Interlocking as to not overload a 75kva

[Electric-Light]

I am not sure of the code interpretation of the device when the whole thing is not UL listed in plug-and-play. Double throw relays are inherently impossible for both sides to turn on at the same time. Water heater elements wiring is a good example. The top element has priority.
Power goes into common, and upper element goes to NO. Lower element goes to NC. When the top thermostat activates, the common flips and breaks NC, and makes NO. Lower element thermostat is a single throw NO.

A setup that use two NO switches in parallel can be set up so it's not SUPPOSED to but not IMPOSSIBLE for both to remain on.

Line/ ladder (?) Diagram above.. I'm not too good with making drawings but I like to work it out this way to make sure things'll work out on site.
The 4 timeclocks control 2 rooms. Only 1 (same) room can be on at a time. Being inductive loads, one room turns on half, 10 minutes before the other half. When that happens it's not possible for the other room to be on, thanks to the auxiliary relays.
Hours later the first half turns off, then 10 minutes later the second half of the room (turns off). Then after about ten minutes, half the second room turns on then the other half another ten minutes later.
If for whatever reason the first room was to turn on; the second would turn off automatically (overide). The second room however can not overide the first but the interlocking /physically not possible part is still achieved.

This is unnecessary. Who told you to do that? A DPDT relay would will work as an A/B selector switch. There is no need to add a 10 minute delay unless the off period is needed for the process, but it adds complexity to the control.

 

[kwired]

I am not sure of the code interpretation of the device when the whole thing is not UL listed in plug-and-play. Double throw relays are inherently impossible for both sides to turn on at the same time. Water heater elements wiring is a good example. The top element has priority.
Power goes into common, and upper element goes to NO. Lower element goes to NC. When the top thermostat activates, the common flips and breaks NC, and makes NO. Lower element thermostat is a single throw NO.

If at room temp, the NC contact is what the top element is connected to. It switches off and connects power to the lower element on the NO contact when set point is achieved, but you knew that

 

[ronaldrc]

If at room temp, the NC contact is what the top element is connected to. It switches off and connects power to the lower element on the NO contact when set point is achieved, but you knew that

OF COURSE ALL ELECTRICIANS KNOW THAT

 

[kwired]

OF COURSE ALL ELECTRICIANS KNOW THAT

I'd guess Electric light does as well, but I feel he had NO/NC turned around for this application. NO on any switching device is the state it is in when sitting there at rest, at normal room temperature or other conditions where there is no external conditions influencing the state of the contact.

You may have a normally open held closed in some applications where a normally open switch more or less reverses it's "normal state" when installed, but is usually designated "held closed" or "held open" in those situations.

 

[Electric-Light]

I'd guess Electric light does as well, but I feel he had NO/NC turned around for this application. NO on any switching device is the state it is in when sitting there at rest, at normal room temperature or other conditions where there is no external conditions influencing the state of the contact.

You may have a normally open held closed in some applications where a normally open switch more or less reverses it's "normal state" when installed, but is usually designated "held closed" or "held open" in those situations.

I guess I did.
Let me rewrite it. A common water heater has two heating elements of the same wattage and two thermostats. The upper element doesn't switch on unless the water exiting the water heater gets low enough. When it is filled with cold water, L1 goes to C. upper element goes NC and L2.
NO goes to L1 input for lower thermostat/lower element. Lower element has its own thermostat which is a simple NC and goes to L2.

What is important is that one arm is shared for completing the circuit and the law of physics dictate it can only be in place at a time and positively enforces break-before-make. A dual relay configuration gives more flexibility and allows you to do A or B, A+B or all off but there is no positive lockout against A+B activation.