# center load voltage drop calculation

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Hello everybody,

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?

Thanks,

Lloyd M Johnston

• posted

"Lloyd Johnston" wrote in message news: snipped-for-privacy@news.supernews.com...

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 program in JavaScript. I wrote such a program in Basic about 15 years ago but lost it somewhere in one of my 8 bit computers when I took them to the dump.

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 551.73(A).
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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.

For example:

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.
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Dean, i feel a little dense and need definition. i understand the math and see how to arrive at 91.78 feet. what exactly does the 91.78 feet tell me? all the loads have a distance in feet from the load center so what exactly is the 91' to the load center tell me? help a brother out

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never mind, i figured it out. if anyone else is wondering, the 91.78' is the length/distance used in any of the voltage drop formulas

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Does this 18 yr old question need some context? like what is the design voltage tolerance error for any load range to every location? My understanding is that this tolerance must be 5% max by design for load regulation error which is the change in the ratio of feed source impedance to load impedance for distribution error. and not just the mean current-distance product.

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