UL 508A Question

[supraman21]

I'm currently taking the UL 508A MTR test and its giving me a headache. I passed the first two sections of the test but I failed the third pretty hard.
One of the questions asks what is the maximum fuse size for a 750 VA control transformer with a 480 volt primary and 120 volt secondary?
Here's my math:
750/480 = 1.5625 X 5 (500%) = 7.8125 Primary
750/120 = 6.25 X 1.67 (167%) = 10.4375 Secondary
Primary is 8 because you round up because its over .5 and secondary is 10 because you have to round down because its under .5.
The answer choices are:
P S
5A 10A
7A 10A
8A 15A
10A 15A
None of the choices line up with my answers. What gives?

 

[Forqnc]

You have to use table 35.2, in the Standards, to adjust the sizing

 

[supraman21]

But it says (Hint: Refer to Table 42.2)...

42.2 is sizing for primary and secondary overcurrent protection for a control transformer. The question is about a control transformer with both a primary and secondary. Why would I use 35.2?

 

[petersonra]

I'm currently taking the UL 508A MTR test and its giving me a headache. I passed the first two sections of the test but I failed the third pretty hard.
One of the questions asks what is the maximum fuse size for a 750 VA control transformer with a 480 volt primary and 120 volt secondary?
Here's my math:
750/480 = 1.5625 X 5 (500%) = 7.8125 Primary
750/120 = 6.25 X 1.67 (167%) = 10.4375 Secondary
Primary is 8 because you round up because its over .5 and secondary is 10 because you have to round down because its under .5.
The answer choices are:
P S
5A 10A
7A 10A
8A 15A
10A 15A
None of the choices line up with my answers. What gives?

7A and 10A.

It does not say you can round up in T42.2.

 

[Forqnc]

But it says (Hint: Refer to Table 42.2)...

42.2 is sizing for primary and secondary overcurrent protection for a control transformer. The question is about a control transformer with both a primary and secondary. Why would I use 35.2?

My apologize 35.2 is for power transformer, I overlooked the question is for a control transformer.
In table 42.2 look at the "a" supplement for secondary current over 9A and rounding up.
So answer 8A and 15A

 

[petersonra]

My apologize 35.2 is for power transformer, I overlooked the question is for a control transformer.
In table 42.2 look at the "a" supplement for secondary current over 9A and rounding up.
So answer 8A and 15A

View attachment 2559592

Secondary current is not over 9 amps in this case.

 

[Forqnc]

Secondary current is not over 9 amps in this case.

I thought it is based on the 750/120 = 6.25 X 1.67 (167%) = 10.4375 Secondary calculated.
Correct me if I am wrong

 

[petersonra]

I thought it is based on the 750/120 = 6.25 X 1.67 (167%) = 10.4375 Secondary calculated.
Correct me if I am wrong

That is not the secondary current. It is that max size of the secondary ocpd.