do you have 220 service in the house?
if you don't have 220 measurable at the main panel you won't get it in
the shed either.
for 220 you need two 110v hots out of phase, not just any 2 hots from a
110v main panel.
please read faq at:
http://www.faqs.org/faqs/electrical-wiring/part1/

wrote:
Hi,
I have recently purchased a new swimming pool heater, installed it next
to the shed in the backyard so naturally had to get electricity to it
from the main panel in the house.
I have a stab-lok panel in the house so I got a 50 amp NC0250cp bracer
that I installed in the house panel then connected the black wire and
the Red wire to each pole of the breaker the white wire to the neutral
of the panes and the Ground to the Ground I am using outdoor 6AWG wire.
I got the wire outside underground and to the shed where I installed a
new panel wired it up by connecting each Black and Reb to each main
connector of the new panel the white to the neutral bar and the ground
to the Ground bar I put in a 15amp single pole breaker for the pool
pump and a 30amp breaker for the pool heater.

with my voltmeter I cam measure 120vac when I go across neutral and
each Black and Red but when I measure across Black and Red I don't get
240vac I find that strange.

when 50amp breaker is on in the house and the 15amp breaker is on in
the shed the pomp works well but I would like to see if the 240 VAC is
ok before I connect up the pool heater.


Can you please help