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#221844 08/18/22 03:37 PM
Joined: Dec 2001
Posts: 2,497
T
Member
Is there an official limit on voltage drop/cable length in domestic circuits in the NEC?

In most European countries there is a hard 3% limit from the main panel to the end of each circuit. Most electricians have rules of thumb for common circuit and wire sizes (e.g. 1.5 mm2 at 16 amps = 17 m maximum length). Yesterday I started to wonder about the comparably large conductors used in US domestic circuits (for the same currents as in Europe) and ran a voltage drop calculation to see whether that might be one reason. Much to my surprise I found out that, assuming the European 3%, the maximum length of a #14 circuit on a 15-amp breaker would be 14 m (roughly 46 feet), which seems very short, if not impossible for larger homes, especially if the panel happened to be in the basement. Is this issue ever considered in domestic wiring in the US?

Joined: Jul 2004
Posts: 9,771
Likes: 14
G
Member
There is no hard and fast rule in the NEC but there are recommendations. (Informational notes)
It is seen as a design issue, not a safety one.
Good electricians do try to have sound design practices tho. In the end, it might mean a lower bid accepted by a less than discerning customer. A lot will really depend on what is available at the service point. I am getting 124/248 so I can take a little more voltage drop than a customer on a utility that is providing voltage on the low end of the accepted nominal. I still try to stay within the recommendation.

210.19(A)
Quote
Informational Note No. 4: Conductors for branch circuits as
defined in Article 100, sized to prevent a voltage drop exceeding
3 percent at the farthest outlet of power, heating, and lighting
loads, or combinations of such loads, and where the maximum
total voltage drop on both feeders and branch circuits to the
farthest outlet does not exceed 5 percent, provide reasonable
efficiency of operation.


215.2(A)
Quote
Informational Note No. 2: Conductors for feeders, as defined in
Article 100, sized to prevent a voltage drop exceeding 3 percent
at the farthest outlet of power, heating, and lighting loads, or
combinations of such loads, and where the maximum total voltage
drop on both feeders and branch circuits to the farthest
outlet does not exceed 5 percent, will provide reasonable efficiency
of operation.


Greg Fretwell
Joined: Jan 2005
Posts: 5,362
Likes: 1
Cat Servant
Member
There’s more to the issue than simple math.
Issue #1 is the rated voltage of an appliance as compared to the actual delivered voltage. For example, if the actual supplied voltage is 121V and the nameplate is marked 115V, you can lose 5% of that 121V and just be making the rated voltage; even a 10% drop would still be well within the design limits of that appliance.

The second issue is more devious. You really need to measure voltage while the circuit is under load. If there was some problem on the supply side — say, power line are abraded by contact with trees — the 121 you read might drop to 95 the instant you apply a load.
Likewise, the supply voltage will vary during the day as the local transformer pics up increased loads. If the transformer is at maximum capacity, the voltage will drop sharply.

What matters is that the supplied voltage be within the design specifications of the load — while it’s running. Typically this means within 5% over or 10% under the nameplate voltage.
Again, detail matters. If the load has a very high starting current there may be problems at startup if the supply is marginal.

Joined: Dec 2001
Posts: 2,497
T
Member
Thanks for your replies!

Design considerations in Europe are slightly different. Mains voltage is specified as x V (usually 230/400) +/- 10% from the distribution network operator and then 0.5% voltage drop from the separation point to the meter and 3% from the meter to the last point of the circuit. Appliances are supposed to operate within those limits if their nameplate rating is 230 or 400 V. For general purpose lighting or outlet circuits, the rating of the fuse/MCB is taken as the design current for voltage drop calculations, i.e. the 3% are at full load regardless of the actual voltage at the service entrance.

Depending on the type of fault protection, voltage drop/loop impedance can become a safety issue. In the event of a line - earth fault the supply is required to be disconnected within 200 ms, i.e. if there is no RCD protection, the magnetic trip of the MCB has to operate. If there's an RCD/GFCI impedance is much less critical.


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