//Juan Luis Hernández (he multiplicado por 1.15 para tener en cuenta las pérdidas
function compute(form){

if (form.transformer.options[form.transformer.selectedIndex].text=="Seleccionar:"){
        return;
}
if (form.voltage.value==null||form.voltage.value.length==0){
       form.voltage.value=0;
}
if (form.current.value==null||form.current.value.length==0){
        form.current.value=0;
}
form.trans_pick.value=form.transformer.options[form.transformer.selectedIndex].text;
form.volt_pick.value=form.voltage.value;
form.current_pick.value=form.current.value;
with(Math) {
if (form.trans_pick.value=="Monofásico") {
  form.kva.value= round((form.current.value*form.voltage.value*1.15)/1000);
}
else if (form.trans_pick.value=="Trifásico")
{
  form.kva.value= round((form.current.value*form.voltage.value*1.732*1.15)/1000);
}
}
return;
}
function clear(form){
// set it all to null
form.transformer.value = "";
form.voltage.value = "";
form.current.value = "";
}

<!-- done hiding from old browsers -->