Wire size for commercial locations
- Thread starter raymack
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Re: Wire size for commercial locations
Hello Kfenn, Your reference to 310.5 per the table and voltages is correct.
Raymack's question was if the NEC prohibited #14 for 120 volt circuits in commercial installations which it does not, but many engineers do prohibit it in their design and contract documents.
Roger
[ September 24, 2003, 10:55 AM: Message edited by: roger ]
Hello Kfenn, Your reference to 310.5 per the table and voltages is correct.
Raymack's question was if the NEC prohibited #14 for 120 volt circuits in commercial installations which it does not, but many engineers do prohibit it in their design and contract documents.
Roger
[ September 24, 2003, 10:55 AM: Message edited by: roger ]
charlie
Senior Member
- Location
- Indianapolis
Re: Wire size for commercial locations
If a home is served from a pad mounted transformer that is close to the service, the service is underground (the electric utility will use larger cable), and the 15 ampere circuit is fed from the service equipment then the fault current available will be significant (close to 7000 at the bushings of the transformer if it is as small as a 25 kVA or 14000 for a 50 kVA). Just how safe is the #14 wire that is close to the service equipment?
The previous information was for residential use. Just think about the large increase of available fault current with large three phase transformers. In our downtown area where we have a network, the available fault current will approach 100000 amperes (yes that is a six figure number). Rather than taking a chance, most engineers will specify no #14 wire is to be used in the structure.
Actually it is an NEC issue. "110.10 Circuit Impedance and Other Characteristics. The overcurrent protective devices, the total impedance, the component short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit-protective devices used to clear a fault to do so without extensive damage to the electrical components of the circuit." If you look at page 50 in Bussmann's SPD you will find a Copper, 75? Thermoplastic Insulated Cable Damage Table. You will find that #14 wire is good for only 3400 amperes Maximum Short-Circuit Withstand Current in 1/4 cycle.
If a home is served from a pad mounted transformer that is close to the service, the service is underground (the electric utility will use larger cable), and the 15 ampere circuit is fed from the service equipment then the fault current available will be significant (close to 7000 at the bushings of the transformer if it is as small as a 25 kVA or 14000 for a 50 kVA). Just how safe is the #14 wire that is close to the service equipment?
The previous information was for residential use. Just think about the large increase of available fault current with large three phase transformers. In our downtown area where we have a network, the available fault current will approach 100000 amperes (yes that is a six figure number). Rather than taking a chance, most engineers will specify no #14 wire is to be used in the structure.
- Location
- Illinois
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- retired electrician
Re: Wire size for commercial locations
Charlie,
The rating for #12 is only 5400 amps. Wouldn't that too be a problem for some commerical installations?
Don
Charlie,
The rating for #12 is only 5400 amps. Wouldn't that too be a problem for some commerical installations?
Don
Re: Wire size for commercial locations
Charlie, very good point. However, with the amount of AFC quoted, wouldn't #12 also be destroyed, along with other sizes?
I have used #14 for fixed load, control and motor branch circuits, on commercial and industrial projects.
Variable load circuits were usually in #12 minimum.
Charlie, very good point. However, with the amount of AFC quoted, wouldn't #12 also be destroyed, along with other sizes?
I have used #14 for fixed load, control and motor branch circuits, on commercial and industrial projects.
Variable load circuits were usually in #12 minimum.
charlie
Senior Member
- Location
- Indianapolis
Re: Wire size for commercial locations
Roger, Don, and Bennie, you guys are thinking and are on it. The conductor damage curves sometimes need to be looked at as well as the nameplate impedance of the transformer that feeds the installation.
From a real world standpoint, the standard fault currents we supply are the maximum anyone can expect on our system (using the lowest impedance on any transformer we have of a particular size, type, and voltage). The fault currents are given at RMS values, assuming a bolted fault and an infinite bus. I think we all know that neither a bolted fault will happen (dynamic impedance will be introduced into the circuit) or an infinite bus will exist (where the primary voltage will not sag with a bolted fault introduced on the secondary side).
Assuming the worst case scenario, #12 Cu. could not be used any more than #14. In the real world, #12 will normally do just fine.
Roger, the EC is responsible for protecting the conductors.
Roger, Don, and Bennie, you guys are thinking and are on it. The conductor damage curves sometimes need to be looked at as well as the nameplate impedance of the transformer that feeds the installation.
From a real world standpoint, the standard fault currents we supply are the maximum anyone can expect on our system (using the lowest impedance on any transformer we have of a particular size, type, and voltage). The fault currents are given at RMS values, assuming a bolted fault and an infinite bus. I think we all know that neither a bolted fault will happen (dynamic impedance will be introduced into the circuit) or an infinite bus will exist (where the primary voltage will not sag with a bolted fault introduced on the secondary side).
Assuming the worst case scenario, #12 Cu. could not be used any more than #14. In the real world, #12 will normally do just fine.
Roger, the EC is responsible for protecting the conductors.
lady sparks lover
Senior Member
Re: Wire size for commercial locations
you know, i was told even while learning in school that "good practice" is using #12 and up. Usually anything below, is for control wiring.
I kind of agree with Charlie, it's really based on the short circuit capacity, not on the code. You can use number #14, but #12 is very common for the short circuiting reasons.
Lady
[ September 24, 2003, 03:21 PM: Message edited by: lady sparks lover ]
you know, i was told even while learning in school that "good practice" is using #12 and up. Usually anything below, is for control wiring.
I kind of agree with Charlie, it's really based on the short circuit capacity, not on the code. You can use number #14, but #12 is very common for the short circuiting reasons.
Lady
[ September 24, 2003, 03:21 PM: Message edited by: lady sparks lover ]
Re: Wire size for commercial locations
Would a motor fed with #12, with a FLA of 6 amps, have an increase in locked rotor current than it would with #14?
I hope this makes sense. I'm trying to say "can a larger than necessary conductor raise the level of damage by a fault?
Would a motor fed with #12, with a FLA of 6 amps, have an increase in locked rotor current than it would with #14?
I hope this makes sense. I'm trying to say "can a larger than necessary conductor raise the level of damage by a fault?
Re: Wire size for commercial locations
Bennie good question. If the conductor is looked at as being an extension of the "start winding" smaller would seem to be better. (higher resistance and lower reactance than run winding)
Roger
Bennie good question. If the conductor is looked at as being an extension of the "start winding" smaller would seem to be better. (higher resistance and lower reactance than run winding)
Roger
- Location
- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: Wire size for commercial locations
But who cares? Motors are not assigned rated values of fault current. A fault upstream of the motor (along the branch circuit) would not damage the motor itself. A fault internal to the motor might be slightly worse (i.e., with the added current being caused by oversized conductors). But future repair costs and down time are not the concern of the NEC. The event would still be terminated by the opening of an upstream breaker. That breaker and the panel in which it is installed must be selected with a fault current rating that exceeds the highest fault they would see (i.e., with a short circuit internal to the panel). A fault in a downstream branch circuit would necessarily have a lower fault current, due to the resistance of the branch circuit conductors - no matter their length and size.
Yes. Reference: Ohm?s Law. But would the higher value of current (in the locked rotor condition) exceed the rated FLA? Only if the applied voltage exceeded the value upon which the rated FLA was based. If you connected a motor rated at 460 volts to a 480 volt panel, and if you oversized the conductors, you could get more than the rated FLA.
Yes. A fault internal to the motor would see higher current, if the applied voltage were higher, which would be the case if the branch circuit conductors had a lower resistance, which would be the case if you oversized them.
But who cares? Motors are not assigned rated values of fault current. A fault upstream of the motor (along the branch circuit) would not damage the motor itself. A fault internal to the motor might be slightly worse (i.e., with the added current being caused by oversized conductors). But future repair costs and down time are not the concern of the NEC. The event would still be terminated by the opening of an upstream breaker. That breaker and the panel in which it is installed must be selected with a fault current rating that exceeds the highest fault they would see (i.e., with a short circuit internal to the panel). A fault in a downstream branch circuit would necessarily have a lower fault current, due to the resistance of the branch circuit conductors - no matter their length and size.
Re: Wire size for commercial locations
Please feel free to correct me if the following is wrong.
A locked rotor is a short circuit, or a line to line fault. The magnitude of current is directly related to the amount of smoke. If the NEC does not care, I do.
Short circuit devices are calculated to open the circuit before heat can damage the winding. The time/trip curve is selected based on the withstand rating of the winding insulation.
Any radical departure from the design values will defeat the protective system.
Please feel free to correct me if the following is wrong.
A locked rotor is a short circuit, or a line to line fault. The magnitude of current is directly related to the amount of smoke. If the NEC does not care, I do.
Short circuit devices are calculated to open the circuit before heat can damage the winding. The time/trip curve is selected based on the withstand rating of the winding insulation.
Any radical departure from the design values will defeat the protective system.
Ed MacLaren
Senior Member
Re: Wire size for commercial locations
Locked rotor current is approximately six times nameplate rated current.
Ed
Not exactly. The motor winding still has some impedance, even without the rotor moving.
Locked rotor current is approximately six times nameplate rated current.
Ed
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Wire size for commercial locations
Not quite true. The windings of a motor have a significant amount of impedance, as compared, say, to a wrench dropped across the motor terminals. The impedance is primarily inductive, and is the result of wrapping the coils of wire. If you physically prevent a motor from turning, and then hit the ?on? button, it will draw between 6 to 14 times the normal running current. This is far below what would be considered a line-to-line fault. A wrench in contact with the terminals could draw more than 1000 times the normal running current. The instantaneous trip feature of the circuit breaker will terminate this event. It?s purpose is personnel safety, not the protection of cables or motor windings.
The devices you are describing here are the thermal overloads. Motor protection in a jammed-rotor condition comes not from the breaker, but from the thermal overloads.
Replace the #12 branch circuit conductors with superconductive bus bars, and you still won?t defeat the protective system. When the thermal overloads see current values high enough and long enough to risk damaging the motor, they trip. When the circuit breaker sees fault-levels of current, it will trip. They will do their jobs no matter what had caused the high current levels.
Re: Wire size for commercial locations
I attended a product service school at the Carrier Corporation plant in California. The compressor motor test lab was quite impressive.
The quality control procedure is to destruction test a motor, from the assembly line, at pre-determined intervals.
One test is to lock the rotor and measure the temperature rise, with operating voltage applied.
When the temperature reaches a value in excess of the insulation rating, this is compared to the parameters set for proper operation of the short circuit protective device. This value is reached much faster than the thermal elements in the running overloads.
This is the reason for exact size of circuit breaker, or fuse being specified for an AC unit.
This is also the reason for the breakers being HACR listed.
[ September 24, 2003, 06:58 PM: Message edited by: bennie ]
I attended a product service school at the Carrier Corporation plant in California. The compressor motor test lab was quite impressive.
The quality control procedure is to destruction test a motor, from the assembly line, at pre-determined intervals.
One test is to lock the rotor and measure the temperature rise, with operating voltage applied.
When the temperature reaches a value in excess of the insulation rating, this is compared to the parameters set for proper operation of the short circuit protective device. This value is reached much faster than the thermal elements in the running overloads.
This is the reason for exact size of circuit breaker, or fuse being specified for an AC unit.
This is also the reason for the breakers being HACR listed.
[ September 24, 2003, 06:58 PM: Message edited by: bennie ]
G
Guest
Guest
Re: Wire size for commercial locations
There are many strategies for protecting a motor load with #14 or other supply conductors:
Starting heaters;
Fuses & heaters;
Circuit breakers;
Thermal sensors.
I found a site that breaks it down quite nicely. At the bottom of this linked page there are five more pages on the different thermal sensors (Temperature Switches, Resistance Temperature Detectors-- RTD's, Thermistors, Thermasentry, & Thermocouples):
Methods of Winding Temperature Protection
I anticipate that this is still cogent to the thread. It's old hat for the old cats but should be helpful to all.
There are many strategies for protecting a motor load with #14 or other supply conductors:
Starting heaters;
Fuses & heaters;
Circuit breakers;
Thermal sensors.
I found a site that breaks it down quite nicely. At the bottom of this linked page there are five more pages on the different thermal sensors (Temperature Switches, Resistance Temperature Detectors-- RTD's, Thermistors, Thermasentry, & Thermocouples):
Methods of Winding Temperature Protection
I anticipate that this is still cogent to the thread. It's old hat for the old cats but should be helpful to all.
- Location
- Massachusetts
Re: Wire size for commercial locations
To me that makes these items sound optional.
All motors need thermal protection.
Sometimes built or designed in to the motor, but if not we must put both thermal and short circuit, ground fault protective devices in place.
You may end up with a motor fed with 14 AWG on a 15, 40, 50 amp breaker by following the rules in 430 and still have a perfectly safe installation.
It is important to remember that breakers and fuses for motors are not overcurrent protective devices.
Breakers and Fuses for motor loads are short circuit and ground fault protective devices, at least the ones feeding the branch circuit conductors.
Fuses may be used at the motor starter to "fine tune" the protection.
[ September 25, 2003, 05:49 AM: Message edited by: iwire ]
Wayne I am curious why you used the term "strategies"?
To me that makes these items sound optional.
All motors need thermal protection.
Sometimes built or designed in to the motor, but if not we must put both thermal and short circuit, ground fault protective devices in place.
You may end up with a motor fed with 14 AWG on a 15, 40, 50 amp breaker by following the rules in 430 and still have a perfectly safe installation.
It is important to remember that breakers and fuses for motors are not overcurrent protective devices.
Breakers and Fuses for motor loads are short circuit and ground fault protective devices, at least the ones feeding the branch circuit conductors.
Fuses may be used at the motor starter to "fine tune" the protection.
[ September 25, 2003, 05:49 AM: Message edited by: iwire ]
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