Oven

[mbrooke]

There was a time when the NEC did limit the need for upsizing the EGC to instances when the ungrounded conductors were increased because of VD considerations. But I think there was a bit of abuse going on, as in, "I didn't use the larger wire because of VD. It was just what I had in the truck at the time." The rule was changed to require upsizing the EGC regardless of the reason that the ungrounded conductors were upsized. The oldest code book I have handy is 2008, and this change happened before then.

But how would down sizing a breaker make this rule apply?

 

[growler]

Dumb enough to ignore in this case.

Ultimate solution: leave the 50a breaker.

Yes it is "Dumb" enough to ignore and I would ignore it.

But I do normally try to install the manufacturer's recommended max breaker size. If there is ever a problem with the appliance a repair man may notice the wrong breaker installed and tell the homeowner some idiot installed the wrong breaker. I doubt if they would even think of the size of the EGC . If it's not marked I also mark the breaker location in the panel.

 

[LarryFine]

But I do normally try to install the manufacturer's recommended max breaker size.

I take the manufacturer's 40a breaker spec as a minimum size, not a maximum.

 

[charlie b]

But how would down sizing a breaker make this rule apply?

That's a very good question. I can see it both ways. You would not be "installing" wires that are larger than ampacity would require. So perhaps the rule might not come into play. On then other hand, reducing the breaker rating will create an installation in which the wire size has higher ampacity than is required. So perhaps the rule would come into play.

But let's change the game completely. Suppose you have one item of equipment powered by a 50 amp breaker and wires rated for 50 amps. Suppose that equipment is replaced by an item that only needs wires that have an ampacity of 40 amps. Whether or not you change the breaker, have you not created an installation in which the wires are larger than the minimum size that has sufficient ampacity for the intended installation?

 

[charlie b]

I take the manufacturer's 40a breaker spec as a minimum size, not a maximum.

I agree. The manufacturers of some equipment (chillers, for one example) will show a Maximum Overcurrent Protection (MOCP) value. Oven manufacturers don't do that.

 

[Dennis Alwon]

Here is something from Vince

Why We Need To Increase The Equipment Grounding Conductor

MR. SAPORITA:··Vince Saporita, Saporita
The panel action may have brought about some ·· · unintended consequences.··Larger phase conductors installed for reason, not just for voltage drop, will result in greater available short circuit currents. · Greater available short circuit currents result in greater currents that must be safely carried by the equipment grounding conductor underground fault conditions.··If too much current flows over the equipment grounding conductor, it will overheat to the point where it begins to anneal.··When that happens, it will no longer be tight under the lug after if cools back down.··When that happens, the ground return path is no longer adequate.··The next ground fault may not be cleared by the over-current device, allowing for the worker that touches the case of the equipment to be electrocuted.··This concept is not new.··It was published decades ago and detailed in the IAEI book on grounding.··The point at which the equipment grounding conductor will no longer be tight under the lug is called its validity rating, and it can be calculated.··Look in the IAEI book on grounding if you want to make the calculation. I urge you to vote for this motion, which may prevent the unintended consequence of an electrocution of an electrical worker.

 

[mbrooke]

That's a very good question. I can see it both ways. You would not be "installing" wires that are larger than ampacity would require. So perhaps the rule might not come into play. On then other hand, reducing the breaker rating will create an installation in which the wire size has higher ampacity than is required. So perhaps the rule would come into play.

But let's change the game completely. Suppose you have one item of equipment powered by a 50 amp breaker and wires rated for 50 amps. Suppose that equipment is replaced by an item that only needs wires that have an ampacity of 40 amps. Whether or not you change the breaker, have you not created an installation in which the wires are larger than the minimum size that has sufficient ampacity for the intended installation?

I don't think nap time is over yet


But in all seriousness an inspector would really have to not like you to nail you on this rule.

 

[growler]

I take the manufacturer's 40a breaker spec as a minimum size, not a maximum.

Well, I was wrong. I said max breaker when what I should have said was recommended. Anyway that's what I use, whatever is recommended.

 

[bwat]

Here is something from Vince

I believe I've seen elsewhere on this forum where someone suggested that the EGC sizing be simply tied to the ungrounded conductor size. I'd vote for this. The available SC current is certainly a valid concern, but with the way that section is written right now, if you upsized the phase conductors because of temperature correction, as whitney pointed out in this thread, you don't have to increase EGC per code. But the SC current and annealling concern should still be just as valid for that, so I would think that you should.

Also, I do agree that most inspectors won't hit you on this. I'd be more concerned with liability if any type of incident occurs. Not to code = negligence.. regardless of whether or not we agree with the code.

 

[synchro]

Here is something from Vince

Why We Need To Increase The Equipment Grounding Conductor

MR. SAPORITA:··Vince Saporita, Saporita
The panel action may have brought about some ·· · unintended consequences.··Larger phase conductors installed for reason, not just for voltage drop, will result in greater available short circuit currents. · Greater available short circuit currents result in greater currents that must be safely carried by the equipment grounding conductor underground fault conditions.··If too much current flows over the equipment grounding conductor, it will overheat to the point where it begins to anneal.··When that happens, it will no longer be tight under the lug after if cools back down.··When that happens, the ground return path is no longer adequate.··The next ground fault may not be cleared by the over-current device, allowing for the worker that touches the case of the equipment to be electrocuted.··This concept is not new.··It was published decades ago and detailed in the IAEI book on grounding.··The point at which the equipment grounding conductor will no longer be tight under the lug is called its validity rating, and it can be calculated.··Look in the IAEI book on grounding if you want to make the calculation. I urge you to vote for this motion, which may prevent the unintended consequence of an electrocution of an electrical worker.

One thing I find interesting in this context is that the length of the conductor to the short circuit point will typically vary the impedance and therefore short circuit current to a much larger extent than changing the wire gauge a step or two. For example, a variation of 10-to-1 in distance from the panel to the short circuit would change the impedance by 10-to-1, but you have to go from 14 AWG to 4 AWG to get the same 10-to-1 impedance change.

Say I had a 10 ft length of #10 AWG phase conductor shorted at its end to a 10 ft length of #10 AWG EGC.
As a crazy example, compare this to another circuit with a 20 ft length of 1000 MCM shorted to a 20 ft length of #10 AWG EGC.
The impedance of both circuit loops and therefore their short circuit currents would be essentially the same. And therefore the 10 AWG EGC would experience the same stress and heating under a short circuit condition.
Just pointing out that it may be useful to look at this issue in quantitative terms with real world situations to see where the critical issues are.

By the way, mbrooke's earlier post that data shows there's often not enough bolted fault current to magnetically trip breakers is a very valid concern. So making sure the EGC is sized up with phase conductors for voltage drop reasons makes total sense to keep the short circuit impedance sufficiently low.

 

[mbrooke]

Here is something from Vince

Under what context though? I'm not necessarily buying it... 8 feet of code minimum circuit can/will produce far more fault current than reasonably up-sizing the current carrying conductors in a 600 foot run.

 

[winnie]

#8 is 16510 cm
#6 is 26240 cm
26240/16510 = 1.6
#10 = 10380
10380 x 1.6 = 16608
#8 is too small so a #6 is needed

Posts 35 and 37 answer this point.

AWG is _defined_ as a geometric series, where the diameter ratio between successive terms is always the same amount. (the 39th root of 92, because some group decided to be particularly annoying. See https://en.wikipedia.org/wiki/American_wire_gauge )

This means, as defined, that the ratio of #10 to #8 should be exactly the same as the ratio from #8 to #6.

However 'rounding' creeps into our math and into the NEC tables. On top of this, once you get to the large conductors specified in 'circular mils' the ratios go off.

IMHO for purposes of 'increased proportionally in size' it is perfectly fine to go by the difference in AWG values, _where all conductors are sized in AWG_.

-Jon

 

[ptonsparky]

Hello -

A customer is thinking about replacing his wall oven. The wire going to the oven is 6-2 with ground on a 50 amp breaker. The new unit requires 40 amps. Would you leave everything as is or would you run an 8 gauge and 40 amp breaker? Thanks

Did you get your answer?

 

[kwired]

Under what context though? I'm not necessarily buying it... 8 feet of code minimum circuit can/will produce far more fault current than reasonably up-sizing the current carrying conductors in a 600 foot run.

And gets even worse with a longer run. With irrigation applications I run into 1000-3000 foot runs quite frequently.

30 amp fuse is pretty common feeder fuse supplying a center pivot machine, direct short on two lines won't necessarily take a fuse out, all because current limitations of the length of the circuit. Give it enough time it still takes out the fuse but doesn't instantly take it out like it does for a short run of circuit conductor. Weak connections sometimes fail before fuse will blow also.

Newer equipment often has class CC fuseholders and those probably have better trip curve for the application. Older equipment typically has "standard" fuseholders and RK5 fuses installed.

 

[mbrooke]

And gets even worse with a longer run. With irrigation applications I run into 1000-3000 foot runs quite frequently.

30 amp fuse is pretty common feeder fuse supplying a center pivot machine, direct short on two lines won't necessarily take a fuse out, all because current limitations of the length of the circuit. Give it enough time it still takes out the fuse but doesn't instantly take it out like it does for a short run of circuit conductor. Weak connections sometimes fail before fuse will blow also.

Newer equipment often has class CC fuseholders and those probably have better trip curve for the application. Older equipment typically has "standard" fuseholders and RK5 fuses installed.

Code really needs to start limiting wire lengths on smaller gauges. But they are to busy trying to fix that with GFCIs and other gimmicks.

 

[winnie]

Code really needs to start limiting wire lengths on smaller gauges. But they are to busy trying to fix that with GFCIs and other gimmicks.

Well a GFCI would fix the problem of not enough current flowing on the EGC to trip the breaker.....

 

[mbrooke]

Well a GFCI would fix the problem of not enough current flowing on the EGC to trip the breaker.....

Sure- but why mandate a device which in theory is less reliable than an EGC? Requiring only one method to meet a practical safeguard when multiple others can suffice is bias and against the code's intent.

 

[winnie]

Sure- but why mandate a device which in theory is less reliable than an EGC? Requiring only one method to meet a practical safeguard when multiple others can suffice is bias and against the code's intent.

Agreed, and thus the smilies.

IMHO we've reached the point in 'practical safeguarding' that really the code should provide a menu of options. For example: AFCI _or_ GFPE in metallic wiring methods _or_ install sprinklers in the house. Increase the size of the EGC _or_ limit the voltage drop in the run _or_ use ground fault detection.

Or, since _practical_ safeguarding must include the cost of the protective devices, set cost thresholds where the protective device is mandated only if the cost drops below a certain level.

-Jon

 

[mbrooke]

Agreed, and thus the smilies.

I know- just adding more to what you said Sorry if I came across as strong- its not directed at you but NFPA 70.

IMHO we've reached the point in 'practical safeguarding' that really the code should provide a menu of options. For example: AFCI _or_ GFPE in metallic wiring methods _or_ install sprinklers in the house. Increase the size of the EGC _or_ limit the voltage drop in the run _or_ use ground fault detection.

Yes! Now that is what I call common sense reasoning :happyyes::happyyes::happyyes::happyyes::happyyes::happyyes:

Or, since _practical_ safeguarding must include the cost of the protective devices, set cost thresholds where the protective device is mandated only if the cost drops below a certain level.

-Jon

Interesting. I vote accept in principal.

 

[kwired]

Throwing cost in the requirements will make a bigger mess than we already have.

Good products that are out there might be exempt because of cost while cheaply made and inferior products are still required because they meet the cost requirement.