Voltage Drop at outlets on a Custom Residential Home

[kwired]

Thank you everyone for the insight you are providing.

At what point would voltage drop start negatively impacting things in a house? Different level for different equipment. Simple incandescent lamp, puts out a little less light at 5% less volts, but you likely won't notice, at same time it runs a little cooler and that extends lamp life, so not necessarily all bad either.

On residential construction, what code determines the size wire that is required for different circuits? AS fare as NEC goes, generally just need to have ampacity to handle the load without overheating the conductor as well as overcurrent protection that will protect the conductor at it's ampacity. Voltage drop is a design issue and reason the recommendation is an informational note.

Is there a code that says there is a maximum, unacceptable voltage drop? NEC I think does have some VD requirements in specific applications, like in some chapter 5 or 6 applications, but non as a general rule of chapters 1-4.

Does a voltage drop of 14.8% on the last outlet in a 15 amp branch circuit using 14-2 wire, with 6 outlets, concern you? Most of the time no, and might even be somewhat expected if drawing a heavy load. Also consider that 120 is nominal and a lot of equipment is designed for 115. Starting out with 123 volts (happens many places) and having a drop to 114 gives you same thing as you would have with 5% drop on 120. They run unloaded volts a little high for this reason.

If one expects there home to be wired to the recommendations in the NEC, then any voltage drop over 5% is unacceptable. Why would the NEC publish this recommendation? As I said above Voltage drop is a design issue and reason the recommendation is an informational note.

..

 

[gar]

190504-1121 EDT

Downtown1121:

You list your occupation as Home Builder. Thus, you may have little electrical background.

If you do not electrically overload a wire, then from the wire standpoint you do not have a fire hazard. This is independent of wire length. There is more to this statement, but if the wire is not coiled up, and some other factors, then it is correct.

However, within safe current limits you have the problem of voltage drop. The wire resistance for a specific wire size (cross sectional area), and material has some constant resistance per unit length, and some not to large temperature coefficient. Voltage drop is resistance times current. Resistance increases linearly with length, and therefore, so does voltage drop. A long wire run will have a greater voltage drop than a short run for the same current. There can be joints (connections, possibly some not very good), actually there have to be joints, in any wire run. These joints will have resistance.

Is voltage drop a problem? Possibly or just annoying. For an incandescent bulb voltage drop won't matter to the bulb, but might bother a person reading a book. Some loads may malfunction, fail, or burn up with too little input voltage, and therefore have a problem.

If I watch closely, or am doing a light intensity comparison on an incandescent bulb, I can probably detect a 1 V change at 120 V. A 10 V change will be real noticeable.

When I design electronic equipment I expect it to work from 95 to 135 V with no problem. Most equipment is not designed with this wide of an input range.

If you put a reading light on the same circuit as a refrigerator or furnace or cycling heater, then you will probably be bothered reading from the cycling load. How much the light changes is a function of wire length and the placements of the various loads on the circuit.

If you have the light on a separate circuit from the varying loads, then there is much less light variation because the light only sees voltage variations from the varying loads that occurs at the main panel. This assumes a fairly stiff system coming to the main panel.

The test instrument mentioned at the start can be useful to detect and find bad electrical joints.

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[infinity]

Thank you everyone for the insight you are providing.

At what point would voltage drop start negatively impacting things in a house?

On residential construction, what code determines the size wire that is required for different circuits?

Is there a code that says there is a maximum, unacceptable voltage drop?

Does a voltage drop of 14.8% on the last outlet in a 15 amp branch circuit using 14-2 wire, with 6 outlets, concern you?

If one expects there home to be wired to the recommendations in the NEC, then any voltage drop over 5% is unacceptable. Why would the NEC publish this recommendation?

Short answer is that the NEC does not care about how much voltage drop you have on a branch circuit. And unless there is a local amendment to the NEC the 5% rule is completely unenforceable. So if this is a design concern by the builder then it should be written into the job specifications.

 

[Downtown1121]

Follow Up Question

Follow Up Question

Again, thank you everyone for the help you are providing me. If you will, a few more questions:

1. Is there any code that dictates what wire is required for different amp circuits?
2. Is there any code that limits the length of wire on different circuits?
3. Is there any code that limits the number of outlets on a circuit?
4. At what point is it reasonable to say "that is too much voltage drop"?
5. What issues happen when we have too much voltage drop?
6. Is voltage drop an issue with DC devices? Is so - at what voltage level?
7. Does it make sense to test voltage drop on a circuit using 100% load or should 80% be used?
8. Have there been any studies done on voltage drop that may help explain why everyone is saying this isn't something to be concerned about?

 

[ptonsparky]

Again, thank you everyone for the help you are providing me. If you will, a few more questions:

1. Is there any code that dictates what wire is required for different amp circuits? The NEC
2. Is there any code that limits the length of wire on different circuits? No, other than Local requirements.. It is a design issue.
3. Is there any code that limits the number of outlets on a circuit?

No, other than Local requirements.. It is a design issue. ​

4. At what point is it reasonable to say "that is too much voltage drop"? Eventually the equipment will not perform as intended. The manufacturer may have min/max requirements and those should be adhered to.
5. What issues happen when we have too much voltage drop?" The smoke gets out of motors, light flicker is excessive, items do not operate as intended.
6. Is voltage drop an issue with DC devices? Is so - at what voltage level? Yes, again, that will depend on the sensitivity of the equipment.
7. Does it make sense to test voltage drop on a circuit using 100% load or should 80% be use?" Use 100% if you expect that to be the norm, There are "rule of thumb" like "increase a wire size for every hundred feet of branch circuit. A residential branch circuit may never see 30% load for any length of time, let alone 80-100%
8. Have there been any studies done on voltage drop that may help explain why everyone is saying this isn't something to be concerned about?

There probably are studies but a good share is just personal experience. Those of us doing residential may never see a problem with VD. A goal for motor feeders at 5% VD would be nice, but I won't loose sleep if we end up with 5.5%.

 

[PaulMmn]

I would venture a guess that for most houses a circuit that runs more than 50' from the breaker box is a rarity. And if wired with 12 gauge wire, you're not going to notice any voltage drop, unless your grow lights start to dim! :angel:

 

[LarryFine]

I would venture a guess that for most houses a circuit that runs more than 50' from the breaker box is a rarity.

And when they do, sub-panels are a consideration.

 

[ActionDave]

....8.Have there been any studies done on voltage drop that may help explain why everyone is saying this isn't something to be concerned about?

I think the fact that we have been wiring houses since the 1920's and have been wiring most of them about the same since the 1960s is evidence enough that there isn't a problem with the way we do it.

I noticed a big increase in the interest in voltage drop along the same time as on line calculators. Back when you did them long hand nobody cared as much.

 

[Coppersmith]

unless there is a local amendment to the NEC the 5% rule is completely unenforceable.

The Florida Building Code has a statewide amendment making the 5% VD max rule mandatory.

 

[oldsparky52]

The Florida Building Code has a statewide amendment making the 5% VD max rule mandatory.

So does that mean that every 15-amp outlet has to support a 15-amp load with no more than a 5% VD? (20-amp and 5% for 20-amp outlets?)

Does the outlet get the whole 5% or is some allotted to the PoCo and service equipment? If so, how much is allotted to the receptacle?

 

[electrofelon]

8. Have there been any studies done on voltage drop that may help explain why everyone is saying this isn't something to be concerned about?

It is absolutely something to be concerned about, however the key is using the correct current in the calculation. Calculating VD for a 200 service using 200 amps is un-necessary and unrealistic, as is using 15/20 amps for most residential circuits.

So then even if using the actual realistic current you show a higher than desired voltage drop, the question is what is the duration and frequency of that current? Is it worth the expense of upsizing the wire for a small appliance circuit feeding a coffee pot and micorwave that will be on for 5 minutes?

Now get into more commercial continuous loads, or the grow lights for those plants in your basement :angel: , it might actually be of concern.

 

[PaulMmn]

Where does Florida say you have to make your measurements? From the breaker to the farthest outlet? From the meter? From the pole?

If the poco sends you 220/110, not much you can do about a complaint that it's not 240/120!

 

[ActionDave]

The Florida Building Code has a statewide amendment making the 5% VD max rule mandatory.

How is that even enforceable? Who decides what the load is on the circuit. My bedroom has two clock radios plugged in and every other recept is open.

 

[hbiss]

So does that mean that every 15-amp outlet has to support a 15-amp load with no more than a 5% VD? (20-amp and 5% for 20-amp outlets?)

Does the outlet get the whole 5% or is some allotted to the PoCo and service equipment? If so, how much is allotted to the receptacle?

I would assume you would only be responsible for the wiring within the premises. This is easy enough to figure out. First do a VD test at the panel. Then subtract that number to whatever you get at the farthest receptacle. So if you got a VD at the panel @15A of 1% and at the last receptacle 5% you would have a combined VD of 4% at that receptacle.

-Hal

 

[electrofelon]

How is that even enforceable? Who decides what the load is on the circuit. My bedroom has two clock radios plugged in and every other recept is open.

And even if they figure the full circuit capacity, how do they decide how the load is distributed throughout the length of the circuit? Worst case with the load at the very end? :rant: I get about 70 feet with #14 with 15 amps to be just under 5%...

 

[hbiss]

Yes, worst case at the last receptacle.

-Hal

 

[mayanees]

Hope he doesnt find out his 200 amp service is a scam since the POCO is using a 25KVA tranny.... :roll:

Now that's funny right there

But the fact of the matter is that the POCO could be using a 7.5, with three services on it ! That's what this loadtest meter is missing; the strength of the source.

 

[growler]

And even if they figure the full circuit capacity, how do they decide how the load is distributed throughout the length of the circuit? Worst case with the load at the very end? :rant: I get about 70 feet with #14 with 15 amps to be just under 5%...

Years ago we had some home inspectors that were writing up voltage drop on home inspection reports. They found out real quick that many receptacles would go up to 10% and they started to write up anything over 10%.

I wasn't sure how accurate their test was so I purchased the Ideal Circuit analyzer and did my own test of these circuits. I found they were a little high because they probably were not following directions and letting the meter cool off between test ( included in directions ). I replaced every receptacle in the circuit and pig tailed all connections. I was still reading up around 12% voltage drop. I started to measure the circuit from receptacle to receptacle and then the home run back to the panel. I measured the voltage drop at the panel to get a base number to subtract (hooked up a receptacle within a foot of the breaker).

I was really surprised by how accurate that circuit analyzer is. Figuring out voltage drop by formula and using the tester were pretty much dead on.

The only thing of interest that I did find out was that most of these circuits are longer than you would think once you add all the sections of wire . Even in a small town house it's easy for the last receptacle in an upstairs bedroom to be 150-200 ft from the panel. Since it's single phase you have to double the distance from the panel to calculate voltage drop.

With the panel in the basement the home run would run from one end of the basement to the other then up the outside wall to the first receptacle and then around one bed room and then jump to a second bed room. It is common practice to put two small bedrooms on one circuit.

What's appliances are going to be used in bedrooms? clocks, stereo, TV, Vacuum, and maybe a space heater. The vacuum will not get used that much and a space heater will not put out quite as much heat and maybe the blower motor won't last as long but the homeowner will never notice.

I learned to used that circuit analyzer to find poor connections so it wasn't a total waste of money.

 

[kwired]

I would assume you would only be responsible for the wiring within the premises. This is easy enough to figure out. First do a VD test at the panel. Then subtract that number to whatever you get at the farthest receptacle. So if you got a VD at the panel @15A of 1% and at the last receptacle 5% you would have a combined VD of 4% at that receptacle.

-Hal

But if an AHJ is using a plug in circuit analyzer, it is going to see net voltage drop of entire system not just your branch circuit being tested.

Could pass test at the end outlet of a circuit when nothing else in the house is running, but come back when cooking, doing laundry, etc, may fail not because of the branch circuit but because source is marginal.

 

[gar]

190514-1153 EDT

Loading is not a factor in the measurement. What is important is that there are no other load changes during the short time the meter is doing its test.

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