Understanding Maximum Cable Lengths for 230V Final Circuits

Understanding Maximum Cable Lengths for 230V Final Circuits

Understanding the maximum cable length for a 230 V final circuit is crucial to ensure that the voltage drop remains within acceptable limits and that the circuit operates safely. Regulation 7.1(ii) of BS 7671 states:

"The maximum length of cables depends on the protective device rating, cable size, and installation method, while also considering factors like the presence of RCDs and the type of earthing system (TN-S or TN-C-S)."

This regulation helps electricians design circuits that are both safe and efficient, taking into account factors like cable size, protective device rating, and installation methods.

Example: Determining Maximum Cable Length for a 32A Ring Final Circuit

Scenario: Suppose you are installing a ring final circuit in a residential kitchen, protected by a 32A Type B circuit breaker (CB/RCBO). The cable size chosen is 2.5mm² for phase conductors and 1.5mm² for the protective conductor. The installation method is Method 100, which typically involves clipped direct to a surface.

Calculation:

• Maximum Length: According to the table, for this configuration, the maximum cable length is 106 meters if the earth fault loop impedance (Ze) is 0.8 Ω (TN-S) or 0.35 Ω (TN-C-S).
• RCD Consideration: If the circuit is protected by a 30 mA RCD, the full 106 meters can be utilized. If no RCD is present, and Ze is closer to the upper limit, you would need to re-evaluate based on the specific Zs values, which might restrict the maximum allowable length further.

Example: Calculating Cable Length for a Lighting Circuit

Scenario: You are wiring a lighting circuit in a hallway using a 6A Type C circuit breaker. The selected cable is 1.0mm², installed using Method 100 (clipped direct).

Calculation:

• Maximum Length: The maximum cable length is 68 meters when considering a standard installation with an RCD. However, due to the higher inrush current of Type C breakers, the length may be limited by the earth fault loop impedance (Zs), especially if no RCD is present.
• Practical Implication: If Ze is high, you may need to reduce the cable length to meet safety requirements or consider upgrading the cable size to maintain the required length.

Key Considerations

• Voltage Drop: The maximum cable length is primarily limited by voltage drop (typically 5% for ring and radial circuits, 3% for lighting circuits).
• Zs Limitations: Ensure that the earth fault loop impedance does not exceed the limits specified for the protective device, particularly in circuits without RCD protection.
• Cable Size: Increasing the cable size can extend the permissible length, but also consider the practical aspects like installation difficulty and cost.
• Installation Methods: Different installation methods (e.g., buried in insulation vs. clipped direct) can significantly affect the allowable cable length due to their impact on cable current-carrying capacity.

By following these guidelines and using Table 7.1(ii) correctly, you can design safe and effective electrical circuits that comply with BS 7671 standards.

Table 7.1(ii) Maximum cable length for a 230 V final circuit using 70 °C thermoplastic (PVC) insulated and sheathed flat cable

Rating (A) For-Different-Types-Of-Circuit(5 % 3 % voltage drop)	Protective device	Cable size (mm²)	Allowed installation methods (NOTE 2)	Maximum length (m) (NOTE 1)	Maximum length (m) (NOTE 1)	Maximum length (m) (NOTE 1)	Maximum length (m) (NOTE 1)

				RCD 30 mA TN-S	No RCD TN-S	RCD 30 mA TN-C-S	No RCD TN-C-S
Ring final circuits (5 % voltage drop, load distributed)
30	BS 3036	2.5/1.5	100, 102, A, C	106	41zs	106	106
30	BS 3036	4.0/1.5	100, 102, A, 101, 103, C	171	48	171	138zs
32	cb/RCBO Type B	2.5/1.5	100, 102, A, C	106	96zs	106	106
32	cb/RCBO Type C	2.5/1.5	100, 102, A, C	106	NPzs	106	56zs
32	cb/RCBO Type D	2.5/1.5	100, 102, A, C	106	NPzs	106	NPzs
32	BS 88-2 (BS EN 60269-2)	2.5/1.5	100, 102, A, C	106vd	32zs	106vd	106vd
32	BS 88-2 (BS EN 60269-2)	4.0/1.5	100, 101, 102, 103, A, C	171	38zs	171vd	127zs
32	BS 88-3	2.5/1.5	100, 101, A, 102, 103, A, C	106vd	19ad	106	95zs
32	BS 88-3	4.0/1.5	100, 102, A, 101, 103, A, C	171	22zs	171	112zs
32	cb/RCBO Type B	4.0/1.5	100, 102, A, 101, 103, A, C	171vd	114zs	171vd	171vd
32	cb/RCBO Type C	4.0/1.5	100, 102, A, 101, 103, A, C	171vd	NPzs	171vd	66zs
32	cb/RCBO Type D	4.0/1.5	100, 102, A, 101, 103, A, C	171vd	NPzs	171vd	NPzs
Lighting circuits (3 % voltage drop, load distributed)
5	BS 3036	1.0/1.0	100, 101, 102, 103, A, C	68	68	68	68
5	BS 3036	1.5/1.0	100, 101, 102, 103, A, C	106	106	106	106
5	BS 88-3	1.0/1.0	100, 101, 102, 103, A, C	68	68	68	68
5	BS 88-3	1.5/1.0	100, 101, 102, 103, A, C	106	106	106	106
6	cb/RCBO Type B	1.0/1.0	100, 101, 102, 103, A, C	68	68	68	65zs
6	cb/RCBO Type C	1.0/1.0	100, 101, 102, 103, A, C	68	68	68	23zs
6	cb/RCBO Type D	1.0/1.0	100, 101, 102, 103, A, C	68	68	68	23zs
6	BS 88-2 (BS EN 60269-2)	1.0/1.0	100, 101, 102, 103, A, C	68	68	68	68
6	cb/RCBO Type B	1.5/1.0	100, 101, 102, 103, A, C	106	106	106	78zs
6	cb/RCBO Type C	1.5/1.0	100, 101, 102, 103, A, C	106	106	106	28zs
6	cb/RCBO Type D	1.5/1.0	100, 101, 102, 103, A, C	106	106	106	28zs
6	BS 88-2 (BS EN 60269-2)	1.5/1.0	100, 101, 102, 103, A, C	106	106	106	106
10	cb/RCBO Type B	1.0/1.0	100, 101, 102, A, C	42vd	42vd	42vd	42vd
10	cb/RCBO Type C	1.0/1.0	100, 101, 102, A, C	42vd	32zs	42vd	32zs
10	cb/RCBO Type D	1.0/1.0	100, 101, 102, A, C	42vd	23zs	42vd	23zs
10	cb/RCBO Type B	1.5/1.0	100, 101, 102, A, C	65vd	65vd	65vd	65vd
10	cb/RCBO Type C	1.5/1.0	100, 101, 102, A, C	65vd	38zs	65vd	51zs
10	cb/RCBO Type D	1.5/1.0	100, 101, 102, A, C	65vd	28zs	65vd	41zs
10	BS 88-2 (BS EN 60269-2)	1.0/1.0	100, 101, 102, A, C	42	42	42	42
10	BS 88-2 (BS EN 60269-2)	1.5/1.0	100, 101, 102, A, C	65	65	65	65
16	cb/RCBO Type B	1.5/1.0	100, 102, C	34	34	34	34
16	cb/RCBO Type C	1.5/1.0	100, 102, C	34	15sc	34	28zs
16	cb/RCBO Type D	1.5/1.0	100, 102, C	34	NPad	34	9zs
16	cb/RCBO Type B	2.5/1.5	100, 101, 102, A, C	49	49	49	49
16	cb/RCBO Type C	2.5/1.5	100, 101, 102, A, C	49	24zs	49	44zs
16	cb/RCBO Type D	2.5/1.5	100, 101, 102, A, C	49	NPad	49	14zs
16	BS 88-2 (BS EN 60269-2)	1.5/1.0	100, 102, C	20	20	20	20
16	BS 88-2 (BS EN 60269-2)	2.5/1.5	100, 101, 102, A, C	49	49	49	49
16	BS 88-3	1.5/1.0	100, 102, C	34	34	34	34
16	BS 88-3	2.5/1.5	100, 101, 102, A, C	34	34	34	34
Radial final circuits (5 % voltage drop, terminal load)
5	BS 3036	1.0/1.0	100, 101, 102, 103, A, C	56	56	56	56
5	BS 88-3	1.0/1.0	100, 101, 102, 103, A, C	56	56	56	56
5	BS 3036	1.5/1.0	100, 101, 102, 103, A, C	88	88	88	88
5	BS 88-3	1.5/1.0	100, 101, 102, 103, A, C	88	88	88	88
6	cb/RCBO Type B	1.0/1.0	100, 101, 102, 103, A, C	56	56	56	56
6	cb/RCBO Type C	1.0/1.0	100, 101, 102, 103, A, C	56	56	56	23zs
6	cb/RCBO Type D	1.0/1.0	100, 101, 102, 103, A, C	56	56	56	33zs
6	BS 88-2 (BS EN 60269-2)	1.0/1.0	100, 101, 102, 103, A, C	56	56	56	56
6	BS 88-2 (BS EN 60269-2)	1.5/1.0	100, 101, 102, 103, A, C	88	88	88	88
6	cb/RCBO Type B	1.5/1.0	100, 101, 102, 103, A, C	88	88	88	78zs
6	cb/RCBO Type C	1.5/1.0	100, 101, 102, 103, A, C	88	88	88	28zs
6	cb/RCBO Type D	1.5/1.0	100, 101, 102, 103, A, C	88	88	88	40zs
10	cb/RCBO Type B	1.0/1.0	100, 101, 102, A, C	35	35	35	36
10	cb/RCBO Type C	1.0/1.0	100, 101, 102, A, C	35	31zs	35	36
10	cb/RCBO Type D	1.0/1.0	100, 101, 102, A, C	35	6zs	35	17zs
10	cb/RCBO Type B	1.5/1.0	100, 101, 102, 103, A, C	52	52	52	52
10	cb/RCBO Type C	1.5/1.0	100, 101, 102, 103, A, C	52	38zs	52	50zs
10	cb/RCBO Type D	1.5/1.0	100, 101, 102, 103, A, C	52	8zs	52	20zs
10	BS 88-2 (BS EN 60269-2)	1.0/1.0	100, 101, 102, 103, A, C	35	35	35	35
10	BS 88-2 (BS EN 60269-2)	1.5/1.0	100, 101, 102, 103, A, C	52	52	52	52
15	BS 3036	1.0/1.0	NP	NP	NP	NP	NP
15	BS 3036	1.5/1.0	NP	NP	NP	NP	NP
15	BS 3036	2.5/1.5	100, 102, C	47	47	47	47
15	BS 3036	4.0/1.5	100, 101, 102, A, C	76	76	76	76
16	cb/RCBO Type B	1.0/1.0	C	18	18	18	18
16	cb/RCBO Type C	1.0/1.0	C	18	13zs	18	13zs
16	cb/RCBO Type D	1.0/1.0	C	18	7	18	NPad
16	BS 88-2 (BS EN 60269-2)	1.5/1.0	100, 102, C	27	27	27	27
16	BS 88-2 (BS EN 60269-2)	2.5/1.5	100, 101, 102, A, C	45	45	45	45
16	BS 88-2 (BS EN 60269-2)	4.0/1.5	100, 101, 102, 103, A, C	74	74	74	74
16	BS 88-3	1.5/1.0	100, 102, C	27	27	27	27
16	BS 88-3	2.5/1.5	100, 101, 102, A, C	45	45	45	45
16	BS 88-3	4.0/1.5	100, 101, 102, 103, A, C	74	74	74	74
16	cb/RCBO Type B	1.5/1.0	100, 102, C	27	27	15zs	27
16	cb/RCBO Type C	1.5/1.0	100, 102, C	27	27	27	27
16	cb/RCBO Type D	1.5/1.0	100, 102, C	27	NPad	27	9zs
Radial final circuits (5 % voltage drop, terminal load) - Continued
16	cb/RCBO Type B	2.5/1.5	100, 101, 102, A, C	45	45	45	45
16	cb/RCBO Type C	2.5/1.5	100, 101, 102, A, C	45	24zs	45	43zs
16	cb/RCBO Type D	2.5/1.5	100, 101, 102, A, C	45	NPzs	45	14zs
16	cb/RCBO Type B	4.0/1.5	100, 101, 102, 103, A, C	69	32ad	69	69
16	cb/RCBO Type C	4.0/1.5	100, 101, 102, 103, A, C	69	16zs	54ad	36zs
16	cb/RCBO Type D	4.0/1.5	100, 101, 102, 103, A, C	69	NPzs	18ad	9zs
20	BS 3036	2.5/1.5	100, 101, 102, A, C	NP	NP	NP	NP
20	cb/RCBO Type B	2.5/1.5	100, 101, 102, A, C	42	42	42	42
20	cb/RCBO Type C	2.5/1.5	100, 101, 102, A, C	42	12zs	42	31zs
20	cb/RCBO Type D	2.5/1.5	100, 101, 102, A, C	42	NPzs	42	8zs
20	BS 3036	4.0/1.5	C, 100, 101, 102, A, C	69	43zs	73	66zs
20	cb/RCBO Type B	4.0/1.5	C, 100, 101, 102, A, C	69	69	69	69
20	cb/RCBO Type C	4.0/1.5	C, 100, 101, 102, A, C	69	14zs	69	36zs
20	cb/RCBO Type D	4.0/1.5	C, 100, 101, 102, A, C	69	NPzs	69	9zs
20	BS 3036	6.0/2.5	100, 101, 102, 103, A, C	105	69zs	105	105
20	cb/RCBO Type B	6.0/2.5	100, 101, 102, 103, A, C	105	107	105	105
20	cb/RCBO Type C	6.0/2.5	100, 101, 102, 103, A, C	105	23zs	105	58zs
20	cb/RCBO Type D	6.0/2.5	100, 101, 102, 103, A, C	105	NPzs	105	15zs
20	BS 88-2 (BS EN 60269-2)	2.5/1.5	100, 102, A, C	42	42	42	42
20	BS 88-2 (BS EN 60269-2)	4.0/1.5	100, 101, 102, A, C	69	43zs	69	66zs
20	BS 88-2 (BS EN 60269-2)	6.0/2.5	100, 101, 102, 103, A, C	105	69zs	105	105
20	BS 88-3	2.5/1.5	100, 102, A, C	42	42	42	42
20	BS 88-3	4.0/1.5	100, 101, 102, A, C	69	43zs	69	69zs
20	BS 88-3	6.0/2.5	100, 101, 102, 103, A, C	105	69zs	105	105
25	cb/RCBO Type B	2.5/1.5	C	33	33	33	33
25	cb/RCBO Type C	2.5/1.5	C	33	2zs	33	22zs
25	cb/RCBO Type D	2.5/1.5	C	33	NPzs	33	4zs
25	cb/RCBO Type B	4.0/1.5	100, 102, A, C	55	47zs	55	55
25	cb/RCBO Type C	4.0/1.5	100, 102, A, C	55	3zs	55	26sc
25	cb/RCBO Type D	4.0/1.5	100, 102, A, C	55	NPzs	55	4zs
25	cb/RCBO Type B	6.0/2.5	100, 101, 102, A, C	83	75zs	83	83
25	cb/RCBO Type C	6.0/2.5	100, 101, 102, A, C	83	5zs	83	43zs
25	cb/RCBO Type D	6.0/2.5	100, 101, 102, A, C	83	NPzs	83	7zs
25	BS 88-2 (BS EN 60269-2)	2.5/1.5	C	31	20zs	31	31
25	BS 88-2 (BS EN 60269-2)	4.0/1.5
32	cb/RCBO Type B	4.0/1.5	C	43	43	43	43
32	cb/RCBO Type C	4.0/1.5	C	43	28zs	43	43
32	cb/RCBO Type D	4.0/1.5	C	43	NPzs	43	NPzs
32	cb/RCBO Type B	6.0/2.5	100, 102, A, C	63	45zs	63	63
32	cb/RCBO Type C	6.0/2.5	100, 102, A, C	63	NPzs	63	26zs
32	cb/RCBO Type D	6.0/2.5	100, 102, A, C	63	NPzs	63	NPzs
32	cb/RCBO Type B	10.0/4.0	100, 101, 102, 103, A, C	105	74zs	105	105zs
32	cb/RCBO Type C	10.0/4.0	100, 101, 102, 103, A, C	105	NPzs	105	42zs
32	cb/RCBO Type D	10.0/4.0	100, 101, 102, 103, A, C	105	NPzs	105	NPsc
32	BS 88-2 (BS EN 60269-2)	4.0/1.5	C	43	9zs	43	31zs
32	BS 88-2 (BS EN 60269-2)	6.0/2.5	100, 102, A, C	63	15zs	63	50zs
32	BS 88-2 (BS EN 60269-2)	10.0/4.0	100, 101, 102, 103, A, C	105	24zs	105	82zs
32	BS 88-3	4.0/1.5	C	43	5zs	43	27zs
32	BS 88-3	6.0/2.5	100, 102, A, C	63	8zs	63	44zs
32	BS 88-3	10.0/4.0	100, 101, 102, 103, A, C	105	14zs	105	72zs
40	cb/RCBO Type B	6.0/2.5	C	46	23zs	46	46
40	cb/RCBO Type C	6.0/2.5	C	46	NPzs	46	15zs
40	cb/RCBO Type D	6.0/2.5	C	46	NPzs	46	NPzs
40	cb/RCBO Type B	10.0/4.0	100, 102, A, C	72	37zs	72	72
40	cb/RCBO Type C	10.0/4.0	100, 102, A, C	72	NPzs	72	25zs
40	cb/RCBO Type D	10.0/4.0	100, 102, A, C	72	NPzs	72	NPzs
40	cb/RCBO Type B	16.0/6.0	100, 101, 102, 103, A, C	118	57zs	118	118
40	cb/RCBO Type C	16.0/6.0	100, 101, 102, 103, A, C	118	NPzs	118	39zs
40	cb/RCBO Type D	16.0/6.0	100, 101, 102, 103, A, C	118	NPzs	118	NPad
40	BS 88-2 (BS EN 60269-2)	6.0/2.5	C	46	NPzs	46	32zs
40	BS 88-2 (BS EN 60269-2)	10.0/4.0	100, 102, A, C	72	NPzs	72	52zs
40	BS 88-2 (BS EN 60269-2)	16.0/6.0	100, 101, 102, 103, A, C	118	NPzs	118	79zs

NOTES to Table 7.1(ii):

Voltage drop is the limiting constraint on the circuit cable length unless marked as follows:

• ▶ ad limited by the reduced CSA of the protective conductor (adiabatic limit);
• ▶ ol Cable/device/load combination not allowed in any of the installation conditions;
• ▶ zs limited by earth fault loop impedance Zs; and
• ▶ sc limited by line-to-neutral loop impedance (short-circuit).

The allowed installation methods are listed, see Tables 7.1(iii) and 7.1(iv) for further description.

NP - Not Permitted, prohibiting factor as NOTE 1.

For application of RCDs and RCBOs, see 3.6.4.

These notes provide important guidance on the limitations and conditions for using the circuit cable lengths outlined in the table. Found in the On-Site Guide 18th Edition, pages 75-83.

Frequently Asked Questions

Q: How does cable size affect the maximum length of a circuit?

A: According to Regulation 7.1(ii) of BS 7671, the maximum cable length is influenced by the cable size, as larger cables can carry more current with less voltage drop. This means a larger cable size can allow for a longer circuit while staying within safe voltage drop limits.

Q: What installation methods are considered in determining maximum cable lengths?

A: Regulation 7.1(ii) includes several installation methods such as clipped direct (Method 100) or buried in insulation, which affect the maximum cable length due to differences in heat dissipation and current-carrying capacity. The allowed methods are detailed in Tables 7.1(iii) and 7.1(iv).

Q: Why is RCD protection important when determining cable lengths?

A: RCD protection is crucial because it affects the maximum cable length permissible under BS 7671. With RCD protection, circuits can be longer as the device provides additional safety against earth faults, thereby allowing for higher earth fault loop impedance (Zs) limits.

Q: What limitations might apply to certain circuit configurations?

A: Table 7.1(ii) includes several notes that outline limitations, such as restrictions due to earth fault loop impedance (Zs), line-to-neutral loop impedance (sc), or the reduced cross-sectional area (CSA) of the protective conductor (ad). These factors can prohibit the use of certain cable lengths or configurations.

Q: How do voltage drop considerations influence circuit design?

A: Voltage drop is the primary constraint on cable length as per Regulation 7.1(ii) in BS 7671. Typically, a 5% voltage drop is allowed for ring and radial circuits, while a 3% drop is the limit for lighting circuits. Ensuring that the circuit remains within these limits is critical for maintaining safe and efficient operation.