I am clueless as far as AMHA rules go but here is the excert from AMHR jumping heights
Jumps: There will be a minimum of four fences
and a maximum of six fences. The fences can
range in height from twelve to twenty-four inches
(in the jump-off this will change to a minimum of
twelve to a maximum of thirty inches). Jumps
should be of attractive design but constructed of a
material so as not to cause danger to the horse. All
jumps must be at least five feet in width, no wider
than six feet, with a minimum of twenty feet
between jumps, with the exceptions of an in and
out. All jumps must have a ground pole. Jump
standards may not be higher than 40”. No wings or
additions are allowed outside the jump standards.
All jumps in hunter and jumper courses must be
numbered on the course in order of sequence.
This is the link to the AMHR rulebook on all classes and registry rules
Page 267 has the jumping classes started on it.
I found the AMHA rulebook, here is their jumper rules for height
Jumps for Hunters and Jumpers will be made of 1-1 1/2”
schedule 40 PVC piping or other suitable lightweight
material with jump cups, not to include pegs, nails, bolts,
etc. All jumps must be a minimum of five (5) feet wide,
with a minimum of eighteen (18 feet between jumps,
with the exception of an in and out. In and outs should
be set approximately twelve (12) feet apart. Jumps thirty-
two (32”) or more in height must have a second rail
added. All post and rail jumps must have a ground pole.
Uprights to be a maximum of forty eight (48 inches in
height. No jumps, including jump-offs to exceed 44”. Any
decorations or jump wings must not protrude more than
twenty four (24) inches from the uprights.
23-02, effective 01-03) (Amended 02-06, effective 01-07)