ACADEMY OF KNOWLEDGE

Dimensioning tables for power cables - NBR 5410

    Explanation

    In order to provide a practical tool to assist designers, installers and others involved in the selection and sizing of the conductors in a low voltage electrical installation, we present below several tables extracted from the ABNT standard NBR 5410 and other useful practical tables. In order to correctly interpret the content of the tables, we make the following observations concerning the various possible ways of installing the conductors:

     

    - Definitions:

    Insulated conductor: conductor provided only with insulation.
    Unipolar cable: cable consisting of a single insulated conductor and provided with a cover over the insulation.
    Multipolar cable: cable made up of several insulated conductors and provided with a cover over all the insulated conductors.

    - Electrical conduits:

    Insulated conductors, single or multi-core cables can be installed in conduits. The use of bare conductors in conduits is only allowed when in exclusive insulated conduit and for grounding purposes.

    - Outdoor facilities

    Outdoor installations are considered to be those in trays, beds, shelves, supports or directly fixed to walls or ceilings.
    Only unipolar or multipolar cables can be installed in outdoor installations.

    - Cable trays:

    Insulated conductors, unipolar or multipolar cables can be installed in cable trays.

    - Directly buried cables:

    Directly buried cables can only be unipolar or multipolar and measures must be taken to protect them from deterioration caused by earth movement, shock of tools from excavation and chemical attack or humidity.

    - Channels in the ground:

    The cables installed directly in the ducts on the ground can only be unipolar or multi-polar or the use of insulated conductors is permitted provided they are contained in conduits inside the duct.

    - About insulators:

    Bare, insulated or bundled conductors can be used on insulators.

    Power Cable Sizing Recommendations

    1-)Knowing the installation

    • Type of installation (underground, overhead, ducts, troughs, etc.)
    • Current to be carried
    • Electrical voltage of the circuit
    • Distance
    • Power factor
    • What is the power supply (direct or indirect)
    • Power supply destination (motor, busbar, oven, etc.)
    • How many conductors per phase
    • Type of system (single-phase or three-phase)
    • Type of current (AC or DC)

    2-) Calculate the ampacity and voltage drop

    • Check the additions in current to be considered, i.e. pooling factor etc.
    • Check the MBR 5410 table for the maximum current per section, according to the installation conditions.
    • Evaluate the maximum voltage drop
    • Extract the values of Rca and XL (AC Electrical Resistance and Inductive Reactance)
    • Consider the values of CosY and SenY (power factor)

    3-) Adjust to voltage drop limits

    • Once the Dvvalue is calculated, divide by the system voltage;
    • Consider limits of up to 4% voltage drop for customers who receive power supply from concessionaires' branches or up to 7% for customers who have their own power supply such as transformers and substations.

    Table 1- Installation methods

    Table 2 - Current carrying capacities, in amperes, for reference methods A1, A2, B1, B2, C and D in Table 1.

    Thermoplastic insulated wires and cables, copper conductor.
    2 and 3 charged conductors.

    • Conductor temperature = 70ºC
    • Ambient temperature = 30ºC and soil temperature = 20ºC

    Table 2A - Current carrying capacities, in amperes, for reference methods A1, A2, B1, B2, C and D in Table 1.

    Thermosetting insulated wire and cable, copper conductor.
    2 and 3 charged conductors.

    • Conductor temperature = 90ºC
    • Ambient temperature = 30ºC and soil temperature = 20ºC

    Table 3 - Current carrying capacities, in amperes, for reference methods e, f, g from table 1.

    Thermoplastic insulated wires and cables, copper conductor.
    2 and 3 charged conductors.

    • Conductor temperature = 70ºC
    • Ambient temperature = 30ºC and soil temperature = 20ºC

    Table 3A - Current carrying capacities, in amperes, for reference methods e, f, g from Table 1.

    Thermosetting insulated wire and cable, copper conductor.
    2 and 3 charged conductors.

    • Conductor temperature = 90ºC
    • Ambient temperature = 30ºC and soil temperature = 20ºC

    Table 4 - Correction factors for ambient temperatures different from 30 ºC for unburied cables and 20 ºC (soil temperature) for buried cables.

    Table 5 - Correction factors for grouping of circuits or multi-core cables.

     

    Notes:
    (a) These factors are applicable to groups of cables, uniformly loaded.
    b) When the horizontal distance between adjacent cables is greater than twice their outer diameter, it is not necessary to apply any correction factor.
    c) The same correction factors are applicable to: (1) groups of 2 or 3 insulated conductors or unipolar cables; (2) multipolar cables.
    (d) If a grouping consists of both bipolar and three-pole cables, the total number of cables is taken equal to the number of circuits and the corresponding correction factor is applied to the tables of 3 charged conductors for three-pole cables.
    e) If a bundle consists of N insulated conductors or single-pole cables, both N/2 circuits with 2 charged conductors and N/3 circuits with 3 charged conductors can be considered.
    f) The values indicated are averages for the usual range of nominal sections, with an accuracy of ± 5%.
    g) The correction factors in items 4 and 5 are generic and may not attend to specific situations. In such cases, tables 10 and 11 should be used.

    Table 6 - Correction factors for cables contained in conduits buried in the ground, with thermal resistivities different from 2.5 K.m/W, to be applied to the current carrying capacities of the reference method D.

    Table 7 - Correction factors for groupings with more than one circuit - directly buried Unipolar cables or Multipolar cables (installation method D of table 1.

    Table 8 - Multipliers to be used to obtain grouping factors applicable to three-phase circuits or multi-core cables in the open air, contiguous cables, in several horizontal layers, in trays, racks and horizontal supports (installation methods C, E, F of Table 1)

    Note: * The factors are obtained by by multiplying the values referring à layout in a plane horizontal by references à layout on a plan vertical plan, which corresponds to number of layers.

    Table 9 - Correction factors for groupings with more than one circuit cables in directly buried conduits (installation method D of Table 1)

    a) Multi-core cables in conduits 1 cable per conduit


    b) Single-core cables in conduits 1 cable per conduit

    Table 10 - Correction factors for groupings of more than one multi-core cable in free air, (installation method E in Table 1)

    Notes:

    a) The values indicated are averages for the cable types and section ranges in Table 3.

    b) The factors are applicable to cables grouped in a single layer, as shown above, and do not apply to cables arranged in more than one layer. The values for such arrangements may be appreciably lower and must be determined by a suitable method; table 8 may be used.

    c) The values are indicated for a vertical distance between trays or beds of 300 mm. For smaller distances, the factors must be reduced.

    d) The values are indicated for a horizontal distance between trays of 225 mm, with the trays mounted bottom to bottom. For smaller spacings, the factors must be reduced.

    Table 11 - Correction factors for grouping circuits consisting of single-pole cables in the open air (installation method F in Table 1.

    Notes:
    (a) The values given are averages for the cable types and section range in Table 3.
    b) The factors are applicable to cables grouped in a single layer, as shown above, and do not apply to cables arranged in more than one layer. The values for such arrangements may be appreciably lower and must be determined by a suitable method; table 8 may be used.
    c) The values are given for a vertical distance between trays or beds of 300 mm. For smaller distances, the factors should be reduced.
    d) The values are indicated for a horizontal distance between trays of 225 mm, with trays mounted bottom to bottom. For smaller spacings, the factors should be reduced.
    e) For circuits containing several cables in parallel per phase, each group of three conductors should be considered as one circuit for the application of this table.

    Table 12 - Presence of harmonics

    FH factor for determining the neutral current.

     

     

     

     

     

     

     

     

     

     

    Notes:
    (a) In the presence of harmonics between 15 and 33% use a multiplier factor of 0.86 for the current conduction tables for all phases and neutral.
    b) In the presence of harmonics, the current at the neutral will be higher than the phases, therefore the neutral section will tend to be larger. To calculate, apply the factors above on the design current (remembering that the design current must contain the harmonics components).
    In = lb x fx
    Where:
    ln = neutral current
    lb = design current
    fh = correction factor
    With this value, check the neutral section in the current capacity tables (use 3-conductor circuit columns).

    Table 13 - Minimum sections of copper conductors according to their use

     

     

     

     

     

     

     

     

     

     

     

     

     

     

    Notes:
    (a) In signalling and control circuits intended for electronic equipment, sections of up to 0.1 mm² are admitted;
    b) In flexible multi-core cables containing seven or more veins, sections up to 0.1 mm² are admitted;
    c) Power outlet circuits are considered power circuits.

    Table 14 - Sections of neutral and protective conductors

    Comments:
    - In the case of colour identification for the neutral conductor, it shall be light blue on the insulation of the insulated conductor or the vein of the multi-core cable.
    - In the same situation for the protective conductor (PE), it must be identified by double colouring, green-yellow or, in the absence of this, the colour green. For the conductor with double function of neutral and protection (PEN), it must be identified in light blue with green-yellow washers at visible or accessible points.
    - In three-phase systems, the neutral conductor section may be smaller than that of the other phase conductors, respecting the minimum values given above, as long as the following two conditions are met simultaneously:
    a) when the presence of harmonics is not foreseen;
    b) the maximum current which may flow through the neutral conductor in normal service is less than the current-carrying capacity corresponding to the reduced section of the neutral conductor

    Table 15 - Rated currents of cage three-phase motors (60 Hz)


    Ob    servation: * For 440 V motors, multiply the currents for 220 V by 0.5

    Voltage drop limits

    At any point of use of the installation, the verified voltage drop must not exceed the following values, given in relation to the nominal voltage of the installation:
    a) 7% calculated from the MT/LV transformer secondary terminals, in case of transformer owned by the consumer unit(s).
    b) 7% calculated from the secondary terminals of the MV/LV transformer of the electricity distribution company, when the delivery point is located there.
    c) 5% calculated from the delivery point, in other cases of delivery point with supply at secondary distribution voltage.
    d) 7% calculated from the generator output terminals, in case of own genset.

    Remarks:
    1 - These voltage drop limits are valid when the nominal voltage of the foreseen usage equipment coincides with the nominal voltage of the installation.
    2 - Delivery Point: Connection point of the electric system of the electricity distribution company with the electric installation of the consumer unit(s) and that delimits the responsibilities of the distributor, defined by the regulatory authority.
    3 - In the cases of lines a), b) and d), when the main lines of the installation are longer than 100 m, the voltage drops may be increased by 0.005% per half meter of line longer than 100 m, without, however, this supplement being greater than 5%.
    4 - For motor circuits, at start-up, the drop must not exceed 10%.
    5 - In no case can the voltage drop in the circuits exceed 4%.
    6 - Voltage drops larger than those indicated in 6.2.7.1 are permitted for equipment with high starting current during the starting period, provided that they are within the limits permitted by their respective standards.

    Table 16 - Voltage drop in V/A.km

     

     

     

     

     

     

    Notes:

    a) The dimensions of the conduit and gutter adopted are such that the area of the cables does not exceed 40% of their internal area;

    b) Temperature in the conductor: 70 ºC.

    Table 17 - Voltage drop in V/A.km

     

    Notes:
    a) Temperature in the conductor: 70 ºC;
    b) Valid for installation in non-magnetic conduit and directly buried;
    c) Applicable to direct fixing, to wall or ceiling, well, building space, tray, shelf, supports on insulators and overhead lines;
    d) Also applicable to Innovcable 750V insulated conductors, on insulators and on overhead lines.

    Table 17A - Voltage drop in V/A.km

    Notes:
    a) Temperature in the conductor: 90 ºC;
    b) Valid for installation in non-magnetic conduit and directly buried;
    c) Applicable to direct fixing, to wall or ceiling, or open, ventilated or closed electro duct, building space, tray, shelf, brackets and on insulators

    Maximum Short-Circuit Currents INNOVCABLE Cables - BWF 0,6/1kV

    Pressed or welded connections
    Maximum conductor temperature in continuous regime: 70ºC

    Maximum short-circuit conductor temperature: 160ºC