I keep getting different interpretations of the NEC for some Fire Pump applications. Let me just put down what I understand right now and you all can let me know if I am completely goofy or what.
We are supplying our pump from a 750 KVA transformer that feeds other loads but has plenty of room to carry the locked rotor current of the fire pump.
We are placing the service rated disconnect for the fire pump separate from any other load and close to the transformer, running the leads to the disconnect directly from the wires coming off of the secondary of the transformer. We are meeting 695.3 (A) (1) I would think.
The confusing point is wire sizing. As I understand 695.4 the leads supplying the disconnect need to be rated for carrying full locked rotor amps. 695.4 (B) (1) also though states that ?The requirement to carry the locked-rotor currents indefinitely shall not apply to conductors or devices other than over current devices in the fire pump motor circuit(s). Where I understand motor circuit(s) to be anything on the load side of our disconnect. I am getting told that the line side to the disconnect can also be the smaller size.
I'm just wondering if I am commiting overkill as everyone says.:-?
I also understood (thought I read somewhere but can?t find again) that the only purpose of any overload device such as we are installing is to protect the line side from a direct short. Of course continuity of power to the fire pump is of primary importance and we are ensuring that the overloads and line side of the disconnect are capable of carrying the fire pump locked rotor current. No other equipment comes off of this disconnect, the jockey pump is being supplied from a separate source.
We are supplying our pump from a 750 KVA transformer that feeds other loads but has plenty of room to carry the locked rotor current of the fire pump.
We are placing the service rated disconnect for the fire pump separate from any other load and close to the transformer, running the leads to the disconnect directly from the wires coming off of the secondary of the transformer. We are meeting 695.3 (A) (1) I would think.
The confusing point is wire sizing. As I understand 695.4 the leads supplying the disconnect need to be rated for carrying full locked rotor amps. 695.4 (B) (1) also though states that ?The requirement to carry the locked-rotor currents indefinitely shall not apply to conductors or devices other than over current devices in the fire pump motor circuit(s). Where I understand motor circuit(s) to be anything on the load side of our disconnect. I am getting told that the line side to the disconnect can also be the smaller size.
I'm just wondering if I am commiting overkill as everyone says.:-?
I also understood (thought I read somewhere but can?t find again) that the only purpose of any overload device such as we are installing is to protect the line side from a direct short. Of course continuity of power to the fire pump is of primary importance and we are ensuring that the overloads and line side of the disconnect are capable of carrying the fire pump locked rotor current. No other equipment comes off of this disconnect, the jockey pump is being supplied from a separate source.