Safety problem with feeders and branch circuits in the same wireway.

[avargasm]

Hi.

The NEC allows running feeders and branch circuits in the same wireway or conduit. My question is: if a feeder with, say, a 100A breaker suffers a short circuit or overload with a 20A neutral of a branch, it is possible that the 100 A breaker won't trip, and the 20 A neutral will probably get hot enough to burn the there. What am I missing?

 

[Eddie702]

Anything is possible.

 

[Patpowers]

230.7 applies to service conductors. There are no OCPD for the service conductors. The cut-out fuse for the transformer is your only protection for service conductors, which is only designed to protect the utility’s equipment.

After your main disconnect you are now using feeders and branch circuits. You can consider 215.2 A (Feeders Not More Than 1000 Volts) (2)Grounded Conductor

this is 2020NEC: the size of the feeder circuit grounded conductors shall not be smaller than that required by 250.122.

Table 250.122 states that a 100A OCPD needs to be protected by 8AWG copper or 6AWG aluminum or copper clad aluminum.

A 20amp branch circuit will have a 12AWG neutral that should provide an effective ground fault path back to the service.

considering an 8AWG is rated for 50 Amps if you’re using 75 degree insulation and terminals. That’s a relatively small conductor to clear a 100 amp OCPD.

If you’re using NM-B 12AWG, the neutral conductor is rated at 30Amps at 90degree, but are only allowed to treat NM-B as a 60 degree conductor, but may derate according to 90 degree.

Even though a 12AWG may be a relatively small conductor, it will be able to clear a short circuit fault with and open a 100A OCPD. There is always the chance that insulation can become compromised; (overheating due to undersized conductors, or insulation damage during installation, or even defective insulation from the manufacture); the conductors may come in some contact that causes arcing and possibly a burn out without opening an OCPD.

But : that’s why it’s in a raceway (to hopefully contain the burn out). By the way feeders in a raceway by themselves can burn up without tripping an OCPD.

Either I gave you some insight or confused the crap out of you. Either way, don’t forget to keep in mind your raceway/ conduit fill, and keep in mind you number of current carrying conductors. After doing those calculations you’ll be sure to limit the amount of branch circuit conductors you try to stuff in with your feeders.

 

[jaggedben]

Hi.

The NEC allows running feeders and branch circuits in the same wireway or conduit. My question is: if a feeder with, say, a 100A breaker suffers a short circuit or overload with a 20A neutral of a branch, it is possible that the 100 A breaker won't trip, and the 20 A neutral will probably get hot enough to burn the there. What am I missing?

You are positing a short circuit of the 100A ungrounded conductor to the 20A neutral. Overload doesn't really figure here since that would be through the loads, which if properly wired would return current on the correct neutral.

It's possible that the short circuit won't trip the 100A breaker, but very likely it will. Or it will arc and blow the short circuit connection open again. For the short circuit to carry a sustained current that is less than 100A would be unlikely. Possible but unlikely. Perhaps a plausible scenario would be nicked insulation in an underground conduit filled with dirty water.

 

[Patpowers]

When you get into motor sizing for branch circuit short circuit and ground fault protection, you’ll find situations where a 14AWG may be protected by a 60A OCPD for branch circuit short circuit and ground fault protection. 430.52 C Table 430.52, single phase motor can have an inverse time breaker at 250 percent of the FLC, and with Exception No. 2 (3) inverse time circuit breaker may be increased up to 400 percent for FLC of 100A or less if needed to allow the motor to start.

This doesn’t apply in your case, but I think the short circuit and ground fault principle does..

 

[avargasm]

You are positing a short circuit of the 100A ungrounded conductor to the 20A neutral. Overload doesn't really figure here since that would be through the loads, which if properly wired would return current on the correct neutral.

It's possible that the short circuit won't trip the 100A breaker, but very likely it will. Or it will arc and blow the short circuit connection open again. For the short circuit to carry a sustained current that is less than 100A would be unlikely. Possible but unlikely. Perhaps a plausible scenario would be nicked insulation in an underground conduit filled with dirty water.

A rat or an arc could cause a 75A overload between the feeder and the neutral branch. Anyway, we are in the situation of having a 100A breaker protecting a 20A wire, and it is evident that this situation is not desirable or safe in any way.

I have seen 10A extension cords being literally evaporated in their insulation under a short circuit and a 20A breaker did not trip; instead, the short circuit extinguished itself by melting the copper of the extension cord. If the cord had been near flammable material, the building would have burned. It is a weak argument to say that a 20A wire (12 AWG) can safely trip a magnetic short circuit protection. It's a thing that will happen only under certain circumstances.

 

[jaggedben]

Can you point to any examples where this actually happened? It's not an important safety issue if it never actually happens.

 

[avargasm]

A rat or an arc could cause a 75A overload between the feeder and the neutral branch. Anyway, we are in the situation of having a 100A breaker protecting a 20A wire, and it is evident that this situation is not desirable or safe in any way.

I have seen 10A extension cords being literally evaporated in their insulation under a short circuit and a 20A breaker did not trip; instead, the short circuit extinguished itself by melting the copper of the extension cord. If the cord had been near flammable material, the building would have burned. It is a weak argument to say that a 20A wire (12 AWG) can safely trip a magnetic short circuit protection. It's a thing that will happen only under certain circumstances.

Errata:

A rat or an arc could cause a 75A overload between the feeder and the neutral branch. Anyway, we are in the situation of having a 100A breaker protecting a 20A wire, and it is evident that this situation is not desirable or safe in any way.

I have seen 10A extension cords being literally evaporated in their insulation under a short circuit and a 20A breaker did not trip; instead, the short circuit extinguished itself by melting the copper of the extension cord. If the cord had been near flammable material, the building would have burned. It is a weak argument to say that a 20A wire (12 AWG) can safely trip a magnetic short circuit protection from a 100A breaker. It's a thing that will happen only under certain circumstances.

 

[avargasm]

Can you point to any examples where this actually happened? It's not an important safety issue if it never actually happens.

I am designing a 7-store building and I am preventing it to happen. Reading the responses, I better "waste" some more conduit, run the feeders apart and don't take this chance.

Anyway, thank you everyone for your time and knowledge.

 

[retirede]

Hi.

The NEC allows running feeders and branch circuits in the same wireway or conduit. My question is: if a feeder with, say, a 100A breaker suffers a short circuit or overload with a 20A neutral of a branch, it is possible that the 100 A breaker won't trip, and the 20 A neutral will probably get hot enough to burn the there. What am I missing?

If you really think this is a critical issue, you should simply make sure any installation for which you are responsible is not constructed this way.
You may achieve your real goal of getting some folks here to continuing to argue with you, but I’m not one of them. I’ll simply wish you a good day, sir.

 

[Patpowers]

A rat or an arc could cause a 75A overload between the feeder and the neutral branch. Anyway, we are in the situation of having a 100A breaker protecting a 20A wire, and it is evident that this situation is not desirable or safe in any way.

I have seen 10A extension cords being literally evaporated in their insulation under a short circuit and a 20A breaker did not trip; instead, the short circuit extinguished itself by melting the copper of the extension cord. If the cord had been near flammable material, the building would have burned. It is a weak argument to say that a 20A wire (12 AWG) can safely trip a magnetic short circuit protection. It's a thing that will happen only under certain circumstances.

Actually you do not have a 100A protecting a 20A cable; (not wire, unless you ran thhn or other individual wires), your wire or cable isn’t 20A, it may be rated for 20A in your application. The cable or wire should be protected by a 20A breaker independent of the feeder’s 100A breaker, they are simple sharing the same raceway.

As for extension cords catching fire and evaporating into thin air: extension cords are usually rated for 12A or less, and have multiple things plugged in such as space heaters that requires an intensity of amps greater than 12A. Your breaker is rated for 20A, therefore extension cord goes up in smoke. Completely different scenarios. If the wiring method makes you feel unsafe, correct it to a method that gives you piece of mind..

 

[avargasm]

Actually you do not have a 100A protecting a 20A cable; (not wire, unless you ran thhn or other individual wires), your wire or cable isn’t 20A, it may be rated for 20A in your application. The cable or wire should be protected by a 20A breaker independent of the feeder’s 100A breaker, they are simple sharing the same raceway.

As for extension cords catching fire and evaporating into thin air: extension cords are usually rated for 12A or less, and have multiple things plugged in such as space heaters that requires an intensity of amps greater than 12A. Your breaker is rated for 20A, therefore extension cord goes up in smoke. Completely different scenarios. If the wiring method makes you feel unsafe, correct it to a method that gives you piece of mind..

The neutral wires are not protected by any OCPD. And yea, the example is with THHN. A 20A wire is usually a 12 or 10 AWG, that is what I mean, for short.

 

[Patpowers]

The neutral wire is bonded to your GES, 12AWG will clear a fault for 100A. I have plugged in a defective toaster and managed to trip the 200A OCPD. That was a 15A circuit on a 15A OCPD with 14AWG NM-B Cable. I fully understand why it may be concerning, but as long as the corresponding ungrounded and grounded conductors are ran in the same conduit for both the feeder circuit and branch circuit and your feeder is protected by 100A OCPD and the branch circuit protected by 20A OCPD, and the GES and EGC are correctly bonded, I wouldn’t be concerned. I myself have used share conduit for feeders and branch circuits and never had an issue. That being said, our NFPA70 is written in blood. Often times a new code is written or clarified by the sacrifice of someone’s life or property. Being that the code has not addressed this as an issue, I’m to assume that critical events haven’t occurred from doing this. Personally, I’ll only do what I feel comfortable with.
Last night our neutral bus bar burned up on our 200A main/meter combo. The utility company allowed me to tie my Grounding Electrode Conductor directly to their messenger. In addition we tied our neutrals together and connected to the messenger. Is this the safest thing to do? Probably not the best, but I have to move a building out of the way to have a new pole put in: the utility company condemned our pole due to a lightning strike a month ago which is probably what damaged my neutral bus. Needless to say, I will sleep comfortably tonight without concern of my house burning down. It’s my place, there’s a licensed electrician on site at all times, ( our shop is here also), and I’m comfortable with the temporary solution.

 

[avargasm]

The neutral wire is bonded to your GES, 12AWG will clear a fault for 100A. I have plugged in a defective toaster and managed to trip the 200A OCPD. That was a 15A circuit on a 15A OCPD with 14AWG NM-B Cable. I fully understand why it may be concerning, but as long as the corresponding ungrounded and grounded conductors are ran in the same conduit for both the feeder circuit and branch circuit and your feeder is protected by 100A OCPD and the branch circuit protected by 20A OCPD, and the GES and EGC are correctly bonded, I wouldn’t be concerned. I myself have used share conduit for feeders and branch circuits and never had an issue. That being said, our NFPA70 is written in blood. Often times a new code is written or clarified by the sacrifice of someone’s life or property. Being that the code has not addressed this as an issue, I’m to assume that critical events haven’t occurred from doing this. Personally, I’ll only do what I feel comfortable with.
Last night our neutral bus bar burned up on our 200A main/meter combo. The utility company allowed me to tie my Grounding Electrode Conductor directly to their messenger. In addition we tied our neutrals together and connected to the messenger. Is this the safest thing to do? Probably not the best, but I have to move a building out of the way to have a new pole put in: the utility company condemned our pole due to a lightning strike a month ago which is probably what damaged my neutral bus. Needless to say, I will sleep comfortably tonight without concern of my house burning down. It’s my place, there’s a licensed electrician on site at all times, ( our shop is here also), and I’m comfortable with the temporary solution.

Thank you for your time.

The neutrals are not protected by any OCPDs. They go right to the neutral bar, together with the feeder's neutrals. Current will flow from, say, live phase A, through the 100A breaker, to the feeder, to the fault point, to the 20A neutral branch wire, to the neutral bar. Earth wires do not help here, indeed they are another possible 20A route burned too by closing the circuit.

 

[avargasm]

The neutral wire is bonded to your GES, 12AWG will clear a fault for 100A. I have plugged in a defective toaster and managed to trip the 200A OCPD. That was a 15A circuit on a 15A OCPD with 14AWG NM-B Cable. I fully understand why it may be concerning, but as long as the corresponding ungrounded and grounded conductors are ran in the same conduit for both the feeder circuit and branch circuit and your feeder is protected by 100A OCPD and the branch circuit protected by 20A OCPD, and the GES and EGC are correctly bonded, I wouldn’t be concerned. I myself have used share conduit for feeders and branch circuits and never had an issue. That being said, our NFPA70 is written in blood. Often times a new code is written or clarified by the sacrifice of someone’s life or property. Being that the code has not addressed this as an issue, I’m to assume that critical events haven’t occurred from doing this. Personally, I’ll only do what I feel comfortable with.
Last night our neutral bus bar burned up on our 200A main/meter combo. The utility company allowed me to tie my Grounding Electrode Conductor directly to their messenger. In addition we tied our neutrals together and connected to the messenger. Is this the safest thing to do? Probably not the best, but I have to move a building out of the way to have a new pole put in: the utility company condemned our pole due to a lightning strike a month ago which is probably what damaged my neutral bus. Needless to say, I will sleep comfortably tonight without concern of my house burning down. It’s my place, there’s a licensed electrician on site at all times, ( our shop is here also), and I’m comfortable with the temporary solution.

As for a 15A circuit tripping a 200A breaker, as I said, depends on some factors. It's not the same with a toaster cord as with a 60 m. circuit. It even depends on ambient temperature, etc. It's like the Challenger accident: will the seal stop the gases before they burn it?==>saved. It's the same here: will the cable resistance (temperature) increase before the OCPD can cut it? ==> accident.

 

[Patpowers]

Where do earth wires come in, EGC’s are not earth wires, they are effective ground fault paths back to your service grounded neutral conductor. We don’t protect neutrals with OCPDs, they all should go back to the same point. I’m not understanding the concern with the 12AWG neutral? Why the fear that it will become energized by the feeders? There seems to be something more going on!

 

[avargasm]

Where do earth wires come in, EGC’s are not earth wires, they are effective ground fault paths back to your service grounded neutral conductor. We don’t protect neutrals with OCPDs, they all should go back to the same point. I’m not understanding the concern with the 12AWG neutral? Why the fear that it will become energized by the feeders? There seems to be something more going on!I

Well, by "earth wires" I mean the green wires that run with the circuits. EGCs. They can act the same as the neutrals, with the same perils.

What I mean is that the earth system won't save you in this example. It will just make things worst.

 

[avargasm]

Where do earth wires come in, EGC’s are not earth wires, they are effective ground fault paths back to your service grounded neutral conductor. We don’t protect neutrals with OCPDs, they all should go back to the same point. I’m not understanding the concern with the 12AWG neutral? Why the fear that it will become energized by the feeders? There seems to be something more going on!

"We don’t protect neutrals with OCPDs, they all should go back to the same point."

That's what I was saying all this time. And that's the risk.

Look: the path is easy: Phase, 100A breaker, 100A feeder (I mean, 1/0 AWG), fault point, 20A neutral (I mean, 12 AWG), neutral bar: short circuit or high load circuit closed. Easy.

 

[Patpowers]

I wouldn’t call then earth wires, that seems to imply that they are to send faults to earth and that’s not the case. Both equipment grounds and neutrals should allow ground fault current and short circuit current to go to the same point: at the service. The only type of fault contact to earth will open is a Class A ground fault protection circuit. Even then the preferred fault path is to the service. I’m not sure of your electrical system, but there must be reasons for your concerns.

 

[avargasm]

I wouldn’t call then earth wires, that seems to imply that they are to send faults to earth and that’s not the case. Both equipment grounds and neutrals should allow ground fault current and short circuit current to go to the same point: at the service. The only type of fault contact to earth will open is a Class A ground fault protection circuit. Even then the preferred fault path is to the service. I’m not sure of your electrical system, but there must be reasons for your concerns.

IKR. I have not talked about ground faults clearing to earth. Not related at all to the discussion.