I think you have a dead short somehow. You need that meter.
Have you checked for Leprechauns under the stairs. Those little bastards will wreak havoc on new homeowners
It sure seems like this is an input issue. Either the dryer outlet has an issue, although the voltage readings appear fine, or your input cable has a problem.
An additional voltage measurement that may be useful is to measure between neutral on the dryer outlet and the ground on a nearby 120VAC receptacle.I have a meter, but don't really know what to do with it! I did measure the voltage in the dryer outlet:
h1 to neutral: 123v
h2 to neutral: 123v
h1 to h2: 248v
....
Perfect. That sounds right to me.
Now set you meter for continuity. It should either peg the needle or beep when you touch the meter probes together.
Turn the spa breaker off and test for continuity between all three stabs on the plug that goes to the dryer outlet. Turn the spa breaker on and test again. You should not have continuity between any stab either way if all loads are disconnected from the gfci breaker.
If that checks out, test continuity from h1 on the plug to red or black in the spa panel, h2 on the plug to red or black in the spa panel and neutral on the plug to the neutral bar in the spa panel.
The two photos above are from the two ends of the SAME input cable? Any reason you are using a 4 prong plug/receptacle (NEMA 14-20) at the input to the spa panel? Other than maybe that's what you had on hand. How is the ground(green) wire isolated/connected in the 3wire end?
Depending on how the spa input plug is wired and if that green wire is shorted to one of the hot conductors, it would result in the exact situation you are experiencing......Same input cable.
I used a 4 prong input for 'future-proofing' should I have access to a 4 prong outlet in the future. Also, followed the accepting wiring diagrams here on HBT.
The ground in the 3wire end is visible in the photo - it's wrapped in black electrical tape.
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