Every time we take a shower the hot water heater trips a relay switch and we run out of hot water. We can reset it but this is annoying, can someone tell me why this is happening? Thank you

The first,(and the easiest) thing I would replace is the breaker. Replace with 30 amp.
If it still gives problems the next thing I would do is replace the elements . Either the breaker's at fault or the elements are drawing too many amps.
Which reminded me of a call exactly like yours back in the 80's. Let me explain,
Fairly new condo. Heater still in warranty. The complaint was that it was blowing breakers.
Working just great when I got there. Everything checked out band I spent the better part of a hour trying to figure out what the problem was. I gave up and on my way out to my truck they called me back. Itt finely tripped a breaker. I'm no electrician but I carry a volt/amp meter just for water heaters.
The elements in a heater are opposing elements. Never both on at the same time.
They pull 19 amps. By themselves and the thermostat would momentarily call for both to be on atv the same time putting a 36 amp. Load on a 30 amp breaker. BINGO!
I swapped out the stats and left a happy family.
This only happened once in all my years out in the field but your complaint was so much like theirs I thought I'd mention it. Good luck, Tom

You didn't say if it was gas or electric. I think most would assume it is electric, but I'm not sure as you say "relay" not breaker or fuse.

If it is electric then the 30 amp circuit breaker is correct.

If its gas and has a power vent, replace with the same size breaker.

In geneal, there are two elements in a hot water heater and in most cases they are not to be on at the same time.

One is going to be used for standard heating. Cold water fills at the bottom of the tank and the bottomis going to be heated first. As you draw from the top the top heater will kick in and the bottom will kick out

So, there are to probablilities:

1. The upper element is bad
2. If the heater is new, it could be wired such that both can be on at the same time.
3. A thermostat is bad