Canadian electrical code confusion.

Discussion in 'Homework Help' started by matt_t, Oct 22, 2012.

  1. matt_t

    Thread Starter Member

    Aug 18, 2012
    30
    0
    Hello, I was wondering if anyone could interpret the following rule from the Canadian electrical code for the following question:

    The following bank of transformer arc welders are to be connected in a shop distribution panel feeder using R90 copper conductors:

    Welder 1 - Primary current rating - 35 Amp Duty cycle - 60%
    Welder 2 - Primary current rating - 65 Amp Duty cycle - 50%
    Welder 3 - Primary current rating - 48 Amp Duty cycle - one hour
    Welder 4 - Primary current rating - 90 Amp Duty cycle - 60% What is the calculated required ampacity of the overcurrent protection device required for the feeder?

    A) 200.5 Amps
    B) 270.9 Amps
    C) 280.2 Amps
    D) 284.5 Amps

    Extract from the code:

    42-006 Supply conductors​
    (1) The supply conductors for an individual transformer arc welder shall have an ampacity of not less than the
    value obtained by multiplying the rated primary current of that welder in amperes by a factor of
    (a) 1.00, 0.95, 0.89, 0.84, 0.78, 0.71, 0.63, 0.55, or 0.45 for welders having a duty cycle of 100, 90, 80,
    70, 60, 50, 40, 30, and 20% or less respectively; or
    (b) 0.75 for a welder having a time rating of 1 h.
    (2) The supply conductors for a group of transformer arc welders shall have an ampacity not less than the sum
    of the currents determined for each welder in the group in accordance with Subrule (1) multiplied by a
    demand factor of
    (a) 100% of the two largest calculated currents of the welders in the group; plus
    (b) 85% of the third largest calculated current of the welders in the group; plus
    (c) 70% of the fourth largest calculated current of the welders in the group; plus
    (d) 60% of the calculated currents of all remaining welders in the group.
    (3) Lower values than those given in Subrule (2) shall be permitted in cases where the work is such that a high
    operating duty cycle for individual welders is impossible.​
    42-008 Overcurrent protection for transformer arc welders​
    (1) Each transformer arc welder shall have overcurrent protection rated or set at not more than 200% of the
    rated primary current of the welder, unless the overcurrent device protecting the supply conductors meets
    this requirement.
    (2) Each ungrounded conductor supplying a transformer arc welder shall have overcurrent protection rated or
    set at not more than 200% of the allowable ampacity of the conductor as specified in Table 1, 2, 3, or 4,
    except that the next higher rating or setting may be used where
    (a) the nearest standard rating of the overcurrent device is less than the rating or setting otherwise
    required by this Rule; or
    (b) the rating or setting otherwise required by this Rule results in too frequent opening of the overcurrent
    device.
    (3) The maximum rating or setting of an overcurrent device protecting a feeder supplying a group of
    transformer arc welders shall not exceed a value calculated by determining the maximum rating or setting
    of overcurrent device permitted by Subrules (1) and (2) for the welder allowed the highest overcurrent
    protection and adding to this value the sum of ampacities as calculated by Rule 42-006 for all other welders​
    in the group.
     
  2. matt_t

    Thread Starter Member

    Aug 18, 2012
    30
    0
    Here is what I worked out for the duty cycles:

    Welder 1: 35A x 0.78 = 27.3A
    Welder 2: 65A x 0.71 = 46.15A
    Welder 3: 48A x 0.75 = 36A
    Welder 4: 90 x 0.78 = 70.2A

    Group demand factor =
    70.2A x 100% = 70.2A
    46.15A x 100% = 46.15A
    36A x 85% = 30.6A
    27.3A x 70% = 19.11A

    Rule 42-008 (3) from what I interpret means to use the largest welder load which is 70.2 x 200% + the rest of the loads which equals 236.26A which is none of the answers. I find the last paragraph of the code completely confusing. Can anyone help me out on this?
     
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