MikeHolt_Section08_Transformers

Unit 12: Transformer (Txfr) Calculations

NEC Article 

Wiring Coloring Standard
120/240V, 1Ø Black, Red, & White
120/208V, 3Ø Black, Red, Blue, & White
120/240V, 3Ø Black, Orange, Blue, & White
277/480V, 3Ø Brown, Orange, Yellow, & Gray, or
Brown, Purple, Yellow & Gray
  • Delta-Connected Txfr’s
    • 3 txfr windings connected in series, end-to-end ([.forming a triangle)
    • Line conductors connected to Txfr where each of the 2 windings meet
    • High-Leg Voltage
      • aka ‘wild-leg’, stinger-leg
      • used to identify delta-connected system conductor having a voltage of 208V to ╧
      • Must be identified with an orange color
      • Panel-boards
        • 408.3(E)(1)
        • 408.3(F)(1)
        • When supplied by 4-wire, delta-connected, 3Ø, must have high-leg conductor terminate on BØ
        • Requires field-marked “Caution 208V to Ground”
      • Disconnects
        • Article 404, Switches
        • Accepted practice is to terminate high-leg conductor on BØ
      • Utility Equipment
        • ANSI standard for meter equipment
        • requires high-leg conductor (208V to Neutral) to terminate on CØ (right-side)
          • Demand meter needs 120V received from BØ
      • Caution: Ensure high-leg conductor is replaced in the original position to avoid danger of connecting 120V circuit to 208V
      • High-leg Voltage = (Line Voltage ÷ 2) * 1.732
  • Wye-Connected Txfr
    • Each of the 3 windings have 1 lead connected to the same common point (forming a Y)
    • The other windings lead are connected to the line conductors
  • Txfr kVA Rating
    • Txfr’s are rated in kilovolt-amperes (kVA)
      • 1 kVA = 1000 VA
  • Txfr Line Current Determination
    • Txfr line current is calculated using either the 1Ø or 3Ø formula below.
    •  1Ø
      • I = VA ÷ E
      • I = VA ÷ (E * 1.732)
  • Txfr Over-Current Protection
    • Table 450.3(A) & (B)
      • Table 430.3(B) (where is this in NEC-2017?)
    • 430.3(B)
      • section covers txfr windings only, not conductors supplying or leaving
    • 240.6(A)
    • Table 450.3(B), NEC-2017 page 70-333
      Transformers under 1000V Primary Winding Over-Current Protection
      Primary Current Rating Maximum Protection
      9A or more 125%, see Note 1
      Less than 9A 167%
      Less than 2A 300%
      Note 1: If 125% doesn’t correspond to a standard fuse/CB rating, then next higher rating is permitted [240.6(A)]
  • Txfr Primary Winding Conductor Sizing
    • Step 01: I = VA / (E*√3) (for 3Ø)
    • Step 02: 450.3(B) (pg 70-333)
    • Step 03: 310.15(B)(16) (pg 70-150)
    • 450.3(B)
    • 210.19(A)(1)
    • 240.4
    • 240.4(B)
    • Table 310.15(B)(16)
    • 310.15
    • Must be no less than 125% of continuous loads, plus
      • 100% of non-continuous loads,
      • based on terminal-temp-rating-ampacity listed in table 310.15(B)(16),
        • before any ampacity adjustment
      • Also, must be protected against ampacity over-current after ampacity adjustment specified in 310.15
        • Next higher standard over-current protection rating permitted provided doesn’t exceed 800A
    •  1Ø
      • Primary Current = Txfr VA Rating ÷ Primary Voltage
      • Primary Protection = Primary Current * Table 430.3(B) %
      • Primary Current = Txfr VA Rating ÷ (Primary Voltage * 1.732)
      • Primary Protection = Primary Current * Table 430.3(B) %
  • Txfr Secondary Winding Conductor Sizing
    • 240.21(C)(2)
    • 215.2(A)(1)(a)
    • 240.21(C)
    • Step 01: Determine rating of device supplied by secondary conductors at 125% of the secondary rating  ??
    • Step 02: Size secondary conductors to have an ampacity of not less than the device rating being supplied
    • Assume intended to carry full capacity continuously
    •  3Ø
      • Secondary Current = Txfr VA Rating ÷ (secondary Voltage * 1.732)
  • Txfr System Bonding Jumper
    • 250.102(C)
    • 250.30(A)(1)
    • 250.28(D)(1)
    • Installed on secondary conductors of Txfrs,
      • must be installed at same location where grounding electrode conductor terminates to neutral point of txfr
      • Completes bonding path back to source (txfr secondary) providing a ground-fault return path
  • Txfr Grounding electrode Conductor
    • 250.30(A)(4)
    • 250.30(A)(5)
    • 250.66
    • Must connect neutral point of a separately derived system to a grounding electrode
    • must be sized accordingly based on the area of the ungrounded secondary conductor
    • Provides only the connection to earth, does not provide a ground-fault return path
  • Txfr Short Circuit Current
    • Step 01: 100% ÷ Impepedance %
      • 100% ÷ 1% = 1÷.01 = 100 (the multiplier)
    • Step 02: I=P/E
    • Step 03: I * Multiplier = Short Circuit Current