Cable current rating

Cable current rating 

cable current rating

Current carrying capacity is defined as the maximum amperage that can flow through an insulated wire or cable before reaching a temperature high enough to melt the insulation or the conductor.

Amperage refers to the strength of the current in its unit of measure. One ampere is the current that one volt of electricity can send through one ohm of resistance.

The amount of current a wire can carry is determined by several factors, including:

Conductor Size - Larger circular mil area increases current carrying capacity.

Insulation – Higher maximum temperature ratings for insulation allow for higher current carrying capacity. Heat generated by the current cannot exceed the maximum temperature rating of the insulation.

Ambient Temperature - Higher surrounding temperatures lower current carrying capacity. Less heat can be produced by the current before reaching the maximum temperature rating of the insulation.

Conductor Number – Increased numbers of individually insulated conductors bundled together decrease heat dissipation, lowering current carrying capacity.

Installation Conditions – Restricted heat dissipation faced by conductors installed in conduit, duct, trays, or raceways lowers current carrying capacity. However, proper ventilation methods like forced air cooling can mitigate this problem.

Basic assumptions and conditions of installation:

1) Maximum Conductor Temperature:  90° C

2) Ambient Ground Temperature:         30° C

3) Ambient Air Temperature:                 40° C

4) Thermal resistivity of soil:                150° C. Cm/W

Depth of laying (to the highest point of the cables laid direct in the ground)

1)    3.3, 6.6 & 11kV Cables:                      90 cm

2)    22 and 33kV Cables:                            105 cm

3) Maximum Conductor temperature at the end of a short circuit:  250° C

To obtain the maximum current carrying capacity of a cable operating at different conditions from the standard, various rating factors are to be multiplied.

Table-1: current rating (ac) for twin 650/1100volt armoured/unarmoured aluminium/copper PVC cable IS: 1554

Size in	Laid direct in the ground		In ducts		In air
mm2	Al	Cu	Al	Cu	Al	Cu
1.5	18	23	16	20	16	20
2.5	25	32	21	27	21	27
4	32	41	27	35	27	35
6	40	50	34	44	35	45
10	55	70	45	58	47	60
16	70	90	58	75	59	78
25	90	115	76	97	78	102
35	110	140	92	120	99	125
50	135	165	115	145	125	155
70	160	205	140	180	150	195
95	190	240	170	215	185	230
120	210	275	190	235	210	265
150	240	310	210	270	240	305
185	275	350	240	300	275	350
240	320	405	275	345	325	410
300	355	450	305	385	365	465

Table-2: current rating (ac) for three-four 650/1100volt armoured/unarmoured aluminium/ copper PVC cable IS: 1554

Size in	Laid direct in the ground		In ducts		In air
mm2	Al	Cu	Al	Cu	Al	Cu
1.5	16	21	14	17	13	17
2.5	21	27	18	24	18	24
4	28	36	23	30	23	30
6	35	45	30	38	30	39
10	46	60	39	50	40	52
16	60	77	50	64	51	66
25	76	99	63	81	70	90
35	92	120	77	99	86	110
50	110	145	95	125	105	135
70	135	175	115	150	130	165
95	165	210	140	175	155	200
120	185	240	155	195	180	230
150	210	270	175	225	205	265
185	235	300	200	255	240	305
240	275	345	235	295	280	355
300	305	385	260	335	315	400

Table-3: current rating (ac) of 1.1 KV XLPE aluminium conductor power cable

Size in	Single core in 3 cables		Multi core
mm2	In ground	In air	In ground	In air
6	45	40	43	40
10	59	53	57	53
16	76	70	78	70
25	99	115	95	99
35	117	140	116	117
50	138	170	140	140
70	168	210	170	176
95	204	255	200	221
120	230	300	225	258
150	265	342	255	294
185	295	385	285	339
240	340	450	325	402
300	390	519	370	461

Table-4: current rating (ac) for three core armoured XLPE cable, IS: 7098

Size	Voltage 3.8/6.6, 6.35/11, 11/11 KV
mm2	Copper conductor			Aluminium conductor
	In ground	In duct	In air	In ground	In duct	In air
25	120	105	135	95	82	105
35	145	125	165	115	97	125
50	170	150	195	130	115	150
70	210	180	240	160	140	190
95	250	215	295	190	165	230
120	280	240	335	220	190	260
150	310	270	380	245	210	295
185	350	305	430	275	240	335
240	400	350	500	315	275	395
300	445	390	570	355	310	450
400	500	440	650	400	350	520

Table-5: current rating (ac) for three single core armoured XLPE cable, IS: 7098 part-2

Size	Voltage 12.7/22,19/33
	Aluminium conductor
	In ground		In duct		In air
mm2	Trefoil	Flat	Trefoil	Flat	Trefoil	Flat
25	100	100	90	90	120	120
35	115	120	100	100	145	145
50	135	140	120	120	170	175
70	165	170	145	150	215	220
95	195	200	170	175	260	265
120	225	225	295	195	300	300
150	250	250	215	215	340	340
185	280	280	240	240	385	385
240	320	315	275	275	450	450
300	355	345	305	300	510	500
400	400	385	345	330	600	570

Book reference :
A practical guide to cable installation and tool box talk

Available with book shop and -

http://www.amazon.in

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Price: Rs. 375/- excluding delivery charges