Transformer Overcurrent Protection, 1000 V and Less
Table 450.3(B) percentages from NEC 2017 [1]; standard ratings from Table 240.6(A), NEC 2023 [2]; nominal voltages from ANSI C84.1 [3]. For winding currents and core size of a small transformer, see the Transformer Sizing calculator.
Maximum Overcurrent Protection — Table 450.3(B)
| Protection method | Primary protection, rated primary current | Secondary protection (Note 2), rated secondary current | |||
|---|---|---|---|---|---|
| 9 A or more | Less than 9 A | Less than 2 A | 9 A or more | Less than 9 A | |
Maximum rating or setting as a percentage of the transformer’s rated current, transformers of 1000 V and less, NEC 2017 [1].
Notes to Table 450.3(B), in Short
| Note | Applies to | Rule (paraphrased) |
|---|---|---|
| 1 | The 125 % entries | Where 125 % of rated current is not a standard fuse or nonadjustable breaker rating, the next higher standard rating is permitted. Other entries may not be exceeded. |
| 2 | Secondary protection | Up to six breakers or six sets of fuses grouped in one location; their ratings added together may not exceed what one device would be allowed. |
| 3 | Primary 250 % entries | With the manufacturer’s coordinated thermal overload protection, arranged to interrupt the primary current, the primary device may be up to 6 × rated current (impedance up to 6 %) or 4 × (impedance over 6 % to 10 %). |
Paraphrased from NEC 2017 [1]; read the code text before applying.
Standard Ampere Ratings — Table 240.6(A)
| Fuses and inverse-time circuit breakers (A) |
|---|
NEC 2023 [2], which added the 10 A rating; the picker’s standard ratings come from this list.