UL508A Table 36.1

[LMAO]

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]

all this does is allow you to use the duty cycle of the resistor to size the conductors.

 

[LMAO]

all this does is allow you to use the duty cycle of the resistor to size the conductors.

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

 

[petersonra]

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]

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]

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.