Technical information

The conductor to the secondary should carry at least 5 x 10 = 50 A.Table

29.1 of UL508A is used to select a AWG 8 (60 A), considerably greater than the

AWG 14 (20 A) that would have been sufficient considering the single current

to the secondary.

•pr

imary and secondary: the protection should be graded to values of no

more than

[a]

if the rating does not correspond with a standard value (see the publication

prior to section “BCP”) the one immediately above the one calculated is

permitted.

Standard UL508A limits itself to increasing the percentage for currents to

the primary below 2 A by imposing:

•Current transformers: 500%

•Power transformers: 300%

The protection of the secondary can be obtained with a special component

or with the sum of the ratings of the protections down line.

In the case of single phase control transformers with one side of the

secondary connected to the equipotent circuit, it is sufficient a single

protection positioned on the side not connected (exactly as in Europe).

The conductors to the primary and the secondary should have a capacity of

no less than the rating of the relative protection.

EXAMPLE

The same single phase transformer of 450 VA as the previous example is

considered.

primaryV

= 240 VI

= 1,88 A

secondaryV

= 24 VI

= 18,8 A

Let’s say it is protected on the primary and on the secondary.

CONTROL CIRCUITS

PrimarySecondary

Current Rating protection Current Rating protection

I1 [A] [percentage of I1] I2 [A] [percentage of I1]

≥ 9250 % ≥ 9125 %

[a]

< 9250 %<9167 %

< 2250 %