Connect this to one servo port on your receiver (with Y-cable if there are no free connectors - battery connector is fine).
Move all your sticks simultaneously or better apply some weight on the moving surfaces (rudders). If you can se red indicators then there is a problem.
Just let the magic smoke come out of my 20a esc on a plane with 5g servos and 2 3.7g servos last week. Normally I would trust that but they were under a lot of load and there was no airflow over the esc. I was testing control surfaces on the ground when it happened
It sucks when it happens to you and you don't expect it.
I am a little curious, how many of your planes have more than 4 servos bigger than 10 gram?
Bigger planes normally have bigger esc with more amps to the bec.
I have one plane with 7 servos - 70A (opto) esc and separate bec but also two planes with 6 servos (one with digital servos) flying fine with the built in bec. The separate bec on the plane with opto esc does not give any more amps than a 70A esc with built in bec woud have done.
The first question does not say what a "pretty large" is - is it a plane of 3,5 - 5 kg / 7-10 lbs then most flyers would say it is normal size. Canards however might indicate a "jet plane" with speed of 200 km/h / 125 mph or more and then even a smaller plane needs more power to the servos and probably separated power to the receiver from the power to the servos.
I am obviously into much smaller planes than our friend ananas ;-)
My ubecs are only 3 and 5 amps.
Not long ago people were competing in F3A class with fast .60-engine pattern planes and used small NiCd batteries like in the picture. 600 mAh would last more than one 10 minute flight. The new 2000 mAh NiMh batteries last "forever" before any need of charging. We are naturally talking different when coming up to big scale models with engines of 100 cc and up.
I had to check the amp draw from a few of my servos:
20 gram servo – stalled 0,5A
50 gram digital servo 1,0 A
I guess that the more normal 10 gram servos used many of in the modern planes will stall at about 0,25 A.
The “normal” draw from the servos are obviously a lot less than the stalled sevo.
I am 16 and have been flying for 1,5 years now. I have not had a single flight with something over 1,5m and more then 100mph but this is just my feel for the amperage the servos draw and as I like to rather be on the safe side with for example ESC´s for motors and rather have 10A too much then 1A to less (and fry that ESC) I got to the conclusion that I WOULD take a little bigger UBEC´s the more valuable the planes get, which they tend to be the bigger they get
I would compare your plane with a 1/5 scale CUB in size, weight and speed.
Your key cameras will not add any weight to notice. The GoPro are 150-200 grams each depending on accessories - a total of approx 1 kg (2 lbs). I think 4 std servos are more than adequate for flight. Add smaller tilt servos to the gopros?
The plane will run fine with the built in bec in the esc if you choose a better esc with 4A bec. Your servos will not draw more than 250 mA each on normal flying, probably a lot less in average.
If assume that you or a helper are controlling the cameras tilt functions with a separate radio and together with FPV?
Most of the guys I know running 50cc planes and larger are using these Smart Fly power systems. They basically run all your connections from your receiver into this and then your servos into it and it powers everything with a separate battery. I honestly don't know much about it but they all swear by it.
What type of radio system do you have? How many channels do you have available? You may be able to do this just fine if you have enough channels. I bought an 8 channel receiver so I wouldn't have to use Y-harnesses for everything. I can run most servos on their own channel and mix them together in the radio. This cuts down on the draw per circuit. I believe this eliminates most of the problem unless maybe you are using high draw servos. Of course, until you put it together and check your voltage and amp draw you won't really know. Things like retracts and flaps aren't going to be drawing all the time though so you can probably get away with a bit more. Things like multiple ailerons and multiple elevator servos are the big concern.
I use spektrum products and they have what they call powersafe recievers where you plug one or more batteries into the receiver for high draw applications. I'm sure whichever system you use may have something similar.