UL 508A conductor size calculation

[David Charles]

I have completed the exam for UL but I did not understand this question. Any help would be great

A 1000 VA control transformer has a 480 volt primary with a 3 amp fuse and 24 volt secondary with no protection. using the 60 C column of table 28.1, what is the minimum conductor size on the secondary of the control transformer

Now in 42.1.2.2 it says "
The secondary conductors of a control transformer having over current protection located on the
primary side only, as described in 42.1.2.1, and with a two-wire single voltage secondary shall be sized
with an ampacity in accordance with Table 28.1 or Table 38.1, that is not less than the rating of the
primary side protective device multiplied by the primary to secondary transformation ratio."

so we have the primary side protective device which is the 3 amp fuse but i don't know what the primary to secondary transformation ratio is that they say to multiply the primary device by.

 

[Rock86]

Primary to secondary ratio is your primary voltage divided by your secondary voltage. In your case, 480V / 24V = 20 times.

 

[David Charles]

Primary to secondary ratio is your primary voltage divided by your secondary voltage. In your case, 480V / 24V = 20 times.

Thank you

so that would mean that 20*3= 60 so it cant be less than 60 so using table 28.1 you would want to use a minimum of 4 AWG conductor size on the secondary side correct?

 

[Rock86]

Using 60 degree column, I would agree with you, #4AWG