UL508A Control Transformer Overcurrent Protection

[DanielRodriguez]

We had a UL inspection recently that caused some confusion regarding the 42.1.3 standard and table 42.2. The situation was as follows.

480VAC Primary, 120VAC secondary, 1KVA single phase control transformer.
Per table 42.2 and units of measurement standard 5.2 the calculations would be as follows.

Primary:
1000 / 480 = 2.083
2.083 * 2.5 = 5.2 (Round Down) = 5A primary CB

Secondary:
1000 / 120 = 8.33
8.33 * 1.67 = 13.91 (Round Up) = 14A Secondary CB

I understood these values to be the maximum sized Protection we could use as implied by the lack of note "a" on the calculations in the table and standard 42.1.3.2. However, The UL inspector said we should be using a 6A Primary CB and a 15A secondary CB? Am I misunderstanding something here, this doesn't seem to match up with the standard.

 

[petersonra]

We had a UL inspection recently that caused some confusion regarding the 42.1.3 standard and table 42.2. The situation was as follows.

480VAC Primary, 120VAC secondary, 1KVA single phase control transformer.
Per table 42.2 and units of measurement standard 5.2 the calculations would be as follows.

Primary:
1000 / 480 = 2.083
2.083 * 2.5 = 5.2 (Round Down) = 5A primary CB

Secondary:
1000 / 120 = 8.33
8.33 * 1.67 = 13.91 (Round Up) = 14A Secondary CB

I understood these values to be the maximum sized Protection we could use as implied by the lack of note "a" on the calculations in the table and standard 42.1.3.2. However, The UL inspector said we should be using a 6A Primary CB and a 15A secondary CB? Am I misunderstanding something here, this doesn't seem to match up with the standard.

I think you are both wrong.

Note (a) that allows you to go to the next size CB only applies to transformer secondaries with a rated current exceeding 9 Amps. You are right that it does not apply to primaries at all.

 

[DanielRodriguez]

I think you are both wrong.

Note (a) that allows you to go to the next size CB only applies to transformer secondaries with a rated current exceeding 9 Amps. You are right that it does not apply to primaries at all.

Since the rated amperes of the secondary are less than 9A you use the 1.67 multiplier (I think I understand this part), but then section 5.2 says that 13.91A should be rounded up to 14A. I know note "a" does not apply here but does that mean section 5.2 does not apply here as well? I cant find anything indicating it would not.

5.2 states: For calculations involving amperes, fractional values of less than .5 shall be rounded down and values greater-than or equal-to .5 shall be rounded up.