I recently took a mater electrician test prep. class where the
instructor introduced imtroduced us to center load voltage drop
calculatins. Upon reviewing my (sketchy) notes, I can't be sure that I
have it right. If I getr an RV park or similar calculation on my test,
I would like to use this formula. Any suggestions or pointers?
Lloyd M Johnston
On 1/31/04 1:39 PM, in article
firstname.lastname@example.org, "Lloyd Johnston"
Step 1: Multiply each load by its distance from the supply end.
Step 2: Add all those answers.
Step 3: Divide the answer from step 2 by the total load.
Load 1: 80', 96 amps. 80 X 96 = 7,680
Load 2: 100', 40 amps. 100 X 40 = 4000
Load 3: 130', 21 amps. 130' X 21 = 2,730
The total of those answers is 14,410. The total load is 157 amps.
14,410 divided by 157 is 91.78 feet to the load center.
The American Electrician's Handbook covers the center of load calculation
for determining voltage drop for normal loads. For RV parks there is a note
in the service feeder section quoted below. This requires that you
calculate the demand for each segment of the feeder then calculate the
voltage drop for that segment using the respective demand load. For a
feeder supplying 15 sites this would require about 15 separate calculations
plus the one from the service to the first site that sees the total demand
load. This can be rather time consuming for a test question and is better
done with a computer program. It would take me about 4 hours to write this
but lost it somewhere in one of my 8 bit computers when I took them to the
From 2002 NEC
551.73 Calculated Load.
(A) Basis of Calculations. Electrical service and feeders shall be
calculated on the basis of not less than 9600 volt-amperes per site equipped
with 50-ampere, 120/240-volt supply facilities; 3600 volt-amperes per site
equipped with both 20-ampere and 30-ampere supply facilities; 2400
volt-amperes per site equipped with only 20-ampere supply facilities; and
600 volt-amperes per site equipped with only 20-ampere supply facilities
that are dedicated to tent sites. The demand factors set forth in Table
551.73 shall be the minimum allowable demand factors that shall be permitted
in calculating load for service and feeders. Where the electrical supply for
a recreational vehicle site has more than one receptacle, the calculated
load shall only be computed for the highest rated receptacle.
(B) Transformers and Secondary Distribution Panelboards. For the purpose of
this Code, where the park service exceeds 240 volts, transformers and
secondary distribution panelboards shall be treated as services.
(C) Demand Factors. The demand factor for a given number of sites shall
apply to all sites indicated. For example, 20 sites calculated at 45 percent
of 3600 volt-amperes results in a permissible demand of 1620 volt-amperes
per site or a total of 32,400 volt-amperes for 20 sites.
FPN: These demand factors may be inadequate in areas of extreme hot or cold
temperature with loaded circuits for heating or air conditioning.
(D) Feeder-Circuit Capacity. Recreational vehicle site feeder-circuit
conductors shall have adequate ampacity for the loads supplied and shall be
rated at not less than 30 amperes. The grounded conductors shall have the
same ampacity as the ungrounded conductors.
FPN: Due to the long circuit lengths typical in most recreational vehicle
parks, feeder conductor sizes found in the ampacity tables of Article 310
may be inadequate to maintain the voltage regulation suggested in the fine
print note to 210.19. Total circuit voltage drop is a sum of the voltage
drops of each serial circuit segment, where the load for each segment is
calculated using the load that segment sees and the demand factors of
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