ECN Forum Forums General Topics of the Trade General Discussion Area How long can a circcuit be?

Isn't that assuming a low impedance circuit.

In the field I see bolted faults on 20 amp breakers that take seconds to open the breaker as the circuit length may may be 400' out and back on 12 AWG.

They may not be designed with that in mind but IMO they are used for that purpose.

Consider the rules in 422.11 that prevent me from suppling an appliance with a circuit that has a much greater capacity than the appliance requires.

For example I can not supply a water heater that has a 24 amp current draw with a 100 amp branch circuit..

It is my view that this rule is in place so that the wiring inside the appliance is also protected.

Or consider the rules in 210.23 that prohibit me from supplying a receptacle (other than 15s) with a circuit of higher rating.

[This message has been edited by iwire (edited 11-02-2006).]

Don,


Of course, if there is not enough fault current to open a breaker instantaneously then it could take seconds for it to operate thermally. But this low of current would be below the damage point of the conductor. And, the thermal curve of a UL489 breaker should mimic that of the conductor when it is at the reference temperature of the breaker (typically 25C or 40C).

The OCPD does not have to clear the fault in 1/2 cycle to protect the conductor it only has to reduce the current below the damage point.

Bob,

I don't believe so. For example a water heater might require a 40A circuit but after the incoming terminal block only 20A conductor might be run to each element.

[This message has been edited by JBD (edited 11-02-2006).]

Than can you explain why the NEC (and UL) will not allow supplying that WH with a 100 amp feeder?

Or why HVAC equipment has a maximum breaker size listed on it?

Just because the 'protection' does not mirror 240 and 310.16 does not mean that the breaker does not protect the appliance wiring from bursting into flames before the OCPD opens.

In your water heater example what would you say does protect the conductors from short circuits if not the branch circuit OCPD?

I am not trying to bust your chops, we just see this differently.

Bob.

Bob,

What I am saying is that a branch breaker is designed and tested to protect branch circuit conductors. The fact that they can also protect some end use equipment is secondary.

I can understand why the manufacturer of end use quipment would design and test their product to use the branch breaker, then they would not need to incur the cost of internal protection.

JBD doesn't the NEC actually require the use of a OCPD for the short circuit and ground fault protection of motors?

As shown in 430.51.

IMO both the NEC and the NRTLs expect the branch circuit OCPD to play a part in the protection of many types of utilization equipment.

I would not call it 'secondary' I would say the tasks are equal.

JMO, Bob

In small conductors 240.4(D) provides overload protection greater than necessary for simple overcurrent protection or short circuit protection.

Bob,

I agree the tasks may be equal, but the design of the breaker is to protect the branch circuit short circuits AND overloads. Supplementary breakers are designed to protect loads. The fact that some loads get to use the features of the branch breaker (i.e. overload for some appliances and short circuit for motors) is a function of the load design not that of the breaker.

JBD I am interested as I do not really understand what your saying.

The breaker does not know where the premise wiring ends and utilization equipment starts.

All the breaker 'knows' is the current passing through it.

It seems to me that saying the design of the breaker is to protect the branch circuit short circuits AND overloads is to limited.

A breaker is designed to open under predetermined conditions of short circuits and overloads. It does not mater if that overload is in a feeder, branch circuit, or utilization equipment.

I could also point to 240.5 that rightly or not extends the job of the OCPD beyond the receptacles and outlets.

Not trying to be a pain I just see a lot in the NEC that requires a branch circuit breaker to protect the equipment beyond outlet as well as the branch circuit itself.



[This message has been edited by iwire (edited 11-03-2006).]

Bob,

The point I am trying to make is not about the requirements of the NEC that the branch OCPD be used to protect certain pieces of equipmentand appliances, which it clearly does.

My point is dealing strictly with the design and testing of branch OCPDs. Their primary purpose is to protect conductors. Their thermal curve is designed to be below the thermal damage curve of a properly rated conductor. Their instantaneous region is tested to protect the conductor from the forces of a short circuit. These design criteria are why it is difficult (and why the NEC includes special allowances) to chose a standard thermal magnetic breaker that will coordinate with motor and transformer inrush currents. A properly operating listed branch OCPD will protect a properly rated and installed conductor during a low impedance short circuit fault.