The BT47 index is a statistical indicator published by INSEE, associated with the series “BT47 – Electricity – Base 2010”. It measures the monthly evolution of costs related to electrical installation work in buildings: wages, materials, energy. Its main role is to serve as a reference for adjusting prices in work contracts, whether they fall under public or private markets.
Composition and Calculation Mechanism of the BT47 Index
The BT47 aggregates several cost items specific to the building electrician profession. It includes labor (wages and charges), electrical materials (cables, equipment, panels), energy, and transport. Each item is weighted according to its actual share in the overall cost of an electrical installation project.
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This weighting is not fixed. The FFIE (Federation of Electrical Equipment Companies) has worked jointly with INSEE for eighteen months of exchanges and data collection to obtain an update of the calculation method. The goal: to ensure that the coefficients better reflect the reality of purchases and expenses borne by companies in the sector.
To learn everything about the BT47 index, it is important to remember that it is not an absolute price, but a number without a unit. Its value only makes sense when compared to itself at another date. It is this differential that allows for calculating a price variation between the signing of a contract and the execution of the work.
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A point often misunderstood: the value of the BT47 is published with a delay of two to three months. The value for January appears at the end of March or early April in the Official Journal. Using the wrong reference date in a contract can distort the entire calculation of the revision.
Price Revision with the BT47: The Formula and Its Pitfalls
The price revision formula that uses the BT47 follows a simple principle. It compares the index value at the time of work execution with its value at the market reference date (often referred to as “month zero”). The ratio between these two values gives a multiplier coefficient applied to the initial price.
In practice, if the index has increased between the signing and execution, the billed price increases in the same proportion. If the index has decreased, the price decreases. The mechanism protects both parties, not just the company.
What Distinguishes Update and Revision
The update applies before the start of the work, when there is a delay between the submission of the estimate and the actual start of the project. The revision, on the other hand, occurs during execution, as work situations progress. Both use the BT47, but not at the same time in the contract.
Confusing the two leads to billing errors. In public contracts, revision is mandatory for contracts exceeding a certain duration, in accordance with the public procurement code. In private contracts, it only exists if explicitly provided for in the contract.
Legally Securing a BT47 Clause in Private Contracts
This is the point that most guides on the BT47 skim over. Reading the index and applying the formula does not pose major technical difficulties. The real area of risk lies in the drafting of the contractual clause itself.
In public contracts, the public procurement code regulates obligations. In private contracts, no regulation imposes a price variation clause. If the contract does not explicitly mention the BT47 as a reference index, along with the original date and the revision periodicity, the clause is difficult to enforce in case of dispute.
Elements to Lock in the Contract
For a revision clause based on the BT47 to hold legally, several elements must be clearly stated:
- The exact reference index (BT47, not “a building index”) and its base (base 2010), with the official INSEE source.
- The original date of the index, known as “month zero,” which serves as a comparison point for any subsequent calculations.
- The periodicity of application: monthly, quarterly, or per work situation, depending on the progress of the project.
- The potential triggering threshold: some clauses specify that the revision only applies beyond a minimum variation of the index.
Omitting any of these elements leaves the door open to contestation. The FFB’s memo on price variation clauses emphasizes this point: the precise drafting of the clause is as crucial as the choice of the index itself.
BT47 Compared to Other Building Indices: Which One to Choose
The BT47 is not the only BT index published by INSEE. The complete series covers dozens of trades. The BT01, all trades index, is the most well-known and most commonly used by default. It reflects a weighted average of all building trades.
Using the BT01 for an exclusively electrical market smooths out the specific variations in the sector. If the price of copper or electrical equipment rises sharply while other materials remain stable, the BT01 will only capture a fraction of that increase. The BT47, on the other hand, reflects it directly.
The choice of index therefore depends on the nature of the work. For an electrical lot in a global market, the BT47 is the most relevant for that specific lot. For a mixed market without breakdown by lot, the BT01 remains a coherent option, but less protective for the electrician.
When the BT47 Is Not Enough
Some contracts combine several BT indices, one for each technical lot. This approach, more cumbersome to manage administratively, offers a finer protection against sectoral variations. It assumes that each lot has its own revision clause, with its own index and its own original date.
In a context where the costs of electrical materials experience sometimes drastic variations (supply tensions, fluctuations in metal prices), the BT47 remains the most suitable tool for electrical installation professionals. The precision of the accompanying contractual clause makes all the difference between a real protection mechanism and a superficial mention without legal effect.