What is the difference between Part 7 of the On-Site Guide and Part 7 of BS 7671?

They are completely different. Part 7 of BS 7671 covers special installations and locations (bathrooms, construction sites, EV charging). Part 7 of the On-Site Guide is a set of standard final circuit tables that let you look up cable sizes, protective devices, and maximum circuit lengths for common domestic circuits.

What is the assumed maximum load for a 32A ring final circuit in the On-Site Guide?

Table 7.1(1) of the On-Site Guide gives an assumed maximum load of 26A for a 32A ring final circuit. This reflects diversity — not every socket and appliance is in use at once — so the circuit is not designed for a full 32A continuous load.

What is the maximum length of a 2.5mm² ring final circuit protected by a 32A MCB?

Using the On-Site Guide standard conditions (TN-C-S, Ze ≤ 0.35Ω, 30mA RCD, reference method 100), the maximum length is 106 metres measured all the way around the ring and back to the consumer unit. Change any condition — earthing system, installation method or protective device — and the length changes.

What do installation reference methods 100, 101, 102 and 103 mean?

These methods apply specifically to flat twin and earth cable. Method 100 means the cable is in or above thermal insulation up to 100mm thick; 101 means insulation over 100mm thick; 102 means the cable is between two plasterboard surfaces (a stud wall); and 103 means the cable is surrounded by thermal insulation on at least three sides — the worst case for heat dissipation and the lowest current rating.

Why can't I just split a ring circuit into two radials?

Ring and radial circuits are treated differently in the tables. A radial with a terminal load has a much shorter maximum length for the same protective device. A 32A ring reaches 106m, but a 32A radial in 4mm² is far shorter — you often need 10mm² cable to get close. Always check the radial tables before splitting a ring.

The On-Site Guide: How to Use the Standard Final Circuit Tables

The On-Site Guide (OSG) is one of the most useful books you can keep in your toolbox — that little brown A5 companion to BS 7671. It condenses the calculations behind standard final circuits into a handful of lookup tables, so you can select a cable size, a protective device, an installation method and a maximum circuit length in seconds.

But it catches people out. The tables are dense, the numbering is unfamiliar, and — confusingly — the section that trips everyone up is called Part 7, which has nothing to do with Part 7 of BS 7671. This guide walks through exactly how those tables fit together, using a worked example, and finishes with a set of memory aids for the installation reference methods that come up again and again.

If you understand this, you’ll answer a whole class of Part 5 exam questions far faster — and you’ll be quicker on site too.

In This Guide

Part 7 of the OSG Is Not Part 7 of BS 7671
The Assumptions Behind the Tables
Step 1: Find the Assumed Maximum Load
Step 2: Check the Maximum Zs
Step 3: Read the Standard Circuit Table
Ring vs Radial — Always Check
Installation Reference Methods Explained
Memory Aids for Every Method
Common Mistakes with the OSG

Part 7 of the OSG Is Not Part 7 of BS 7671

This is the single biggest source of confusion, so let’s clear it up first.

Book	”Part 7” Means
BS 7671 (the brown Wiring Regs)	Special installations and locations — bathrooms, construction sites, EV charging
On-Site Guide	Standard final circuit tables — cable sizes, protective devices, maximum lengths

Important: When someone says “Part 7 of the On-Site Guide”, they mean the standard final circuit section — not special locations. Keep the two books straight in your head or you’ll waste time flicking to the wrong tables.

Part 7 of the OSG deals only with final circuits using copper conductors, and for ring circuits it assumes the circuit starts and finishes at the consumer unit or distribution board.

The Assumptions Behind the Tables

Every OSG table is built on a set of assumptions (set out on the first page of the section). If your installation doesn’t match them, the table result doesn’t apply and you need to calculate from first principles instead.

Assumption	Value
Conductors	Copper only
Earthing systems	TN-C-S, TN-S, or TT (with 30mA RCD/RCBO)
Ambient air temperature	Not exceeding 30°C (cooler is better)
Disconnection time	≤ 0.4s for circuits up to 63A, with Cmin of 0.95
Voltage drop	3% for lighting, 5% for all other circuits
Installation methods	As listed in the OSG method tables

Key point: The 3% and 5% voltage drop limits come straight from Regulation 525 of BS 7671. On a 230V single-phase circuit, 5% is 11.5V — that’s the budget the length tables are working to.

Step 1: Find the Assumed Maximum Load

Start with Table 7.1(1). You need three things: what the protective device is, its rating, and the type of circuit.

Take our worked example — a 32A BS EN 60898 Type B circuit breaker protecting a ring final circuit. The table gives:

Parameter	Value
Protective device	32A BS EN 60898 Type B MCB
Assumed maximum load	26A
Maximum Ze	0.35Ω
Maximum measured Zs	1.1Ω

The assumed maximum load of 26A (not 32A) reflects diversity — in a domestic setting, not every socket is loaded and not every appliance runs at once. The circuit isn’t expected to carry a continuous 32A.

Exam tip: A common trick question asks for the “assumed maximum load” of a standard ring final circuit. The answer is 26A, not the 32A rating of the protective device. Diversity is doing the work here.

Step 2: Check the Maximum Zs

The maximum measured Zs for the protective device comes from the OSG’s earth fault loop impedance table (Table B6). Find the device type down the left, the rating along the top, and read off where they cross.

For a Type B 32A device, the maximum measured Zs is 1.1Ω.

Remember: This 1.1Ω is a measured value — it has already been adjusted for a Cmin of 0.95 and the 80% temperature correction. There is nothing further for you to do to it. Don’t apply the 80% rule a second time.

That last point catches people out constantly. If you’re comparing an on-site loop impedance reading against a value from the tabulated measured Zs column, the correction is baked in. The 80% adjustment only applies when you’re working against the maximum Zs values in Table 41.3 of BS 7671, which are quoted at operating temperature. For a full walkthrough of that distinction, see our guide on Testing Zs: Earth Fault Loop Impedance Explained.

Step 3: Read the Standard Circuit Table

Now to the main event — the large standard circuit table, Table 7.1(2). It’s full of choices, so work through it in order.

Find the ring final circuits section, then locate your protective device. For a 32A Type B MCB, the table specifies:

Column	Result
Cable	2.5mm² twin and earth with 1.5mm² CPC
Permitted installation methods	100, 102, A and C
Earthing / RCD	TN-C-S, Ze ≤ 0.35Ω, 30mA RCD
Maximum circuit length	106 m

That 106m is measured all the way around the ring — from the consumer unit, around the circuit, and back to the board. Choose reference method 100 (clipped direct to joists, thermal insulation not more than 100mm), pick the earthing system, confirm the 30mA RCD, and the length falls out of the table.

Key point: Every input changes the output. Swap the earthing system, the installation method or the protective device and the maximum length shifts. The tables aren’t a single answer — they’re a decision tree.

Ring vs Radial — Always Check

Here’s where people get burned. Assume that because a 32A ring reaches 106m, a 32A radial will be similar. It won’t.

Radial circuits with a terminal load (all the load at the far end — think of a shower or a cooker) are on a separate page of the same table. Look at what happens to a 32A radial:

Circuit Type	Cable	Approx. Max Length
Ring final (32A)	2.5mm²	106 m
Radial, terminal load (32A)	4mm²	Much shorter
Radial, terminal load (32A)	6mm²	Shorter still than the ring
Radial, terminal load (32A)	10mm²	~105 m

The only way to get a 32A radial close to the ring’s 106m is to jump up to 10mm² twin and earth. That’s a big difference in cable cost and installation effort.

Warning: Never split a ring into two radials without checking the radial tables first. Sometimes it simply won’t work safely with the existing cable. Ring and radial circuits must be treated separately — the maths behind them is different.

For more on why the two circuit types behave so differently, read Ring vs Radial Circuit Testing: Why the Methods Differ.

Installation Reference Methods Explained

The installation method decides how well the cable can shed heat — and therefore how much current it can carry. The OSG lists the methods and their current ratings (this data comes straight from Appendix 4 of BS 7671).

Look at a single 2.5mm² twin and earth cable across two methods:

Reference Method	Description	Current Rating
C	Clipped direct	27 A
103	Completely surrounded by thermal insulation	13.5 A

Same cable — but surrounding it in insulation halves its capacity. This is exactly why installation method matters as much as cable size. For a deeper look at reading these tables, see Installation Reference Methods and Cable Sizing.

Memory Aids for Every Method

The reference method letters and numbers are hard to keep straight under exam pressure. Here’s a set of visual prompts that stick:

Method	Memory Aid	What It Is
A	A for architrave	Cable in a gap behind an architrave, or conduit in a thermally insulated wall
B	B for box	Cable in conduit or trunking (a round or square “box”) clipped to a surface
C	C for clipped direct	Clipped direct, chased just below plaster, or on non-perforated cable tray
D	D for dug in	Armoured cable or conduit buried in the ground
E / F	Edge, end or flat	Cables on a perforated cable tray (or in concrete trenches)

The 100-series methods are specific to flat twin and earth cable (the Regs call it “70°C thermoplastic insulated and sheathed flat cable”) — the ones you meet most in domestic work:

Method	Memory Aid	Meaning
100	Insulation up to 100 mm	Cable in or above thermal insulation ≤ 100mm thick, touching joists/plasterboard
101	101 is more than 100	Insulation over 100mm thick
102	Two plasterboard surfaces	Cable between two plasterboard faces — a stud wall
103	At least three sides covered	Surrounded by insulation on three or more sides — worst case for heat

Exam tip: Method 103 gives the lowest current rating because the cable can’t shed heat. Whenever a question describes a cable “surrounded by insulation” or “in a thermally insulated wall for its whole length”, reach for the 103 column.

Common Mistakes with the OSG

The pitfalls that cost marks — and cause problems on site:

Mistake	Why It’s a Problem
Confusing OSG Part 7 with BS 7671 Part 7	You end up in special locations instead of the standard circuit tables
Applying the 80% rule to a tabulated measured Zs	The correction is already built in — you’d double-count it
Assuming a radial matches the ring length	Radials with a terminal load are far shorter for the same device
Ignoring the installation method	Insulation can halve a cable’s rating — 27A down to 13.5A
Using the tables outside their assumptions	Above 30°C ambient, or non-standard earthing, the results don’t apply
Forgetting diversity	The assumed maximum load (26A) is not the device rating (32A)

Bottom line: The On-Site Guide is a shortcut, not a substitute for understanding. It works brilliantly when your installation matches the standard assumptions — and misleads you badly when it doesn’t. Always confirm the conditions before trusting the number.

Practice and Further Study

Standard final circuits, cable selection and installation methods are core Part 5 territory — and they appear on the 18th Edition exam every sitting. Test yourself across the topics:

Our app includes 690+ practice questions covering all 8 parts with detailed explanations referencing specific regulation numbers, plus full mock tests with the same weighted question distribution as the real exam — so you can practise reading tables under time pressure exactly as you’ll need to on the day.

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