I have a manufactured home 2006 model built by Norris homes. I have to install the pahse coupler in the external box because my internal breaker box is full.
My Breaker Box is full, but I resolved this problem like this:
I moved all the circuit breakers which required to receive X10 signals on the same phase
So, Washing Machine, MicroWave, Heating system, Soulder Machine, Saw, Compressor, Exterior outlet, Frezzing and many other can't receive X10 signal, but all other can ... No needing to use Phase Coupler !
Note that you can use a 120Volt Double Breaker (available to 15A + 15A only) for increase your Breaker Box capacity !
My Breaker Box is made by SquareD ! Maybe that your brand manufacturing manufacture these Double Breaker 120V 15A.
Note that I have 3 Breakers like this, and install them on a Phase side that X10 is necessary !
So These 3 Double Breakers gave me 6 breakers 15A !
So you could remove 2 singles 120V 15A, and install 1 Double Breaker and 1 slot would be ready for receive another breaker 120V
or
you could remove 4 singles 120V 15A, and install 2 Doubles Breakers and 2 slot would be ready for receive another breaker 240V
Like this photo, note that a
Breaker 38-40 and 26-28 are 240 Volts (20A and 40A)
Breaker 36 and 32 are a single Breaker 120 Volts 15A
Breaker 34 and 30 are a double Breaker 120 Volts 15A