Breaker size allowances

[rosebud]

John K.

John K.

I would seem that every body is missing the point here. Sure we can easliy say the breaker at some point in its connection had poor contact. How can we prove it though? How can it be proven that the breaker failed to trip? Had the breaker been protected via proper fuse size would this have protected this circuit from failure? There must be a forensic Electrician out there who is able to look at this whole picture.

 

[rosebud]

John K.

John K.

Certainly. It todays worls every body wants to sell a new panel. I belive we have a responsibilty to understand what caused the failure correct it and then proceed forward.

 

[don_resqcapt19]

Had the breaker been protected via proper fuse size would this have protected this circuit from failure?

Why do you think that a downstream OCPD should protect the upstream OCPD? I don't understand that concept.
Don

 

[don_resqcapt19]

I belive we have a responsibilty to understand what caused the failure correct it and then proceed forward.

If the failure was at the bus connection on the line side of the breaker, about the only possible cause is a poor connection between the breaker and the bus. The breaker will limit excessive current flow and the only way to get excessive heat without excessive current is a high resistance connection. I would not expect that a breaker would trip in time to prevent this type of incident as the current is below the trip point. It may eventually trip when the heat from the poor connection works its way into the thermal trip sensor in the breaker, but at that time the damage is already done. OCPDs, cannot in general, prevent damage or fires that result from poor connections. The device has no way to know what the current is doing...it only knows that the current, limited by the reistance, is within the normal range.
Don

 

[haskindm]

A standard breaker or fuse will not protect against the heat caused by a loose conection. Periodic checking of connections, or thermal imaging is the only way to do that. An arc-fault device MAY protect against loose connections, if it is loose enough that it causes an arc (but I don't want to SPARK any more arc-fault controversy here). No matter what size or type fuse had been installed in the disconnect, the breaker would have been destoyed by the heat from a loose connection. There was never any overload, so neither the breaker or fuses would have had any reason to trip. There is no blame to place here, unless it is to the owner for not servicing his electrical equipment periodically (who does?) or having the connections checked via thermal imaging or an infrared thermometer. The connection to the breaker may have been properly tightened at installation and worked loose over time or it may have never been tightened properly. The loose connection may have been at the buss and was a function of age and/or corrosion. At this point it is almost imposible to say. Sometimes "stuff happens". Connections fail, that is why the NEC demands that they be accessible. I don't think you will ever have a definitive answer of exactly why this failed at this time and in this manner. It appears from all that you have told us that the installation complies with the NEC and the manufacturer's requirements. Chalk it up to equipment failure and move on...

 

[don_resqcapt19]

An arc-fault device MAY protect against loose connections, if it is loose enough that it causes an arc (but I don't want to SPARK any more arc-fault controversy here).

I'm sorry, but I can't not comment. An AFCI never looks at a series arc...it only looks at parallel arcs. It may clear a fault caused by a poor connection if the heat of the fault causes the insulation to fail and the poor connection becomes a parallel fault or more likely a ground fault.
Don

 

[dlhoule]

Why do you think that a downstream OCPD should protect the upstream OCPD? I don't understand that concept.

Don

I do Don. For example: 30A fuses in Buss Plug feeding disconnect with 20A fuses for whatever. I want these downstream OCPD to protect the upstream OCPD just as a matter of convenience if there should be a problem. Those 20A fuses are reacheable from ground level. The fuses in the buss plug require finding a lift to get to them.

I just can't help myself sometimes!

 

[don_resqcapt19]

Larry,

I do Don. For example: 30A fuses in Buss Plug feeding disconnect with 20A fuses for whatever. I want these downstream OCPD to protect the upstream OCPD just as a matter of convenience if there should be a problem. Those 20A fuses are reacheable from ground level. The fuses in the buss plug require finding a lift to get to them.

I see that as selective coordination and not as protecting an upstream OCPD. In the posters case the rating of both the breaker and the fuse was 60 amps, so in that application, how does the down stream fuse protect the upstream breaker?
Don

 

[rosebud]

John K.

John K.

Don, why dont we just eliminate the secondary OCPD then. If we are going to match it's amperage to the breaker why don't we just install a disconnect?

 

[don_resqcapt19]

why dont we just eliminate the secondary OCPD then. If we are going to match it's amperage to the breaker why don't we just install a disconnect?

There would be only 2 cases where you need a fused disconnect. 1) if the nameplate calls for fuses as the OCPD or 2) if you have a feeder for multiple units and you have to provide a smaller OCPD for the unit and its branch circuit. If you don't have either of those conditions, then you use an unfused disconnect.
Don

 

[dlhoule]

Don, In the posters case it doesn't. Will you explain what the difference is in selective coordination and protecting an upstream OCPD?

I see that as selective coordination and not as protecting an upstream OCPD. In the posters case the rating of both the breaker and the fuse was 60 amps, so in that application, how does the down stream fuse protect the upstream breaker?

 

[stew]

larry the manufacturer lists the min cicuit ampacity to size the conductor and the maximum breker size to size the ocpd. thes are motor loads and are sized as such. the breaker may thus be much larger than would normally be the case . we put 12 on 30 amps all the time and this is perfevtly fine on these applications and is code compliant.the manufacture uses the takes the current draw of the hermetic compressor which is the largest laod X1.25 and adds the fan amps to get the circuit ampacity(conductor size) then they calculate the ocpd at usually 2 or 3 times the nameplate of the largest load,again the compressor motor. Nice that the do all the math for us lowly electricians eh?

 

[don_resqcapt19]

Larry,

Will you explain what the difference is in selective coordination and protecting an upstream OCPD?

I don't know of any cases where a downstream OCPD is used to protect an upstream OCPD. Selective coordination is when the OCPD closest to the fault opens and the upsteram OCPDs do not open.

In the posters case the rating of both the breaker and the fuse was 60 amps, so in that application, how does the down stream fuse protect the upstream breaker?

It doesn't and it is not intended to. If you use a fused disconenct the fuses are to protect the equipment, not the upstream breaker.
Don

 

[electric_instructor]

I'm aware of article 440 HOWEVER, - are YOU aware of article 110.14(C), and article, 240.4 (D) some of the most BASIC codes? The Codes that are actually placed in the VERY beginning of the book?

The above mentioned articles fall well within the beginning of the codes. And as such, being an expert electrician, I'm sure that you understand the very layout/design of the codes.

I've had this discussion too many times before. I feel that too many fail to understand the "intent" of the code. Article 440 is meant to apply to the disconnecting means, whip, and internal wiring of the actual unit, and NOT the feeder circuit that is a part of the "premises" wiring!

Lets NOT forget the limitations of Nm cable as well! If this is a residential application? Article 334.80, or (60 Deg C)!LMAO

I'll again refer you to the above mentioned articles.

I suppose that if ALL connections are rated for the temps. required, INCLUDING the breaker, and that you choose to IGNORE article 240.4(D), AND article 334.80, then you would be right?

Can you show me WHERE in article 440 it says to ignore article, 110,14 (C), 240.4(D), OR article 334.80??


SWEET!

 

[infinity]

Can you show me WHERE in article 440 it says to ignore article, 110,14 (C), 240.4(D), OR article 334.80??

240.4(D) Small Conductors. Unless specifically permitted in 240.4(E) or 240.4(G), the overcurrent protection shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper; or 15 amperes for 12 AWG and 25 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied.

240.4(G) Overcurrent Protection for Specific Conductor Applications. Overcurrent protection for the specific conductors shall be permitted to be provided as referenced in Table 240.4(G).
Table 240.4(G) Specific Conductor Applications
Conductor Article Section
Air-conditioning and refrigeration equipment circuit conductors 440, Parts III

I agree with you on using the 60 degree column for NM cable, but 240.(D) does not apply to the branch circuit supplying an AC unit. And forget about 110.14(C), just about every piece of electrical equipment now has 75 degree rated connections.

 

[George Stolz]

I agree with Trevor. The presence of "Air Conditioners" in 240.4(G) means that you read no further as far as 240.4(D) is concerned. It does not apply.

Of course, that doesn't mean we have all the information at that point - but at that point it's clear that 240.4(D) no longer applies.

There is no relief from 334.80, but that only applies to romex. Even SE cable doesn't use that section.

 

[infinity]

There is no relief from 334.80, but that only applies to romex. Even SE cable doesn't use that section.

That's a good point. Switching to SE or MC cable will relieve you of the 60 degree limitation. AC cable not within thermal insulation would provide the same relief.