# How does the European Union (EU) Carbon Border Adjustment Mechanism (CBAM) overvoltage method for aluminium smelter gases work, and what is the duct collection correction?

Canonical: https://senseible.earth/answers/eu-cbam-aluminium-overvoltage-method-and-duct-collection

The overvoltage method gives carbon tetrafluoride (CF4) as a coefficient times anode effect overvoltage divided by current efficiency, times tonnes of aluminium, times 0.001. The guide prints coefficients for centre worked and side worked prebake only. Gas measured in the duct is divided by the duct's collection efficiency to include what escapes.

## Two methods, same job

A smelter pot sometimes has a short upset, the anode effect, and gives off two strong warming gases, CF4 and hexafluoroethane (C2F6), together the perfluorocarbons (PFCs). CBAM is the EU's charge on the carbon built into imported goods. The Commission's aluminium guide gives two ways to count them. The slope method counts the minutes of upset. The overvoltage method counts how far the voltage rose above its target, like counting how hard the engine was revving instead of how long it ran. The guide calls both methods equivalent and says to pick the one that suits your process control equipment.

## The overvoltage equations

- CF4 in tonnes = coefficient x (overvoltage / current efficiency) x tonnes of primary aluminium x 0.001.
- The coefficient is in kilograms of CF4 per tonne of aluminium per millivolt (mV) of overvoltage. The guide prints 1.16 for centre worked prebake and 3.65 for side worked prebake.
- Overvoltage (AEO) is worked out per cell in mV: the area of time multiplied by voltage above the target voltage, divided by the length of the data collection.
- Current efficiency (CE) is the average current efficiency of aluminium production, written in per cent.
- C2F6 in tonnes = CF4 tonnes x the C2F6 share. The guide prints 0.121 for centre worked prebake and 0.252 for side worked prebake in this table.

## Two tables, two prints of the share

The slope table in the same guide prints 0.280 as the side worked prebake C2F6 share, and the overvoltage table prints 0.252. Both are printed in the guide. Our reading: use the share that belongs to the method you report. The guide gives no rule, so ask your verifier which the monitoring plan names.

## The duct correction

Some gas escapes the roof of the pot room instead of going up the duct (our background explanation). The guide says fugitive PFC emissions are taken into account from the gas measurable in a duct or stack: total PFC emissions equal the duct emissions divided by the collection efficiency of the duct. The collection efficiency shall be measured when the installation-specific emission factors are determined.

The guide does not spell out how this correction fits with the two equations above for a smelter that has no duct measurements. Ask your verifier before you apply it.

## Minimum and better

The minimum is the technology factors in the Methodology Act. The recommended improvement is your own factors from field measurements at least every 3 years or after significant changes, such as a change in the spread of anode effect durations or in the control program that decides how an anode effect ends, with an uncertainty of at most plus or minus 15% each. The guide points to the International Aluminium Institute's most recent best practice guidelines.

## Overvoltage method on a centre worked prebake line, 100,000 tonnes (example)

- Overvoltage per cell: 10 mV. Current efficiency: 94%. Coefficient: 1.16
- CF4: 1.16 x (10 / 94) x 100,000 x 0.001 = 12.34 tonnes
- C2F6: 12.34 x 0.121 = 1.49 tonnes
- Carbon dioxide equivalent (CO2e): 81,817 + 16,574 = about 98,391 tonnes
- Duct example: 12.00 tonnes of CF4 measured in the duct, collection efficiency 0.98 (written as a fraction): 12.00 / 0.98 = 12.24 tonnes in total, so 0.24 tonnes escaped

Voltage, efficiency, output and duct readings are example numbers. The guide does not say whether to write collection efficiency as a fraction or per cent, so this example uses a fraction.

## What Senseible does here

Senseible reads fuel invoices and electricity bills into monthly records. Reading pot voltage or duct measurements is custom, on request. Senseible builds the verifier-ready file and checks every figure in-house; an accredited verifier Senseible partners with signs it off, or the client sends it to their own.

- [Build your aluminium CBAM file](https://senseible.earth/cbam-suite)
- [EU slope factors by pot type](https://senseible.earth/answers/eu-cbam-aluminium-slope-factors-by-pot-type)
- [Smelter PFC count for CBAM](https://senseible.earth/answers/richards-bay-smelter-pfc-emissions-cbam)
- [Verifier checks at an aluminium plant](https://senseible.earth/answers/what-cbam-verifier-checks-aluminium-plant-india)

Sources: [European Commission, CBAM Guidance No. 5e, aluminium (14 August 2026)](https://taxation-customs.ec.europa.eu/document/download/590e983b-c937-41ea-bb9f-810c29e70e1e_en?filename=Guidance%20No.%205e%20-%20Sector-specific%20guidance%20document%20on%20aluminium.pdf)

Last reviewed 2026-11-07. Check Tables 3-2 and 3-3 and Equation 20 of the guide against Annex II, section B.7 of Implementing Regulation (EU) 2025/2547 before you file. The guide says it is not legally binding.