SECTION 9-1
GENERAL DESCRIPTION

The reliability of a ship' s electronic distribution system depends largely on its cable installation. Current carrying capacity of cables, their insulation strength, and their ability to withstand all kinds of service exposure, including heat, cold, dryness, bending, crushing, vibration, twisting, and shock are important - but the workmanship that goes into the cable installations is just as important.

The ratings and characteristics of the various classes of Navy shipboard cable are given in General Specification S62-2, NavShips 250-660-23 and supplementary matter. This section contains all the cable information needed by the electronics installation worker.

2. TYPES OF CABLES.

The principal classes of cable are briefly described here, with notes on the application of each.

a. HEAT AND FLAME RESISTANT CABLES.-Most of the permanent wiring on shipboard is done with heat-and flame-resistant cables having protective armor. These cables use synthetic resin or varnished cambric as primary insulation supplemented by asbestos for heat-and flame-resistance and covered by a braided, protective armor. They are designated by the letters "HFA" and include the following types:

Multi-conductor cables are available with up to 44 conductors, each conductor having a cross-sectional area of 2828 circular mils.

Twisted-pair telephone cables are available with up to 60 pairs of conductors, each conductor having a cross-sectional area of 703 circular mils.

SHFA, DHFA, THFA, and FHFA type cables are available in various sizes of circular mils and are used mostly with the power and lighting system.

b. REDUCED-DIAMETER CABLES. - The new, reduced-diameter, type "SGA" cables are smaller and lighter weight than the "HFA" types but have design features that give them greater insulation strength and equivalent heat-and flame-resistance as the larger cables and will let them be used for the same service as the "HFA" types. Widely used "SGA" cables are:

Multi-conductor cable in this class (Type MSCA) is made up of stranded conductors, each having seven strands of 0.016 inch diameter copper for a total area of 1779 circular mils per conductor.

c. CABLES FOR REPEATED FLEXING SERVICE.-Many applications aboard ship require cables that can be bent and twisted again and again without damaging the conductor, insulation, or protective covering.

Flexible cables have synthetic rubber or synthetic resin insulation and a sheath that is resistant to water, oil, heat, and flame. They are not as heat-and-flame-resistant as armored "HFA" and "SGA" cables. Flexible cables are of the following types:

New flexible cables, types SHOF, DHOF, THOF and FHOF are identical in diameter and weight to corresponding sizes of the COP cables. The only difference between these two types is in the grade of insulating material on the individual conductors. The HOF cable uses a special heat-resistant synthetic (butyl) rubber insulation and may be operated continuously at temperatures above the limits permissible for type COP cables. The result is a higher current-carrying capacity and a reduction in cable weight, since it is possible to use a smaller size in type HOF cable than in type COP for a given application.

More information such as current rating, resistance per foot, and stuffing tube sizes is given for all of these types in the tabulated data section.

c. WATERTIGHT AND NON-WATER-TIGHT CABLE. - The heat-and-flame-resistant, armored cables under Specification MIL-C-915 (Ships) are made watertight by filling all voids within the stranded copper conductors and all spaces between insulated conductors within the cable core with compounds which block the entrance of water. The new reduced diameter cables are of watertight construction.

Conductor sizes are designated by numbers that are to the nearest thousand of the actual circular mil area. These numbers follow the type and class designation.

EXAMPLE: TSGA-60 is a reduced-diameter, three-conductor, armored cable for general shipboard use, with each conductor having a cross-sectional area of 60,090 circular mils.

4. MULTIPLE -CONDUCTOR -CABLE DESIGNATIONS.

Multiple - conductor - cable types and class designations are followed by a number that indicates the number of conductors.

EXAMPLES: MSCA-30 is a heat- and flame-resistance, armored cable with 30 conductors.

For telephone cable, the number indicates twisted pairs.

5. SELECTION OF CABLE SIZE.

Current-carrying capacity and voltage drop limitations, determine the size cable for a particular application. The current capacity is dependent upon the type and size of the conductor, the permissible temperature rise, and the character of the space in which the cable is installed,

6. BASIC CABLE INSTALLATION DATA.

a. CABLE CONNECTIONS. -All connections to cables and at normal breaks in a cable, should be made in standard appliances and fittings. Don't make splice connections.

b. CABLE ENTRANCES. -Cables entering water-tight equipment should be brought into the equipment through Navy Standard Stuffing tubes. When cables

c. PASSING THROUGH DECKS AND BULKHEADS.- Where cables pass through decks and water -tight bulkheads, watertight stuffing tubes should be used. Where cables pass through non-watertight bulkheads 1/4 inches thick, or more, no stuffing tubes are used; simply pass cable through drilled, smoothed holes. Where the bulkheads are less than 1/4 inches in thickness, bushings should be used. All cables passing through decks should be protected from mechanical injury by kickpipes or riser boxes. A kickpipe is a length of steel pipe welded to the deck and having a stuffing tube threaded to the upper end.

d. CABLE BENDS.- Bending cables too sharply will damage them. The correct minimum radius for each type of cable is given in table 9-1.

e. SUPPORTING CABLE ON DECKS AND BULKHEADS. -The methods for supporting cable depend upon the number and size of cables in a particular run, thickness of bulkhead or deck, obstructions to direct runs, heat and moisture conditions, structural material, water-tight conditions and other conditions taken up later in the chapter.

SECTION 9-2

CHOOSING PROPER SIZE AND
CALCULATING VOLTAGE DROP
 

Cable installations are rated according to the ambient temperature the cable operates in. Ratings are: general, restricted and isolated corresponding to normal, poor and good heat conditions.

a. GENERAL CABLE RATINGS. -General ratings apply to cables installed under the conditions that are most common in Naval service. (Ambient Temperature 40° to 50°C) Typical examples are:

Cables installed in racks, with not more than three cables next to each other carrying current at the same time. Cables spaced one-half inch apart no matter how many of the cables are carrying current at once.

Cables installed in armored trunks, when the load is intermittent and the armor is more than two inches thick.

The current-carrying capacities given in the tables apply to general cable ratings.

b. RESTRICTED CABLE RATINGS. - Restricted ratings apply to cables installed where the ambient temperature is greater than normal (50°C or greater) A typical example is:

Gables installed next to each other in racks, with three or more cables carrying current at the same time.

When cables are installed so that restricted ratings have to apply, their current carrying capacities are 15% less than the values given in the tables for general ratings. Every effort should be

c. ISOLATED CABLE RATINGS.- Isolated ratings apply when there are no hot objects or loaded cables close by or when heat insulation reduces the amount of heat that can reach the cable. Typical examples are:

Single cables, in free air or clamped to steel or aluminum decks or bulkheads.

Cables, in groups of two, clamped to decks or bulkheads of steel, aluminum, or other material that carries heat away rapidly.

Cables in racks where none of the cables run hot.

Under such favorable conditions, cables can be loaded 10% above the current-carrying capacities given in the table for general ratings.

2. CHOOSING CABLE SIZE.

To choose the right size cable for a particular application, it is necessary to know the following:

The demand factor is 1.0 for a power cable supplying a single load; it is 0.9 for a power cable supplying a group of several loads. The current value to use in choosing the conductor size is reached by adding the maximum connected load to the allowance for future load and multiplying this total by the demand factor. This value of resultant load amperes is used to find, in Table 9-1, a cable of the desired size that is safely rated to handle the load. If the cable is to have a restricted service rating, the resultant load amperes figure should be increased 15% before the conductor size is chosen from Table 9-1. If the isolated

The cable chosen on the basis described above will be large enough to carry the maximum load without overheating, but it is also necessary to be sure the voltage drop is below the allowable maximum. Percentage voltage drop is the difference in voltage between any two points on a circuit expressed as a percentage of the rated switchboard or transformer secondary no-load voltage.

For all electronic installations, the maximum allowable percentage voltage drop between switchboard or transformer panel for circuits above 100 volts is 2%. For circuits of less than 100 volts, such as control and interlock, the maximum allowable percentage voltage drop is 5%. Since the majority of circuits use voltages above 100, Table 9-2 shows how many feet of each standard cable size can be used at different loads without exceeding the 2% voltage drop. These results are approximate. A method of computing length of cable run for a desired percentage voltage drop is shown in Section 2, Paragraph 4.

Table 9-1 is a cable comparison chart in which all the type cables the electronics installation worker may work with are divided into four major groups, as follows:

Heat and flame resistant, non-flexing service.

Special purpose cables.

Twisted pair telephone cable, armored.

Heat, flame, oil resistant, repeated flexing service.

Cable sizes up to 30,000 circular mils are included in most cases. Larger sizes are not listed since their use in electronic installations is limited. A table on steel stuffing tube information is included here giving tube clearance drill size and the inner diameter of the gland nut for all the sizes to make the table 9-1 as complete and useful as possible.

It should be noted that where a maximum DC voltage of 1000 is specified, the maximum AC voltage is 600.

The reduced diameter cables (DSGA, TSGA, FSGA, and MSCA) are being procured for use on new construction and certain electronics installations where HFA types were formerly used.

Similarly, the HOF type cables (SHOF, DHOF, THOF and FHOF) replace the COP types (SCOP, DCOP, TCOP, and FCOP) and are now called for on new construction.

Cable types TTRS and TTRSA, twisted pair telephone cable, have properties which make them useful as R.F. cable. These properties are listed in the table.

TABLE 9-1a. HEAT AND FLAME RESISTANT, NON-FLEXING SERVICE

NOTE: SSGA replaces Type SHFA, SDGA replaces Type DHFA, TSGA replaces Type THFA, FSGA replaces Type FHFA, MSCA replaces Type MHFA.

TABLE 9-1b. HEAT AND FLAME RESISTANT, NON-FLEXING SERVICE

* NOTE. Maximum voltage AC between conductors is 300.

TABLE 9-1c. HEAT AND FLAME RESISTANT, NON-FLEXING SERVICE

TABLE 9-1d. SPECIAL PURPOSE CABLES

*Cables suitable for use through the pressure-proof hull
NOTE: HOF Types replace COP Types

TABLE 9-1f. HEAT, FLAME, OIL RESISTANT, REPEATED FLEXING SERVICE

*Cables suitable for use through the pressure-proof hull
NOTE: HOF Types replace COP Types

TABLE 9-li

TABLE 9-2
MAXIMUM ALLOWABLE LENGTH OF CABLE RUN FOR A GIVEN LOAD WITH A 2% VOLTAGE DROP

Table 9-2 shows the maximum lengths of cable run permissible at a given load current. The table may be applied to all cable types but the results are approximate.

To demonstrate the use of Table 9-2, two typical problems and their solutions are worked out.

Problem 1: Find the most suitable cable size for a 12-amp. , 117V. , single-phase load, to be run 69 feet. Percentage voltage drop to be less than 2%.

Solution: Refer to Table 9-1 and find the smallest cable size which allows a maximum load current of 12 amps. This would be size 3. Therefore our selection must be size 3 or larger.

Refer to Table 9-2 now, and under the vertical column 12 amps. , read down until a figure slightly greater than 69 is reached (in this case 73) and read horizontally to the left to get correct cable size, in this case 9.

Problem 2: Find the proper cable size for a 10 ampere, 117 volt, single phase load to run 25 feet. Percentage voltage drop to be less than 2%.

Solution: Since there is no column computed for 10 amperes, it is necessary to make use of the base column, computed on 1 ampere. The run in feet is a linear function of current, so the base column may simply be divided by the load current to obtain the maximum allowable run for that load.

For DSGA-3, a 1 -ampere load permits a run of 276 feet. Therefore, a 10-ampere load allows a run of 276 y 10 or 27.6 feet which is within the limits required. DSGA-3 has a maximum rating of 12 amperes at 50°C, so that the cable is not overloaded with this choice.

An example calculation is given below showing the method for determining the length of cable that can be used for a maximum of 2% voltage drop using a given conductor size at a temperature of 45°C.

It is known that:

The formula for finding the percent voltage drop is:

Percent voltage drop =
(R x 2l- x I x 100) / C.M. x E
(for single phase operation).

l= Length of cable

R = Resistivity of copper in circular mil feet at a given temperature (see Figure 9-1)

C.M.= Circular mil area of one conductor

E = Terminal voltage

(1) Therefore:

l=((% V.D.)(C.M.)(E)) / ((R)(2)(I) x 100)

The C. M. area may be found from Table 9-1 as corresponding to the standard conductor size (opposite any DHFA 3, THFA 3, etc.) as being, in this case 2828 C. M.

The resistivity (R) is in ohms per C. M. ft. at 45°C and is found from Figure 9-1,

(1) l =
((2 x 2828 x 117) / (12 x 2 x l x 100))
= 275.72 feet

The same procedure may be followed for voltages of 220 and 440 or if the length of feet is calculated for 117 volts; and entered in a chart, multiply the length by 1.88 or 220 volts and 3.76 for 440 volts at whatever load is to be used.

(2) l =

As can be seen, the differences between formulas (1) and (2) is the squareroot(3) substituted for 2l. For this reason, to obtain 3 phase length, if those for single phase operation have already been calculated, multiply by 1.15. These lengths of course, represent the lengths for corresponding conductor size and load.

Copyright © 1997-2007, Historic Naval Ships Association.
All Rights Reserved.
Legal Notices and Privacy Policy
Version 3.00