UL508A Table 36.1

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LMAO

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Texas
I am a little confused by the last sentence in UL508A section 36.2.4. Paragraph says that conductors connected to resistors that do not run at 100% should be sized based on motor FLA and the the percentage in Table 36.1 fourth column... fair enough. But then it says circuits with different off/on times should be using the "percent ON time".

So for example, if a resistor is running at 5s on 75s, off the conductors ampacity should be at least 35% of motor FLA (OK, I get that part), but if the off/on time is different the conductors should 6.25% of ON time....???? Makes no sense...
 

petersonra

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Northern illinois
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engineer
So I can pretty much ignore the table and size the conductor based on resistor "average" current?

no. you have to use the on time percentages in the table if the on/off times are not in the table.

for instance, 5 seconds on, 15 seconds off is not in the table. it is however 25% on time (refer to 3rd column), so you would have to use an ampacity of 65 percent as found in the 4th column for 25% on time.
 

LMAO

Senior Member
Location
Texas
no. you have to use the on time percentages in the table if the on/off times are not in the table.

for instance, 5 seconds on, 15 seconds off is not in the table. it is however 25% on time (refer to 3rd column), so you would have to use an ampacity of 65 percent as found in the 4th column for 25% on time.

Oh,OK. But that makes the first two columns of the table pretty much useless, doesn't it? Numbers in the third column is the duty cycle corresponding to the first 2 columns... What is the point of the first 2 columns?!
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Oh,OK. But that makes the first two columns of the table pretty much useless, doesn't it? Numbers in the third column is the duty cycle corresponding to the first 2 columns... What is the point of the first 2 columns?!

good question to ask UL some time.

there are a lot of things in UL508a that are curious.
 
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