sparkytools

Cette page n’est pas encore entièrement traduite. Les passages qui manquent s’affichent en anglais pour l’instant. Lire la version anglaise →

Feuille d’exercices : chute de tension

Twelve voltage-drop problems: the drop in volts, what percent of the supply that is, and how long a run can be before it uses up a budget. Half are in feet and ohms per 1000 ft, half in metres and ohms per kilometre. The conductor resistances are practice figures, made up for the arithmetic — not any real conductor’s. On the job, the resistance comes from your own code book’s table or the maker, and so does the percentage you are allowed.

Voltage drop worksheet · sheet #1

NomDateScore/ 12

single-phase: VD = 2 × L × R × I ÷ 1000three-phase: VD = √3 × L × R × I ÷ 1000% = VD ÷ V × 100L = VD × 1000 ÷ (2 or √3 × R × I)

  1. Two-wire single-phase, 600 V, 6 A, 90 ft one way, conductors 2.6 Ω per 1000 ft (practice figure). Find the voltage drop and the percent of supply.

  2. Two-wire single-phase, 480 V, 15 A, 45 m one way, conductors 3.85 Ω/km (practice figure). Find the voltage drop and the percent of supply.

  3. Three-phase, 277 V, 40 A, conductors 1.9 Ω per 1000 ft (practice figure). Longest one-way run that stays within a 2% drop?

  4. Three-phase, 277 V, 14 A, 20 m one way, conductors 1.8 Ω/km (practice figure). Find the voltage drop and the percent of supply.

  5. Two-wire single-phase, 240 V, 19 A, 120 ft one way, conductors 0.55 Ω per 1000 ft (practice figure). Find the voltage drop and the percent of supply.

  6. Two-wire single-phase, 347 V, 7 A, conductors 1 Ω/km (practice figure). Longest one-way run that stays within a 5% drop?

  7. Three-phase, 277 V, 13 A, 110 ft one way, conductors 1.4 Ω per 1000 ft (practice figure). Find the voltage drop and the percent of supply.

  8. Three-phase, 277 V, 12 A, 50 m one way, conductors 3.1 Ω/km (practice figure). Find the voltage drop and the percent of supply.

  9. Two-wire single-phase, 480 V, 40 A, conductors 3.15 Ω per 1000 ft (practice figure). Longest one-way run that stays within a 2% drop?

  10. Two-wire single-phase, 208 V, 20 A, 25 m one way, conductors 1.5 Ω/km (practice figure). Find the voltage drop and the percent of supply.

  11. Three-phase, 208 V, 26 A, 40 ft one way, conductors 3 Ω per 1000 ft (practice figure). Find the voltage drop and the percent of supply.

  12. Three-phase, 208 V, 27 A, conductors 0.5 Ω/km (practice figure). Longest one-way run that stays within a 5% drop?

sparkytools.net/worksheets/voltage-drop — free practice sheet. Practice numbers only; nothing here is a code-book value.

Answer key · Voltage drop worksheet · sheet #1

Show the answers and the working
  1. 2,81 V (0,47 %) VD = 2 × 90 × 2,6 × 6 ÷ 1000 = 2,81 V; 2,81 ÷ 600 × 100 = 0,47 %
  2. 5,2 V (1,08 %) VD = 2 × 45 × 3,85 × 15 ÷ 1000 = 5,2 V; 5,2 ÷ 480 × 100 = 1,08 %
  3. 42,1 ft 2% of 277 = 5.54 V; L = 5.54 × 1000 ÷ (√3 × 1.9 × 40) = 42.1 ft
  4. 0,87 V (0,32 %) VD = √3 × 20 × 1,8 × 14 ÷ 1000 = 0,87 V; 0,87 ÷ 277 × 100 = 0,32 %
  5. 2,51 V (1,05 %) VD = 2 × 120 × 0,55 × 19 ÷ 1000 = 2,51 V; 2,51 ÷ 240 × 100 = 1,05 %
  6. 1 239,3 m 5% of 347 = 17.35 V; L = 17.35 × 1000 ÷ (2 × 1 × 7) = 1,239.3 m
  7. 3,47 V (1,25 %) VD = √3 × 110 × 1,4 × 13 ÷ 1000 = 3,47 V; 3,47 ÷ 277 × 100 = 1,25 %
  8. 3,22 V (1,16 %) VD = √3 × 50 × 3,1 × 12 ÷ 1000 = 3,22 V; 3,22 ÷ 277 × 100 = 1,16 %
  9. 38,1 ft 2% of 480 = 9.6 V; L = 9.6 × 1000 ÷ (2 × 3.15 × 40) = 38.1 ft
  10. 1,5 V (0,72 %) VD = 2 × 25 × 1,5 × 20 ÷ 1000 = 1,5 V; 1,5 ÷ 208 × 100 = 0,72 %
  11. 5,4 V (2,6 %) VD = √3 × 40 × 3 × 26 ÷ 1000 = 5,4 V; 5,4 ÷ 208 × 100 = 2,6 %
  12. 444,8 m 5% of 208 = 10.4 V; L = 10.4 × 1000 ÷ (√3 × 0.5 × 27) = 444.8 m

How to use it

  1. Print the sheet (or save it as a PDF from the print dialog). The answer key prints on its own page, so you can hand the sheet out and keep the key.
  2. Work every problem on paper, with the formula strip at the top, before you look at anything.
  3. Mark it from the key. For each miss, read the working line and find the exact step you did differently.
  4. Press New sheet for another twelve; the sheet number is in the link, so the same sheet can be printed again.

Worked example

A 240 V run, in volts and percent

  1. A two-wire 240 V circuit carries 20 A to a load 150 ft away. The conductor’s resistance is 1.6 Ω per 1000 ft (a practice figure).
  2. The current goes out and comes back, so the wire in the loop is 2 × 150 = 300 ft.
  3. 300 ft is 0.3 of 1000 ft: 0.3 × 1.6 = 0.48 Ω for the loop.
  4. VD = I × R = 20 × 0.48 = 9.6 V. As a percent: 9.6 ÷ 240 × 100 = 4%.

Three-phase swaps the 2 for √3 (1.732): the three lines share the return, so the loop is not a simple out-and-back.

Questions fréquentes

Why are the resistances made up?

Because the real ones are code-book table values, and they change with the code cycle and the country. The arithmetic is the same whatever the number, so the sheet uses practice figures and says so on every problem. Look up the real value for the job in your own book.

Is the length one way or both ways?

One way on every problem here — the distance from the supply to the load. The 2 (or √3) in the formula takes care of the return.

Can I print a different sheet?

Yes. Press New sheet and you get another twelve problems with their own answer key; the sheet number goes into the link, so you can print it again later or send it to someone and they get the same sheet. Sheet #1 is always the one this page opens on.

Does it cost anything or need an account?

No. Every worksheet is free, prints from your own browser (or saves as a PDF from the print dialog), and needs no sign-up.

Voir aussi

Vous voyez une erreur sur cette page? Signalez-la

Nous joindrons cette page et les nombres que vous avez tapés. Nous lisons chaque signalement, mais nous ne pouvons pas y répondre.

Merci — votre signalement a été envoyé.

L’envoi n’a pas fonctionné. Veuillez réessayer plus tard.