overload protection/short circuit protection

[alixenos]

Hello,

I've a question regarding article 240


In 240.4.A, if power loss hazard is more objectionable than the overload, then overload protection is not required, however, short circuit protection is required.

My question is, is the means of overload protection (CB, fuses) the same as the means of short circuit protection? If this is the case, how can we NOT provide an overcurrent protection but DO provide short circuit protection if the mechanism of implementing both is the same?

Thank you

 

[alixenos]

The other question is if the ampacity of a conductor is 800A or less, say 380A, then according to 240.4.B, we use table 240.6(A) and look for the next larger standard Amp rating of the CB, in this case we protect 380A conductor with 400A CB.


The question is: won't we provide better protection for 380A conductor if we choose 350A CB? we we are allowed to choose the next larger standard size in this case?

Thank you again.

 

[JoeStillman]

If you are taking advantage of 240.4(A), you don't have any overload protection. You only have short-circuit and ground-fault protection. An example of short-circuit protection would be a mag-only motor circuit protector. It has a continuous current rating and a short-circuit trip rating, but no thermal overload element. It won't trip on overload.

Just because you are allowed to go to the next size larger OCP doesn't mean you are required to. But when you say "better", you're in value judgement territory. Conventional wisdom is that actual overload conditions, especially for properly selected conductors, are so rare that the next-size-up rule doesn't pose any significant risk. Especially when you consider that most of the time, we are applying conductors at ampacities lower than the temperature rating.

 

[Dennis Alwon]

A fuse or circuit breaker is protecting the circuit from being overloaded. It is also giving ground fault and short circuit protection. Ground fault is a short to ground and short circuit is a short between conductors.

Overloads are usually entered in to the equation when there is a motor involved. In that case the overloads that are either built into the motor or install in a separate enclosure, will give the equipment protection from overload. These overloads are rated a lot lower than a circuit breaker or fuse so that the overcurrent protective device will not be needed to protect for overload but it will protect for ground fault and short circuit.

You may have run across a situation where there is #12 wire run to a heat pump or a/c unit that min cir amp is 20 amps and there is a 30 amp overcurrent protective device being used. #12 on a 30 amp breaker is allowed because the a/c units have overloads built in and it will protect the circuit from currents larger than their setting which would be less than the 20 amp min. cir amp.

 

[jim dungar]

My question is, is the means of overload protection (CB, fuses) the same as the means of short circuit protection? If this is the case, how can we NOT provide an overcurrent protection but DO provide short circuit protection if the mechanism of implementing both is the same?

If you size the fuse or breaker so that the long time portion of its Time Current Curve is beyond the normally expected overload point (often 6X for 20secs) you still have protection from the rest of the curve.

 

[Jraef]

... An example of short-circuit protection would be a mag-only motor circuit protector. It has a continuous current rating and a short-circuit trip rating, but no thermal overload element. It won't trip on overload.

Not a good example, you cannot use a mag-only breaker for anything EXCEPT a factory assembled motor starter that is listed as a whole unit.
The issue has to do with the DIFFERENCE in the SCPD and the OCPD rules. An OCPD for a basic non-motor load (motors come under article 430) must be per the wire ampacity from the applicable charts, rounded up to the nearest value. But unfortunately, the NEC doesn't do a good job of nailing down SCPD rules, other than for Service Entrance (230.208), Transformers (240.92) and Motors (430.52). So if your load (such as the magnet in the example) needs ONLY the SCPD, I would use the lowest values on those options; 300% for fuses, 250% for circuit breakers. The OCPD must be significantly smaller.

That said, in my 40+ years in this business, I have never had the occasion to come across a circuit that did not need an OCPD, other than a fire pump.

 

[Carultch]

My question is, is the means of overload protection (CB, fuses) the same as the means of short circuit protection? If this is the case, how can we NOT provide an overcurrent protection but DO provide short circuit protection if the mechanism of implementing both is the same?

Overload usually means moderate amounts of overcurrent, as opposed to extreme amounts that a short circuit would draw. For instance, if an adversarial user deliberately turns on all the loads of a panelboard at once, and attempts to draw 300A on a 200A service. Compare this with a short circuit due to a fault that draws 3000A. A 3000A fault will trip the breaker quickly, while a 300A overload will trip the breaker slowly. Usually, a device that protects against overcurrent, also protects against overload, as it's meant to eventually trip on any current in excess of its rating.

Motors add their own difference to these terms. Motors draw a locked rotor current, if you stop the rotor from spinning. If you look at a torque/speed/current graph for a motor, you'll see that locking its rotor draws the maximum possible current, and the least possible current is drawn when it can freely spin unloaded at maximum speed. A motor overload protection could trip on too much mechanical load torque, too much electrical current, or both, and is usually part of the motor, instead of separately installed.

Motor loads commonly have overcurrent devices, in excess of what you'd expect. It has to do with them having their own overload protection to accommodate the specifics of what they need as motors, while avoiding nuisance tripping the branch circuit breaker on the inrush current at startup. The overload protection protects against moderate amounts of overcurrent, while the branch circuit breaker is primarily there to protect against faults like short circuits.

 

[JoeStillman]

Not a good example, you cannot use a mag-only breaker for anything EXCEPT a factory assembled motor starter that is listed as a whole unit.
The issue has to do with the DIFFERENCE in the SCPD and the OCPD rules. An OCPD for a basic non-motor load (motors come under article 430) must be per the wire ampacity from the applicable charts, rounded up to the nearest value. But unfortunately, the NEC doesn't do a good job of nailing down SCPD rules, other than for Service Entrance (230.208), Transformers (240.92) and Motors (430.52). So if your load (such as the magnet in the example) needs ONLY the SCPD, I would use the lowest values on those options; 300% for fuses, 250% for circuit breakers. The OCPD must be significantly smaller.

That said, in my 40+ years in this business, I have never had the occasion to come across a circuit that did not need an OCPD, other than a fire pump.

A better example might be a piece of air conditioning equipment that has MCA and MOCP on the nameplate. These values are calculated per article 440 and MOCP is always greater than MCA. But I always get in an argument when the wire ampacity is less than the breaker it's connected to. The overloads are part of the AC machine and the breaker is only for SC & GF. Part III of 440 even leaves out "overload" protection for the conductors. You don't get to that until Part IV.

It's hard to persuade some people that the breaker is not the overload protection.

 

[Jraef]

It's hard to persuade some people that the breaker is not the overload protection.

So true. I’ve seen a lot of people try to do it with fuses too, usually resulting in motor thermal damage. Seems silly to me to try to save the relatively minor cost of a proper OL relay and risk the high cost of rewinding a motor plus the associated down time. Yet, the practice persists.