def _t_hs(h, s):
    """Define the boundary between Region 2 and 3, T=f(h,s)

    Parameters
    ----------
    h : float
        Specific enthalpy [kJ/kg]
    s : float
        Specific entropy [kJ/kgK]

    Returns
    -------
    T : float
        Temperature [K]

    Raises
    ------
    NotImplementedError : If input isn't in limit
        * 5.048096828 ≤ s ≤ 5.260578707
        * 2.563592004e3 ≤ h ≤ 2.812942061e3

    References
    ----------
    IAPWS, Revised Supplementary Release on Backward Equations p(h,s) for
    Region 3, Equations as a Function of h and s for the Region Boundaries, and
    an Equation Tsat(h,s) for Region 4 of the IAPWS Industrial Formulation 1997
    for the Thermodynamic Properties of Water and Steam,
    http://www.iapws.org/relguide/Supp-phs3-2014.pdf. Eq 8

    Examples
    --------
    >>> _t_hs(2600, 5.1)
    713.5259364
    >>> _t_hs(2800, 5.2)
    817.6202120
    """
    # Check input parameters
    if s < 5.048096828 or s > 5.260578707 or \
            h < 2.563592004e3 or h > 2.812942061e3:
        raise NotImplementedError("Incoming out of bound")

    nu = h/3000
    sigma = s/5.3
    I = [-12, -10, -8, -4, -3, -2, -2, -2, -2, 0, 1, 1, 1, 3, 3, 5, 6, 6, 8, 8,
         8, 12, 12, 14, 14]
    J = [10, 8, 3, 4, 3, -6, 2, 3, 4, 0, -3, -2, 10, -2, -1, -5, -6, -3, -8,
         -2, -1, -12, -1, -12, 1]
    n = [0.629096260829810e-3, -0.823453502583165e-3, 0.515446951519474e-7,
         -0.117565945784945e1, 0.348519684726192e1, -0.507837382408313e-11,
         -0.284637670005479e1, -0.236092263939673e1, 0.601492324973779e1,
         0.148039650824546e1, 0.360075182221907e-3, -0.126700045009952e-1,
         -0.122184332521413e7, 0.149276502463272, 0.698733471798484,
         -0.252207040114321e-1, 0.147151930985213e-1, -0.108618917681849e1,
         -0.936875039816322e-3, 0.819877897570217e2, -0.182041861521835e3,
         0.261907376402688e-5, -0.291626417025961e5, 0.140660774926165e-4,
         0.783237062349385e7]

    suma = 0
    for i, j, ni in zip(I, J, n):
        suma += ni * (nu-0.727)**i * (sigma-0.864)**j
    return 900*suma