Voltage drop on residential branch circuits

I

IanO3

Member
Location
Florida
Occupation
Electrician
A 15A branch circuit is being upsized to 12/2 romex for a long run (130'). Can you pull the homerun in 12/2 romex and have jumpers to other receptacles downstream be in 14/2 Romex?
Question is from a Code standpoint to begin with. But also from an electrical theory standpoint.

The voltage dropped is across the whole circuit (someone could potentially pull 12A off that 15A circuit at the last receptacle. Would this cause an issue to say, the 10' jumper from box containing the HR (with 12/2 HR) to next receptacle (14/2 from HR box onward).

Bonus Question - If you do a voltage drop calculation on this (2x12.9x130'x12A = 40240/3.6V(3% VD on 120V circuit) = 11,180cm) (#10 = 10380cm). This leads me to believe, to properly stay below 3% Vd, you should run 8/2 NM to these receptacles. Is this "Correct" and are people doing this?

What am I missing/ not understanding?
 
IanO3 said:
A 15A branch circuit is being upsized to 12/2 romex for a long run (130'). Can you pull the homerun in 12/2 romex and have jumpers to other receptacles downstream be in 14/2 Romex?
Click to expand...
Sure. Voltage drops add, so if you have 120' of 12/2 and 10' of 14/2, that gives you 12/13 of the benefit over the baseline of all 14/2 that you would get by doing all 12/2. If that slightly lesser reduction in voltage drop is still sufficient for your design goals, no problem.

IanO3 said:
Bonus Question - If you do a voltage drop calculation on this (2x12.9x130'x12A = 40240/3.6V(3% VD on 120V circuit) = 11,180cm) (#10 = 10380cm). This leads me to believe, to properly stay below 3% Vd, you should run 8/2 NM to these receptacles. Is this "Correct" and are people doing this?
Click to expand...
Is your receptacle actually going to have a 12A load on it, and is that load so sensitive to voltage drop that over 3% on just the branch circuit would be a problem? If so, you had best follow your calculation. But that's a rare collection of circumstances.

Cheers, Wayne
 
IanO3 said:
A 15A branch circuit is being upsized to 12/2 romex for a long run (130'). Can you pull the homerun in 12/2 romex and have jumpers to other receptacles downstream be in 14/2 Romex?
Question is from a Code standpoint to begin with. But also from an electrical theory standpoint.

The voltage dropped is across the whole circuit (someone could potentially pull 12A off that 15A circuit at the last receptacle. Would this cause an issue to say, the 10' jumper from box containing the HR (with 12/2 HR) to next receptacle (14/2 from HR box onward).

Bonus Question - If you do a voltage drop calculation on this (2x12.9x130'x12A = 40240/3.6V(3% VD on 120V circuit) = 11,180cm) (#10 = 10380cm). This leads me to believe, to properly stay below 3% Vd, you should run 8/2 NM to these receptacles. Is this "Correct" and are people doing this?

What am I missing/ not understanding?
Click to expand...
Yes, you can do that. Some inspectors, even though they don't have an amendment, will require a tag at the breaker that says do not replace with a larger breaker.
 
wwhitney said:
Sure. Voltage drops add, so if you have 120' of 12/2 and 10' of 14/2, that gives you 12/13 of the benefit over the baseline of all 14/2 that you would get by doing all 12/2. If that slightly lesser reduction in voltage drop is still sufficient for your design goals, no problem.


Is your receptacle actually going to have a 12A load on it, and is that load so sensitive to voltage drop that over 3% on just the branch circuit would be a problem? If so, you had best follow your calculation. But that's a rare collection of circumstances.

Cheers, Wayne
Click to expand...
I highly doubt my receptacle will draw 12A, but at what point am I required to protect against the potential load? (Always right?). This is for general use receptacles in a particular area of a dwelling unit, I don't know the load for certain. Its not practical but someone could certainly plug 12A in at the end of the line (80% of my 15A circuit).

I'm pretty sure nobody is pulling 8/2 NM to gen use recepts when the circuit is only 130' long, but why not if that's what a max potential Vd calculation gives?
I'm really trying to figure out what proper procedure would be for gen use circuit voltage drops, the constant "Rule of Thumbs" that get spoken of is all I can find (upsize 1 wire size for every 100' or upsize circuit when length exceeds nominal voltage), other than this being a decent educated guess, whats the proper practice?
 
retirede said:
From an NEC standpoint, there is no requirement for voltage drop. As long as the conductors are properly protected, it’s fine.
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And as long as breaker size is correct for the smallest wire size throughout the branch circuit, than no code violations have been made? I get to thinking about someone performing a service change on a house and thinking "its #12 wire so it goes on a 20A breaker", then suddenly theres a code violation for the portions of the branch that are ran in 14/2 after the HR.
 
IanO3 said:
I highly doubt my receptacle will draw 12A, but at what point am I required to protect against the potential load? (Always right?).
Click to expand...
The NEC has no voltage drop compensation requirement for these circuits. What will someone be plugging in that may draw 12 amps?
 
IanO3 said:
And as long as breaker size is correct for the smallest wire size throughout the branch circuit, than no code violations have been made? I get to thinking about someone performing a service change on a house and thinking "its #12 wire so it goes on a 20A breaker", then suddenly theres a code violation for the portions of the branch that are ran in 14/2 after the HR.
Click to expand...
That’s a valid concern.
 
infinity said:
The NEC has no voltage drop compensation requirement for these circuits. What will someone be plugging in that may draw 12 amps?
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A 10A vacuum cleaner with a 2A standing fan plugged into the same outlet? I'm obviously making up an uncommon scenario, but that's what I mean by the potential draw. I don't know what someone will do in their bedrooms a year from now after the house has been wired.
 
IanO3 said:
I highly doubt my receptacle will draw 12A, but at what point am I required to protect against the potential load? (Always right?).
Click to expand...
For a general use receptacle? I guess never. For a known specific load? You need to provide a circuit of the necessary ampacity, but there is no voltage drop requirement in the NEC. There may be on in your jurisdiction's energy code. If you want the known load to work, you obviously have to ensure sufficient voltage under load, but that is not as simple as just saying "max 3% VD on the branch circuit."

IanO3 said:
A 10A vacuum cleaner with a 2A standing fan plugged into the same outlet?
Click to expand...
OK, so then the question is during what portion of the operating cycle does the vacuum actually draw 10A, and what is the voltage at the branch circuit breaker when the load is drawing 12A, and what is the minimum allowable voltage for the vacuum while it is drawing 12A, and does the difference between those two voltage allow for more than 3% VD on the branch circuit or not?

If the answers are "frequently during normal vacuuming, 112V, and 108V," then yes, if you want to plug your vacuum into that receptacle, you better size the branch circuit for at most 4V of VD. While if they are "only at initial startup, 120V, and 100V," then in your example there's no need to provide for only 3% VD on the branch circuit.

Cheers, Wayne
 
Mixing wire gauges in residential is not a great idea. Too many people will see the #12 at the breaker and want to upgrade to 20A. Just avoid the problem by making this a 20A branch circuit and use #12 throughout. 15A duplex receptacles are allowed on a 20A circuit.
 
Unless I knew specific loads that might cause an overload on a 15A circuit, I wouldn't even consider 130' a problem in a dwelling. Some of these "McMansions" I have wired, I'm sure had runs over 130' and I've not had a single complaint.
 
infinity said:
A space heater with a large voltage drop would just take longer to heat up. Certainly not a safety issue.
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The NEC wont restrict voltage drop the way product listings do.

115 Volt TV, Computer, Modem/Router, & Printer are typical general purpose (GP) loads with Nameplate minimum of 100vac, which black out with a vacuum, space heater, paper-shredder, or room air conditioners running on same circuit.

IMHO Gambling a mortgage on typical construction buys junk not compatible with occupancy, including xFCI breakers, and GP receptacles on #14 AWG.
 
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IanO3 said:
?
I'm really trying to figure out what proper procedure would be for gen use circuit voltage drops, the constant "Rule of Thumbs" that get spoken of is all I can find (upsize 1 wire size for every 100' or upsize circuit when length exceeds nominal voltage), other than this being a decent educated guess, whats the proper practice?
Click to expand...
I've been wyring for 25 years now, and I don't think I have ever upsized a resi branch circuit for VD.
 
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