Voltage Drop - A Closer Look!
John M. Birkby ~ July, 1999
National Electric Code Articles 210-19(a) FPN No. 4 and 215-2(d) FPN No. 2 state
in part, "...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."
Section 90-5, Mandatory Rules, Permissive Rules, and Explanatory Material is defined as follows:
(a) Mandatory Rules. Mandatory rules of this Code are those that
identify actions that are specifically required or prohibited and are characterized by the use
of the terms shall or shall not.
(b) Permissive Rules. Permissive rules of this Code are those
that identify actions that are allowed but not required, are normally used to describe options
or alternative methods, and are characterized by the use of the terms shall be permitted
or shall not be required.
(c) Explanatory Material. Explanatory material, such as references
to other standards, references to related sections of this Code, or information related
to a Code rule, is included in this Code in the form of fine print notes (FPN).
Fine print notes are informational only and are not enforceable as requirements of this Code.
The SureTest®branch circuit wiring analyzer is becoming more and
more, the tool of choice for many state, county, municipal and private electrical inspection agencies.
The SureTest® is a true impedance tester and utilizes a patented full
15 ampere load test to analyze supply power and measure voltage drop and ground impedance. It
is a hand-held, microprocessor controlled device with a digital display. The intent of the SureTest®
is to identify and help isolate hazardous conditions in electrical circuitry. It will display
the full load voltage drop in percent at the receptacle under test. A much higher than normal increase
(2% or more) in voltage drop between adjacent receptacles on a circuit could indicate a potentially
hazardous condition. These might include poor splices, high resistance or corroded connections, damaged
conductors, improper wiring or inadequate connections at "back-stabbed" receptacles. A gradual,
small increase in voltage drop along a circuit would not be indicative of a hazardous condition. This
would be a normal increase in the total impedance of the conductors, splices, wiring devices, circuit
breaker, service cable, etc.
In the NEC Fine Print Notes regarding voltage drops, it states a 5 percent maximum
voltage drop "will provide reasonable efficiency of operation". Nowhere does it imply
that a voltage drop in excess of 5 percent is deemed a hazard. Much to the chagrin of the electrical
contractors, some inspectors have decided to enforce the 5 percent voltage drop as the maximum allowable
in their area of jurisdiction. In NEC Article 90-4, it does state "...The authority having
jurisdiction for enforcement of the Code will have the responsibility for making interpretations
of the rules, for deciding on the approval of equipment and materials, and for granting the special
permission contemplated in a number of the rules." Fine Print Notes (FPN) are not rules
and, as defined, are clearly not enforceable.
Now, letís take a closer look at a voltage drop in excess of 5 percent. If concern
is for "reasonable efficiency of operation" and protection of equipment, the initial line
voltage should be taken into consideration. For example, with a nominal voltage of 120 volts at the
receptacle under test, an 8 percent voltage drop under full load would result in an operating voltage
of 110.4 volts. 125 volts would drop to 115 and 114 volts to 104.9. Obviously, an 8 percent voltage
drop at 120 to 125 volts is of little concern, while an 8 percent drop at 114 volts could seriously
compromise the operation of equipment such as room air conditioners, refrigerators, high-amp vacuum
Some discretion should be considered when analyzing the voltage drops in a branch circuit.
The SureTest® pulses a full fifteen ampere load to analyze the
circuit, not just the receptacle under test. NEC Table 210-21(b)(2) allows a maximum
total cord and plug connected load in amperes of 12 for a 15 ampere rated receptacle and 16 for a
20 ampere rated receptacle. Although there are appliances, such as hair dryers, available today with
15 ampere nameplate ratings, most UL listed appliances are rated at 12 amperes, maximum. Therefore,
it may be anticipated that the maximum current flowing between the last receptacle on the branch circuit
and the previous receptacle should be 12 amperes. Inasmuch as the SureTest®
imposes a full 15 ampere load at the last receptacle, a reading of 6.25 percent voltage drop would
be equivalent to 5 percent at 12 amperes.
Many inspector members of the IAEI (International Association of Electrical Inspectors),
ASHI (American Society of Home Inspectors) and NAHI (National Association of Home Inspectors) have
reported feeling comfortable with gradually increasing voltage drops as high as 8 to 10 percent where
the line voltage is near the nominal 120 volts. Most of the controversy over the 5 percent maximum
voltage drop occurs during residential inspections. Some very capable contractors who are required
to comply with the 5 percent drop, have reported experiencing a great deal of difficulty trying to
achieve this in homes of 3,000 square feet or more. Even after all connections have been inspected
and tightened, all receptacles "pig-tailed" and circuit breakers checked, the voltage drop
remains in excess of 5 percent and yet, below 8 percent. One means of compliance would be to install
sub panels, but in many instances, this would be physically impractical and cost prohibitive.
The SureTest® has proved to be an invaluable tool for
inspectors and contractors alike. In addition to performing a full 15 ampere load test, the model
ST-1D will also display percent voltage drop for a 20 ampere load. Also provided, are readouts of
line voltage, ground-to-neutral voltage, estimated load on line in amperes, ground impedance in ohms
and indications of false grounds or ground-to-neutral shorts. A unique GFCI test will verify the trip
point of the device within the milliamperes-to-ground vs. time parameters set forth by Underwriters
Laboratories, Inc. When used for its intended purpose, the SureTest®will identify hazardous
conditions including poor splices, high resistance connections, damaged conductors, false grounds,
poor grounding, improper wiring, undersized wire, overextended circuits and faulty or mis-wired GFCIs.
Visual inspections alone, cannot detect the hidden flaws in electrical circuitry which could result
in a catastrophic fire or electrocution!
As for the 5 percent voltage drop, itís still as it should be --- a Fine Print Note
- with the inspector also considering wire gauge, length of run, no of receptacles in the circuit,
This material has been reviewed and approved by Mike Holt of Mike Holt Enterprises,
Inc Telephone: 1.888.NEC.Code (632.2633) ~ Website: www.mikeholt.com
Comment No. 1
I respectfully beg to differ with the suggestion of using the 3% and 5% voltage drop FPN's in the NEC;
this from the perspective of an Electrical Designer at the design phase of a project. One or two old-timers
have told me 2% max. I personally shoot for between 1-2% at the design phase.
First, electrical design drawings - in particular conduit routing drawings - are schematic in nature.
We are *suggesting* a route. We have *no control over the final installed Length. My point: the final
installed Length could be much different than what we can scale off of a drawing. Particularly height,
since our drawings are 2D.
Second, source Voltage. We use the NEC nominal voltages, but typically have no idea what the source
voltage really is. Especially in industrial facilities, who knows what the transformer taps are set
at? I've witnessed once or twice voltage checks on 3-phase panels, and none of the readings were NEC
nominals. If you have to take a feed from a heavily loaded 480V MCC, go check the voltmeter for the
3 phases on the front. You likely won't see all 3 reading 480V. Good 'ol phase imbalance.
With the highly variable Length and Voltage numbers, I'm more comfortable with much lower percentages.
Mike Holtís Comment:
If you want to download (free) a Windows voltage drop calculator or if you whant to learn more about
this subject, click here.
Note: A book/video is available from my office: Mike Holt of Mike Holt Enterprises, Inc Telephone:
1-888 NEC-CODE ~ Order
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