Wind in the wire: Abergwyngregyn

It was a windy day when we walked under the wires and there was a steady roar that I worked out was the sound of the wind in the wires. I'll estimate 100Hz. Let's assume that it has set up the first harmonic of a stationary wave. The equation for calculating the frequency is:

Where L is the length of the wire (1000m as measured on the map), T is the tension of the wire and mu is the mass per metre. The wire is aluminium with steel core. Let's round up the density of aluminium to 3000 kg per cubic metre and estimate circular cross-section of 4cm diameter. Cross-sectional area = 0.001 square metres and mass per unit length of 3kg. That would give a tension of 120 billion Newtons. Breaking stress for the steel is 7.9 x 10^8 Newtons per square metre. This figure would exceed that so some of my estimates or assumptions must be wrong.