Primary ocp can be responsive to secondary short circuits but is not always responsive to secondary overcurrent caused by? PP294
an unbalanced condition
Transformer "charging current" for smaller transformers is what compared to the ampere rating of the transformer? PP298
Proportionally larger for the ampere rating of the transformer. the va is always larger than the IxE rating, therefore rising and collapsing AC field in VARs is and therefore VA rating is always larger. The field suffers power loss due to hysteresis. Where the magnetic domains rise and fall two times in a cycle.
The overcurrent device protecting the primary of a transformer may be located where? PP300
But must do what?
May be located at other than than the tranformer, but must....
protect the primary windings (T450.3B) and the conductors (240.4)
According to 240.21(C)1 of the 2017 NEC, the transformer secondary is not permitted to be protected by the primary ocpd unless it is one of the following transformer connections. PP310
- single-phase, 2 wire, single-voltage secondary
- Delta-Delta, 3 phase, 3 wire, single-voltage secondary
When primary protecton includes sencondary protection, what ratio must be used? Controls what? PP311 NEC 240.21C1
Transformer ratio (must not exceed the transformer secondary conductor ampacity by the secondary to primary transformer voltage ratio). and controls the size of the conductors used on the secondary of the transformer.
The basic rule for feeder taps on transformers (240.21) states that the OCPD is required to be connected where? PP313
Where the conductor recieves its supply
Tap rules allow what? PP313
Exceptions that allow the tap to not have an OCPD at the point where the conductors recieve thier supply
The length of a tap is from what to what? PP314
Secondary conductors are sometimes refered to as tap conductors? T/F PP 315 the
from the feeder to the OCPD, and not the transformer
- ...however in ECM magazine "Understanding the Rules for Feeder Taps" Mike Holt | Jun 01, 2002. He claims that all transformer secondaries are tap conductors and "apply to every transformer installation". Which is true if the transformer secondary does not exceed 10 ft, or 25ft in supervised industrial installations. But primary OCPD would use a secondary to primary voltage ratio (pri/sec) to protect the secondary as well. Often from a Panelboard or switchboard. Referring to the definition of tap conductor we see that it cannot be a service conductor and has over current protection ahead of its supply that exceeds the value usually perminted elsewhere in art. 240.4.
Tap conductors allow what regarding over current protection and ground fault protection location in a circuit? (Upstream and downstream)
- Overcurrent protection is downstream and short circuit and ground fault protection is upstream.
- (Art 240.2 handbook notes regarding the definition of tap conductor) 2017
What rule is the only rule that permits secondary conductors to land directly onto main lug only-type switch boards or panelboards. pp316
The 10ft tap rule as it pertains to the 10ft secondary conductor rule NEC 240.21C2.
regarding fire pumps, what type of overcurrent protection is allowed on the secondary of the transformer? PP327
None. Primary ocpd only and it must be sized at 125% of the pump, and all accessories in the circuit and be able to provide locked rotor currents indifinetly. NEC 695.5B
Note 1 on table NEC 450.3(B) for OCPD for primary and secondary conductors for transformers under 600v allows rounding up to the nearest OCPD and what for conductor ampacity?
rounding down for conductor ampacity