug my SureTest into the outlet and see that it shows 22% voltage drop due to the long run of #14 wire and multiple backstabbed outlets in series. I then can suggest that qualified electrician be used to assure that it’s really OK or to get it fixed. That gets the monkey off my back and lets me feel better about the situation.

Ron, I think Roy’s point was that there was no monkey on your back to begin with…

but the way I see it…if it lets you feel better about the situation, and you want the additional role, then go for it. :smiley:

I always get a long laugh hearing agents tell how an inspector with his/her brand new SureTest checked the outlets at a home inspection they were on - found a 7%-10% voltage drop and recommended calling out an electrician for electrical review, etc. AND the electrician called the HI a nincomboob.

The buyer ran away from the sale - the agents lost $18,000 commission.

The agents then go on to tell how they’re passing the inspectors name around at sales meetings to alert other agents so they can keep this inspector out of any of their sales transactions.

Gosh life is humorous.

A. I won’t call out a 7-10% voltage drop but when it hits 12-15% I do.

B. Once when I called out several outlets with 15-22% drops, I got a call from an “electrician” who said he measured the voltage at the panel at 120 volts with his meter so there was “no problem”. I spent 10 minutes trying to explain to him how to test for voltage drop. Not sure if he got his license in a cracker jack box or what or if he really had a license. Anyway somtimes the nincompops are right.

C. I don’t try and please realtors. :twisted:

Here we have a problem.

What method are you using to come up with the voltage drop?

If you are using a voltage tester and coming up with a difference in voltage in a house the problem is something different than voltage drop due to the length of the circuit.

A simple calculator can tell you what the voltage drop is for a given circuit.
Using this calculator and the load of a voltage tester of let’s say one tenth an amp we can see that the voltage drop on a 15 amp circuit installed with #14 NM cable would need to be at least 6716.82 feet to have a 3% drop.

As you can see the meter is not enough of a load to make a difference in the voltage in a normal house.

If you are finding a 15% difference in the circuit of a house using nothing more than a voltage tester then there is a serious problem with the circuit and it is far more than a simple voltage drop due to the size of the conductor.

The formula for voltage drop can be found here
As we all can see one of the major players of voltage drop is the load on the circuit (twice the resistance of the conductor times the load or amperage times the length of the run)

It would be better to call out a difference in the voltages of a circuit as something else other than a “Voltage Drop”.
As a contractor the first thing I am going to ask for in the event of a HI calling out a voltage drop is the math he used. No math equals no voltage drop.

As I said in post #15, and verified by your calculator, about 75 ft. of #14 wire will have a 5% drop at 15 amps. Hence the previous comment that long runs (much greater than 75ft) of #14 are a problem. I use a Suretest which puts a load on the circuit and calculates the voltage drop under load. Measuring the voltage under no load proves nothing.

The Suretest measures the voltage no load, then puts a momentary 12 amp load on the circuit, measures the voltage again with the load, then calculates the % voltage drop at 12 amps, extrapolated to 15 amps, and extrapolated to 20 amps. That takes about 3 seconds for the tester to perform. I use the 15 amp value as that is what the outlets are rated for.