Various ways exist to represent a design fire curve for tunnels. These can include different fire growth rates or combinations of fire growth rates with constant levels of Heat Release Rate (HRR) coupled to a decay period. This means that the curve has to be represented with different mathematical expressions for different time periods. A more convenient way is to describe the design fire curve with a single mathematical expression. Such a curve has been presented by the author [1] but it does not include a constant HRR period. This paper presents a new, single exponential, design fire curve with a constant maximum HRR. A presentation of available design curves is given as well.