Sizing Circuit breaker & cable for DC Power Feed equipment

[subinstaller]

Hello all, hope I can get some advice on sizing circuit breaker and power conductors in a DC power application (specialized telecom application, -54 volts normal operating voltage, -40.5 volts minimum operatiing voltage).

The equipment I'm trying to size the cb and conductors for has a maximum power input of 700 amps DC (at -40.5 volts). But under normal system operating conditions, the equipment will not consume more than 400 amps DC (at -40.5V). So 400 amps is the expected load, the only way the equipment would require the 700 amps would be if some technican inadvertanly went into the system parameters and changed things, there is no mechanical or software safe guards to prevent this from happening. The operating procedures are written to keep the system at a level where it will only need 400 amps DC input, but again, aside from written procedures and training, a technican would be physically able to ramp things up where the equipment would consume the 700 amps input.

So my question to the forum is.................based on NEC, do I need to size my circuit breaker and power cabling based on the 700 amps the equipment is capable of consuming or the 400 amps that it will normally be operatiing at?

Any advice, especially pointing to NEC section(s) would be appriciated. (I realize most outages are caused by human error so not looking for that type of input, really need to understand the code for this situation)

Thank you

 

[don_resqcapt19]

In general, you have to protect the conductors at or below their ampacity. If you have 400 amps of conductors, you will need a 400 amp OCPD at the supply end of the conductors. As far as I know there are no special rules for the sizing and overcurrent protection of conductors used on DC systems.

 

[subinstaller]

Thanks, so if I understand your response correctly, I can base the sizing off the 400 amp load even though the equipment is capable of a higher output that would require 700 amp input power?

400amp input power is what's expected of the equipment to produce the expected output, but with no mechanical or other means to limit someone from cranking up with power output inadvertantly and requiring the 700 amp input power, I'm trying to figure out which input power to use for circuit breaker and power cable sizing?

I understand the common sense part (I think), money savings of course to size for 400 amps but would that violate any NEC, NFPA, type of codes?

 

[don_resqcapt19]

Where is the breaker in respect to the power source? I don't see any issues with using a 400 amp breaker and 400 amp wire on the load side of the breaker. There may be some issues with the conductors between the power source and the line side of the breaker, but I don't have enough details to know.

 

[Tony S]

This is a situation where the equipment manufacturers should be offering the information and taking the lead.

If it?s possible to raise the power level to >400A you have to allow some contingency for that. If >400a is going to damage the equipment then limiting the current should be the manufacturers responsibility.


I would be like me trying to apply BS7671 to the induction furnace I redesigned. I had to control the load current. The 2.6MVA 11/.66KV transformer would just keep pumping the power out until something went bang, it was quite happy in its own little world.

 

[Tony S]

Where is the breaker in respect to the power source? I don't see any issues with using a 400 amp breaker and 400 amp wire on the load side of the breaker. There may be some issues with the conductors between the power source and the line side of the breaker, but I don't have enough details to know.

I do, look at the breaker manufactures limitations on DC operation. Some will specify it some won?t, avoid them.

If at all possible I would look at DC fault detection operating the AC breakers first. Not an easy task.

 

[don_resqcapt19]

I do, look at the breaker manufactures limitations on DC operation. Some will specify it some won?t, avoid them.

If at all possible I would look at DC fault detection operating the AC breakers first. Not an easy task.

I assume that if you are working on a DC system, you would use an OCPD suitable for use on a DC system.

 

[Tony S]

I assume that if you are working on a DC system, you would use an OCPD suitable for use on a DC system.

Many think they can use any circuit breaker rated for a certain current. You didn?t make that clear.

High speed DC breakers are very expensive which is why I suggested breaking the AC feed a fraction before breaking the DC. Some may not agree with me.

The load control in this instance the equipment manufacturers should be taking the lead.



Breaking DC under fault conditions is spectacular. I?ve still got scars on my right leg from the burns caused by a DC air blast ACB opening at several KA. The ammeter had gone of scale at 1600A, I haven?t a clue what current it opened at.