Fuse/ Breaker Sizing 2

[pyro214]

To exaggerate the scenario I'm thinking about: if we have a 10A load that is connected using a 1000A rated cable and 500A rated fuse/ circuit breaker (F/CB), if the 10A load short circuits and draws 200A:

[ol 1]
[li]The F/CB will not open[/li]
[li]The cable will not catch on fire[/li]
[li]We 'hope' that the load has an internal fuse that will open, otherwise the load will catch on fire[/li]
[/ol]

My question is, wouldn't it be advantageous to have a ~15A F/CB on this 10A load to catch a wider range of short-circuit scenarios?

To my knowledge, the National Electrical Code (NEC) does not define a maximum gap between the load amperage and the F/CB rating; HOWEVER, the NEC does say to use a F/CB that is equal to (or 125% greater if dealing with a continuous load) the load amperage or the next standard size up. My section question is, why would the NEC bother saying 'next standard size up'... why not just say that we can use any size up as long as the cable is rated equal to or greater than the F/CB.

Thanks in advance for any insight.

 

[waross]

Are you confusing "load" with cable ampacity?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[pyro214]

No. I'm referring to the load (equipment at the end of the cable).

Source to 1000A cable to 500A rated F/CB to 1000A cable to 10A load.

 

[VEBill]

Fuses and CBs protect wiring, not necessarily loads.

 

[pyro214]

Hmm, alright. I feel like it would be optimal if they protected both but I'll stick to just focusing on protecting cables.

 

[SceneryDriver]

Many devices, especially permanently installed and wired ones, like air conditioner units, furnaces, etc... list a Max Circuit Ampacity on their nameplates for exactly this reason.

In the case of protecting receptacle-connected loads, a receptacle will have a MCA listed as well. For instance, it's against code, and just a damn bad idea anyway, to wire a 15A receptacle to a 50A breaker. The breaker would not trip before the current carrying parts of the receptacle would be overloaded.


SceneryDriver

 

[pyro214]

SceneryDriver, I wouldn't mind reading the code that explains the scenario you outlined. Any chance you recall what section it talks about breakers being too big for receptacles?

Thank you for your input.

 

[SceneryDriver]

NFPA 70, the National Electrical Code (not trying to be pedantic about the NEC; I'm not sure where in the world you're located), states in 110.3 (B):

"Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling."

For instance, a NEMA 5-15 receptacle is described as supplying 120v at up to 15A. There are differing, and non-matable, pin configurations for other amperages and voltages. If you look at the manufacturer's instructions for the 5-15 receptacle I'm using in my example, it will also give a max wire size the receptacle is capable of being connected to - in this case, it's almost always #10 wire. Since you need wire larger than that to safely carry 50A, you'd be using it outside of it's listing. It would also be outside its listing since it could supply much more than the 15A it was designed and listed for.

Same goes for the air conditioner I mentioned in my earlier post. It was UL listed with the Max Circuit Ampacity spelled out on its name plate. If connected to a circuit with a larger breaker/fuse, you're using it outside of its listing, and the Authority Having Jurisdiction can/should red flag you. I believe you MAY be able to violate the listed max circuit ampacity with engineering and AHJ approval, but I think you'd have to have a darn good and darn strange reason to do so.

Not to say that there are instances where that rule is bent slightly. I've temp-ed a NEMA 5-15 receptacle onto the main lugs of a meter base for power for a hammer drill while doing a panel change. In that case, it was VERY temporary, I was the only one using it, and it was removed before I left for the day. It's just that something like that should not be left in the hands of of amateurs. How many end users could compute the total amperage draw on a circuit? Most just plug stuff in until the breaker trips

SceneryDriver

 

[pyro214]

Awesome, thank you for that detailed explanation. Much appreciated!

 

[jraef]

... if we have a 10A load that is connected using a 1000A rated cable and 500A rated fuse/ circuit breaker (F/CB), if the 10A load short circuits and draws 200A:

Highlighted is the flaw in your thinking.

If there is a short circuit (or Ground Fault), ALL of the available current at that point in the circuit attempts to flow to it. You cannot pick the short circuit amps as being 200A based on something inherent in the load. A short is a short, it is NOT related to the load. So if your circuit is connected to a transformer that has an available fault current of 10,000A, then when there is a short circuit, the current that tries to flow will be 10,000A, minus the effect of resistance of the wire up to the point at which the short circuit takes place. The current will flow until something stops it, in this case the fuse, but during the time it takes the fuse to clear, ALL of that current is flowing. So in reality if you have wire rated for 1000A on a circuit fused for 500A that is (somehow) connected to a 10A load, there will be VERY LITTLE resistance added to the circuit and it might actually make it WORSE!


"Will work for (the memory of) salami"

 

[LionelHutz]

Following the code, 10A x 1.25 = 12.5A and rounding up to the next available size gives a 15A breaker as the largest breaker you can use. So, why is this even a question?

It's assumed loads below 15A will use a 15A breaker so they either have to pass the failure testing with a 15A breaker or use a smaller rated fuse.

 

[VEBill]

Perhaps one could install what is sometimes called a "pony panel".

 

[jmbelectrical]

SceneryDriver,

In the case of air conditioning equipment, doesn't NEC Article 440 allow the overcurrent protective device to be sized greater than the ampacity of its associated branch circuit conductors?

 

[SceneryDriver]

jmb,
Article 440 certainly does. I was using the air conditioner as an example; years ago, when I was an apprentice electrician (when I wired more residential AC units than I care to remember), it was the example used when I asked a question similar to the OP's about overcurrent protection. My point was that many devices have as part of their listing a max circuit ampacity, and connecting that device to a source capable of sourcing more current would violate its listing. Couple that with things like max wire size, and max rated current through the device leads to limitations on the size of OCPD that they can be served by.


SceneryDriver

 

[iceworm]

Hmm, alright. I feel like it would be optimal if they (fuse/CB) protected both but I'll stick to just focusing on protecting cables.

With a few exceptions, examples as noted above, most of this is design decisions. Again, not knowing where you are located, the NEC is not a design manual. It is a set of minimum specs that may not work well. have poor reliability, and only practible safety.

A couple of points - that I'm sure you already know:
Over Current Protection does not protect the load. For example:
Overloads do not protect the motor from overload. The design does that. The overloads may not even protect the motor from burning up under inadvertant overload. If something has gone wrong with the process, the motor loads up until the overloads trip. Operator resets and tries again - motor trips again. Somewhere around the third or fourth trip within an hour the motor lets the smoke out. It's tough to protect against stupid.

How about the conductors:
The overloads should be set to trip inside of the conductor thermal damage curve. And hopefully in the event of abject stupidity (such as multiple ovld resets) the motor fries before the conductors catch fire.

Motor specific failures:
If the motor fails, internal short, there is mo motor to save - it's dead. The job for the CB is to put out the fire.

The conductors:
Say the conductors fail from overhating or forklift attack. Again there are no conductors to save - their dead. The job for the CB is to put the fire out.

OCP is about protecting the structure - not the equipment.

ice

Harmless flakes working together can unleash an avalanche of destruction

 

[iceworm]

A couple of points - that I'm sure you already know:
Over Current Protection does not protect the load. For example:

(NEC referenced)
This one is my favorite: Consider a 20MW tranformer out in the yard, XFM secondary unlimited length outside tap, OCP at the far end of the conductors - 500feet away.

The question is asked, "The OCP is 500 feet away from where the conductors receive their supply. The OCP protects everything downstream, what protects the conductors from the xfm to the OCP?
A: The OCP at the end. Whatever current goes in the xfm end comes out the OCP end.
Q: What happens if there is an overload in the middle of the run?
A: You mean like a crane or backhoe attack? Well in that case there are no conductors to protect. They are dead. Eventually the xfm primary OCP will trip. If it is utility owned, hopefully they set the OCP low enough the xfm does not catch fire. If one wishes to protect the conductors that is a function of concrete and steel - not the OCP.

Again the OCP is to protect the structure - put the fire out.

ice

Harmless flakes working together can unleash an avalanche of destruction

 

[iceworm]

To my knowledge, the National Electrical Code (NEC) does not define a maximum gap between the load amperage and the F/CB rating; HOWEVER, the NEC does say to use a F/CB that is equal to (or 125% greater if dealing with a continuous load) the load amperage or the next standard size up. My section question is, why would the NEC bother saying 'next standard size up'... why not just say that we can use any size up as long as the cable is rated equal to or greater than the F/CB.

Might want to read those NEC (2011) sections again:
210.3 Branch circuit rating
210.19 Conductor ampacity not less than maximum load
240.4 CB selected to protect conductors

So why does the NEC give one the option to use the next size up CB?
Well, one picks the conductors such that the ampacity is equal or more than the load requires. Generally economics limits how much larger conductors one might choose to install.

Next the CB is selected such that it protects the conductors. Up to 800A (with some exceptions)if the ampacity falls between two CB ratings, the NEC allows one to select the next size up strictly for economics.

For example: Say the load is above 225A and below 230A. 4/0 Cu is required (230A ampacity). The available CBs are 225A and the next up is 250A. NEC 240.4.B allows one to use the 250A CB to protect the 230A conductors

Example 2
Consider a 480 primary, 3ph, 75kva xfm. Primary FLA 90A.
Generally, one picks the primary conductors to at least supply 100% FLA continuous, or ampacity = 125%FLA = 113A. So the load requires #2CU = 115A. and I can use the next size up, 125ACB. (this is in article 450).

But this xfm is known to have high inrush and requires a larger primary CB. NEC allows up to 250% FLA or 180A. Ah, but I can not use next size up for this one, so I am limited to 175A CB. The feeder conductors have to be protected by the 175A CB, so I am limited to at least 2/0 CU - bigger is okay. But it would be good if the conductors would fit in the CB lugs. This is a case where neither the conductors nor the CB were picked to fit the load (xfm FLA X 125%)

So, an answer to your query is: Sure, most times bigger is okay. Some times bigger conductors are required - say like for Vd. As noted, the NEC is not a design guide. It is a set of minimum specs to where the system may not work well and only has practicable safety. Everything after that is a design decision - right up to the ridculously absurd.

Although, it is true most customers would prefer you not put in a $40K circuit when a $500 circuit will do fine.

ice

Harmless flakes working together can unleash an avalanche of destruction