Why 2/0 EGC with 4/0, 4/0, 4/0, 2/0 Mobile home feeder?

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Hv&Lv said:
Please do.
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Now, 1,500 amps. 120 volts / 1,500 amps = 0.08 ohms loop impedance.


According to the values in chapter 9 table 9-

250 AL has an X of 0.041 and an R of 0.085 per 1000 feet

2/0 AL has an X of 0.043 and an R of 0.16 per 1000 feet

0.0378 ohms at 300 feet

0.0609 ohms at 300 feet

Adding to 0.0987 ohms or 1,215 amps

A simple 300 foot run easily places typical breakers into a 5-10 second clearing time which exceed the short time withstand of code minimum EGCs.

To put it bluntly a half size EGC simply can not overheat, where as 250.122 assumes the breaker will trip within its magnetic threshold as with most runs inside a building. Now If you've got the fault current there is nothing wrong with 250.122 and thats why code lets you go that small. Otherwise adiabatic limts come into play.
 
mbrooke said:
Now, 1,500 amps. 120 volts / 1,500 amps = 0.08 ohms loop impedance.


According to the values in chapter 9 table 9-

250 AL has an X of 0.041 and an R of 0.085 per 1000 feet

2/0 AL has an X of 0.043 and an R of 0.16 per 1000 feet

0.0378 ohms at 300 feet

0.0609 ohms at 300 feet

Adding to 0.0987 ohms or 1,215 amps

A simple 300 foot run easily places typical breakers into a 5-10 second clearing time which exceed the short time withstand of code minimum EGCs.

To put it bluntly a half size EGC simply can not overheat, where as 250.122 assumes the breaker will trip within its magnetic threshold as with most runs inside a building. Now If you've got the fault current there is nothing wrong with 250.122 and thats why code lets you go that small. Otherwise adiabatic limts come into play.
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The 12 1/2 % ECG rule of thumb comes to mind, or just 1/8th of the feeders
~RJ~
 
No explanation for the grounding, but at one time RV park feeders required full size neutrals. Southwire makes it both ways and, conjecture only, the RV requirement might be a reason.
 
augie47 said:
No explanation for the grounding
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because the NEC does not address VD in ECG's , but the physics exist Augie

Consider a recent thread, and UL standard of 75' for a GEC, being a similar animal to a ECG

They seemed to have figured it out via some study......

with ECG's we are looking at extended mag trip times, if they trip at all

~RJ~
 
You can post all the calculations you want but that still doesn't prove all the manufactures of type SE cable decided it and only it needs an oversized bare conductor to be used as an EGC.

Type SE cable = Service Entrance cable. It was originally designed for service entrance applications that don't have an EGC. The bare conductor was and still is used as a grounded conductor. In previous additions of the NEC the grounded conductor for residential services was allowed to be 2 sizes smaller than the ungrounded conductors. 338.10(B)(2) exception still recognizes the bare conductor being use for a grounded conductor for existing installations for branch circuits and feeders..

The bare or green conductor in NM, MC or any other cable has never been recognized as approved for use as the grounded conductor.
 
curt swartz said:
You can post all the calculations you want but that still doesn't prove all the manufactures of type SE cable decided it and only it needs an oversized bare conductor to be used as an EGC.

Type SE cable = Service Entrance cable. It was originally designed for service entrance applications that don't have an EGC. The bare conductor was and still is used as a grounded conductor. In previous additions of the NEC the grounded conductor for residential services was allowed to be 2 sizes smaller than the ungrounded conductors. 338.10(B)(2) exception still recognizes the bare conductor being use for a grounded conductor for existing installations for branch circuits and feeders..

The bare or green conductor in NM, MC or any other cable has never been recognized as approved for use as the grounded conductor.
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Correct. And that was indeed one major reason why the bare wire was sized larger than 250.122.

Though IMO I think it is unlikely that the bare wire in 5 conductor SER would be used as a grounded conductor-

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Mobile Home Feeder Cable is primarily designed to feed Mobile Homes (550) but the Conductor/Cable is Type USE and covered by Article 338.
 
kwired said:
Doesn't specifically cover multiplex assembly commonly called mobile home feeder cable but rather contains the general requirements for all wiring methods that might be used.
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Not sure of issue you are alluding to why this reference not legitimate. A mobile home feeder cable is simply different for 1, specifically and biggest difference, due to requirement for an insulated ground conductor per that required in 550.33(A)(1), thus terminology of MHF applied to the 4 insulated conductor type cable as it is the common use for such.
A multiplex is a type of service conductor and can be used as a feeder as well.
Refer back to 338.2 definition....... or multiconductor assembly with or without an overall covering, .....
And then 338.10(B)(1)
330.10 Uses permitted.
330.10(B) Branch Circuits of Feeders
330.10(B)(1) Grounded Conductor Insulated.
Based on this information MHF multiplex cable falls into this category and meets the definition and permited use.
The "general requirements" of 550.33 does apply, no matter the type of conductors or cord must meet the listed requirements of the Article.
Additional requirements found in 550.4(B), 550.10(A), 550.10(I)(1) insulated ground conductor; 550.18, 550.32. These all reference service or feeder conductors.
This section is not particularly laid out efficiently for ease of getting the information coherently addressed within the NEC 2017, it tends to jump around a lot.
 
Fred B said:
Not sure of issue you are alluding to why this reference not legitimate. A mobile home feeder cable is simply different for 1, specifically and biggest difference, due to requirement for an insulated ground conductor per that required in 550.33(A)(1), thus terminology of MHF applied to the 4 insulated conductor type cable as it is the common use for such.
A multiplex is a type of service conductor and can be used as a feeder as well.
Refer back to 338.2 definition....... or multiconductor assembly with or without an overall covering, .....
And then 338.10(B)(1)
330.10 Uses permitted.
330.10(B) Branch Circuits of Feeders
330.10(B)(1) Grounded Conductor Insulated.
Based on this information MHF multiplex cable falls into this category and meets the definition and permited use.
The "general requirements" of 550.33 does apply, no matter the type of conductors or cord must meet the listed requirements of the Article.
Additional requirements found in 550.4(B), 550.10(A), 550.10(I)(1) insulated ground conductor; 550.18, 550.32. These all reference service or feeder conductors.
This section is not particularly laid out efficiently for ease of getting the information coherently addressed within the NEC 2017, it tends to jump around a lot.
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Not saying it isn't legitimate, but what many market as "mobile home feeder cable" is nothing more than 4 conductors multiplexed that have individual rating of USE/RHW/RHH. It is not a stand alone NEC wiring method like NM, MC, UF, or SE cables are. Pull 4 single RHW conductors of same sizes that are in the multiplex assembly and you essentially have the same thing as far as NEC is concerned.

MH feeder cable is nothing more than a convenience in some ways of having all four conductors in a ready to pull assembly. Great if you pulling through raceway only or direct burying with sleeve where you come out of the ground. Can be a pain to impossible to work with if you have pull boxes or conduit bodies in the run though, vs running individual conductors instead of a multiplexed assembly.
 
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