Help with Multiple Level Regression

I need help with a Regression Problem I have. I have two independent variables (Amperage and Diameter) and one dependent variable (Temperature). I would like to be able to have one equation that predicts the dependent variable. None of the relations ships are linear, but they are some what exponential. I have limited amount of data collected, but it should yield enough to make a prediction. I have attached both a graph showing the results and the data. The results do not have to be to a certain confidence level, as I am only looking for a ball park equation. I am looking for advice on using excels regression tool. Ex. I would like to predict the temperature of a .175 diameter at 30 amperage with the equation.