Ceramic resistors. Translation:
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I won’t have mains power to my enclosure but I’m thinking my winter low temps will be -5/-10 F. Summer temps will be a different story. I’ll likely put a two screened, filtered ports on the bottom to draw in/exhaust out fresh air. Perhaps an aluminum white sun shield for the summer months. Sadly, that’s still 5 months away (snow storm today in New York).
Ceramic resistors.
20 words
The 24V 2.5A power supply and POE is used in this example.
КРН150 ceramic heating element
The connection picture is not correct. The elements in the middle need to be:
- Spaced apart, not touching each other, so the heat can be dissipated
- EACH resistor element connection needs to have TWO nuts, one on each side of the connection plate, to make good electrical contact.
In the picture, the middle ones don’t have a good electrical contact (the long bolt is barely touching the inside of the hole).
Of course in the real design should be metal washers.
Power 150 W. Allowable voltages in the range of 6-220 volts (32 volts max. per plate). R = 6.4 ± 1.1 Ohm
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No long rods going through all holes either 
Otherwise it is not going to be in series.
Paralel connection works too… if you have the right voltage and ampacity capability.