Electrical Design Software | Elecdes Design Suite by Scada Systems Ltd

How to Use the NEC Code to Calculate Raceway Filling

NEC Introduction

Paneldes can now use the NEC code to calculate raceway fill.

The NEC fill calculations affect conduits. Due to conduit pulling requirements NEC reduces the percentage of the cross sectional area of a conduit that can be filled according to the cable count in the conduit.

The NEC fill calculations affect instrument and control cables and trays. Due to weight and heat considerations a maximum fill depth for any instrument / control tray is set to a proportion of the actual tray depth when calculating cross sectional fill capacity. A maximum depth is also set by NEC

The NEC fill calculations affect power cables and trays. Due to weight and heat considerations a maximum fill depth for any tray is set when calculating cross sectional fill capacity. This fill depth varies slightly between single and multi core cables and between types of tray.

The NEC fill rules affect large power cables. Due to weight and heat considerations certain cables may only be placed in tray in a single layer. This single layering also affects the power tray fill calculations.

To accommodate NEC, EDS now allows 2 cable factors for any tray and 2 for any cable. These NEC required values originate from the catalog data for your cables and trays.

You will also need to tell EDS of the power numbers you have used for your "signals" cables.

The preferences for filling calculations may be selected as "NEC" and all of the NEC functions will be activated.


NB1: "Channel" tray is assumed to be no different to the "Ventilated" and "Closed Bottom" trays used in all the EDS calculations. This is a customer approved variation from NEC.


NB2: Single and multi core cables are handled together by the EDS calculations below. The calculations used by EDS are however the NEC multi core calculations in all cases. This is a customer approved variation from NEC.

NEC Signals Power Levels, Preferences

You can set the power numbers that identify control or instrumentation (signals) cables and trays.

See How to Configure NEC Filling.

NEC Catalog settings

There are two changes to the CABLEFACTR field in the "Catalog" files.

The changes will also ripple through the EDS system and be present in the downstream DBF files loaded from the Catalog's. The Catalog's can now have 2 cable factors associated with any cable or tray.

Tray

For Trays, within the catalog, the CABLEFACTR field will contain a number as before:

e.g. 3.6000

This will be the POWER tray cable factor calculated by the user.


If the same tray is needed to be used for Signals cables also, then an optional "SIGNALS" cable factor can be placed in the AUX1 attribute as follows.

SIGNALSCF: 5.7000

"SIGNALSCF:" indicates this AUX1 attribute data is being used for the "Signals" cable factor.


The tray's user defined cross sectional area for filling will be 3.6000 square inches unless the tray is capable of carrying cables with Power Level Numbers for NEC Signals. If capable of carrying signals cables then the cross sectional area for filling will be 5.7000 square inches rather than 3.6000. The 5.700 number is the "Signals cable fill cross section". The 3.6000 number is the "non-signals" (power) fill cross section.


For metric users the CF number must be based upon square mm and would be more likely to be a number of the order of magniture of 20,000 (sqmm).


NB: A tray with a power number of 0 will not use SIGNALSCF, even though power 0 is capable of carrying any cable, including control and signals. This differs from early versions of Paneldes.

NB2: The user has the responsibility to calculate the 2 values.

NB3: Example tray power numbers with capability of carrying signals: "1" , "1,8" , "8" , "1-8" , also "1,2" or "7-9" except these last two are mixing power and control/signals cables which is not usually appropriate.


If the user enters only one CABLEFACTR number for trays, then it should be placed in the CABLEFACTR field. This is acceptable however it will restrict a tray used for signal cables to the fill of the same tray if it were being used for power cables.

Cables

For Cables, in the catalog, the CABLEFACTR field could contain:

e.g.1: 2.5000

e.g.2: 2.5000 d 1.500"

e.g.3: 1600 d 40mm

The letter code separator for trays is the letter "d" indicating the number that follows is a "required large power cable diameter".


In the second example the cable's user defined cross sectional area for general raceway filling will be 2.5000 square inches. This would be used when filling a conduit.

For metric users the CF number must be based upon square mm and would be more likely to be a number of the order of magniture of 1600 (sqmm).

If the cable were being placed in tray, then the D code indicates that it is a "large power cable" and the diameter (1.500 inches) should be used when filling.

For metric users the D (diameter) number must be based upon m, if it is not explicitly followed by units, and would be more likely to be a number of the order of magniture of 0.040 (m).

Do not include the D or the second number UNLESS it is a "large power cable". Unlike the SIGNALSCF code, the D code dictates the cables filling behaviour by its presence.


NB: The user has the responsibility to calculate these 2 values.


In the first example there is only one number.

"Signals" cables AND "small" power cables should only have 1 number in the catalog.

NEC Conduits

The NEC fill calculations affect conduits. Due to conduit pulling requirements NEC reduces the percentage of the cross sectional area of a conduit that can be filled according to the cable count in the conduit.


The conduit fill calculation in EDS is calculated as follows:

  • Conduit Cross Sectional area = CCF; (Single CF value in EDS tray catalog)

  • Cable Cross Sectional area = XCF; (First CF value in EDS cable catalog)

  • Fill % = XCF / (CCF * NecFactor)


    Where the "NecFactor", which reduces the amount of cable in the conduit, is:

    • If 1 cable is in the conduit = 0.53

    • If 2 cables are in the conduit = 0.31

    • If 3+ cables are in the conduit = 0.40

NEC Signals Trays & Cables

The NEC fill calculations affect instrument and control (signals) cables and trays. Due to weight and heat considerations a maximum fill depth for any instrument / control tray is set to a proportion of the actual tray depth when calculating cross sectional fill capacity. A maximum depth for calculation is also set by NEC at 6 inches.

Signals trays will have a control or instrumentation power number set (1 or 8).


The signals trays fill calculation in EDS is calculated as follows:

  • Tray Cross Sectional Signals fill area = TCF; (AUX1 value in EDS tray catalog)

  • Cable Cross Sectional area = XCF; (First CF value in EDS cable catalog)

  • Fill % = XCF / TCF


    Where TCF should be calculated BY THE USER as:

    • TCF = Tray Width * Allowed Tray Height.

    Where "Allowed Tray Height" should be calculated as:

    • Allowed Tray Height = X% of; the smaller of; 6 inches / 150mm or Tray height.

    Where " X%" should be:

    • 40% for closed bottom tray

    • 50% for ventilated tray.

NEC Power Trays & Small Cables

The NEC fill calculations affect power cables and trays. Due to weight and heat considerations a maximum fill depth for any tray is set when calculating cross sectional fill capacity. This fill depth varies slightly between single and multi core cables and between types of tray.


The power trays "small cable fill" calculation in EDS is calculated as follows:

  • Tray Cross Sectional power fill area = TCF; (CF value in EDS tray catalog)

  • Cable Cross Sectional area = XCF; (First CF value in EDS cable catalog)

  • Fill % = XCF / TCF


    Where TCF should be looked up BY THE USER as:

    • TCF = From NEC table 318-9.

      (NB: this is NOT the actual cross sectional area of the tray)

    Inside NEC table 318.9:

    • Column 1 is for ventilated tray.

    • Column 2 for closed bottom tray

NEC Power Trays & Large Cables

The NEC fill rules affect large power cables. Due to weight and heat considerations certain cables may only be placed in tray in a single layer. This single layering also affects the power tray fill calculations.

Wall thickness: 1.5 Inches or 40mm are allowed for the total tray wall thickness during this calculation.


The power trays "large cable fill" calculation in EDS is calculated as follows:

  • Tray Width = TW; (From the tray catalog)

  • Cable Diameter = XCD; (Second CF value in EDS cable catalog)

  • Fill % = XCD / (TW - 1.5 inches). NB: These values should both be scaled as inches

Notes on Tray TEEs and Xs

Due to the ability of Tees and Xs to contain more cable than their dimensional width, the width used by the LARGE NEC POWER CABLES calculation will be double the dimensional width (TW x 2.0).

For small power and signals tray the user should calculate the CABLEFACTR and AUX1 fields accordingly. Tees and Xs should have these numbers set to 2x the cable factors of the equivalent standard tray. This is the responsibility of the user.

Power Trays & Mixtures of small and large Cables

When we encounter a mixture of small and large power cables, as per the 2 previous sections, fill will be calculated by combining the diameters of the large cables (in a single layer) with the cross sectional area of the combined small cables, as defined by NEC:

NB: This calculation uses the "1.2 x diameter" for imperial and "1.2 x 25.4 x diameter" for metric, "Multi-core Ventilated", tray calculation, which is the most conservative strategy for filling. The most aggressive strategy may fit from 1% - 20% more cable in the tray.


The power trays "mixed" calculation in EDS is calculated is as follows:

  • Tray Cross Sectional power fill area = TCF; (CF value in EDS tray catalog)

  • Cable Cross Sectional area = XCF; (First CF value in EDS cable catalog)

  • Cable Diameter = XCD; (Second CF value in EDS cable catalog)

  • Fill % = (sum(XCF) + (1.2 x (metric also x 25.4) x sum(XCD) ) ) / TCF


    sum (XCD) will be the sum of all of the large cable's diameters (inches).

    sum (XCF) will be the sum of all of the small cable's cross sect areas (sq. inches).


    Where TCF should be looked up BY THE USER as:

    • TCF = From NEC table 318-9.

      (NB: This is NOT the actual cross sectional area of the tray)

      Inside NEC table 318.9:

    • Column 1 is for ventilated tray.

    • Column 2 for closed bottom tray.

See also

Panel ducting

Raceway

Routing functions description

Route optimisation errors and warnings

Power numbers for raceway

Cable filling factor for raceway

Keeping power separated raceway on different layers