Why 2/0 EGC with 4/0, 4/0, 4/0, 2/0 Mobile home feeder?
- Thread starter jchelm24
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Could it be for 3 phase and a reduced neutral?
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- New Jersey
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Could be for paralleling multiple sets to make a larger feeder.
The 2/0 EGC is colored green in the cable assembly.
One of the 4/0's has a white stripe to denote the neutral. I would think single phase installs would be the common use?
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- Chapel Hill, NC
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- Retired Electrical Contractor
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- New Jersey
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- Journeyman Electrician
That's another possiblity. I didn't do the calculation but the ratio of #4/0-#2/0 seems like a big increase in the EGC size.
Even with 250.122(B) the egc would be 6awg copper or 4 awg aluminum. It's just confusing as to why the egc is so big in this case.
mopowr steve
Senior Member
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- NW Ohio
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- Electrical contractor
Me too,
When I get Mobile Home Feeder at supply house it has #4 EGC but when I go to Lowe’s they have it with the larger EGC
When I get Mobile Home Feeder at supply house it has #4 EGC but when I go to Lowe’s they have it with the larger EGC
mbrooke
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mbrooke
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Cable manufacturers found out that with longer runs, breaker opening times increased, and with that EGC heating.
In short its to prevent the EGC from melting.
curt swartz
Electrical Contractor - San Jose, CA
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- San Jose, CA
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I'm 99.999% sure that has nothing to do with it. If it was they would use larger EGC's for NM and MC cable.
I have always been under the impression the EGC in SE cable is sized for use as a grounded conductor. I doubt 4 conductor cable gets used that way often so not sure why they don't offer 2 options.
Sometimes I install pipe and wire instead of 4/0 SER to avoid having to deal with the oversized BARE wire.
mbrooke
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They already do. #14 -#10 comes with a full size EGC.
But to answer larger sizes of NM and MC cable is rarely run more than a few hundred feet. Thus it is unlikely a manufacturer has ever had such an issue brought to their attention.
curt swartz
Electrical Contractor - San Jose, CA
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- San Jose, CA
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- Electrical Contractor
The NEC requires full size EGC for circuits 30 amp and less.
There is no reason to bring anything to the manufactures attention. The NEC and UL have no issue with current EGC sizing. You are the only one that thinks its a big problem.
kwired
Electron manager
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Could be single phase where full size neutral is required. (maybe 120/208 three wire)
Or corner ground feeder, white stripe is for grounded phase.
Either case though typically you would only need a 2 AWG EGC unless upsized for VD.
mbrooke
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After reduced size NM proved insufficient.
No one has to bring anything to the manufacturer's attention, they already know and have known for decades. Hence why up sized EGCs have been the norm for URD and mobile home cables for some time now.
NEC and UL has zero issue with a minimum limit as forth in 250.122.
However, what many fail to realize is that 250.122 is just that, a minimum limit. It is not my consensus, rather that of major organizations including the IEEE Green book.
mbrooke
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If I may, citing one of my sources, the IEEE Green Book:
Real world example of an overheated EGC:
I can show you the math if you like, but to keep things simple the more time an EGC carries current the hotter it will get. Long runs like those found in mobile home parks and rural pedestals increase breaker opening time due to the added impedance of the wire run itself.
Please do.
mbrooke
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Copper or AL?
I'll start with copper.
According to typical time current curve a 200 amp breaker takes 6 times the rated current to open in 5 seconds. That gives us 1,200 amps of fault current.
Thus, we turn to this equation:
Assuming a starting temperature of 75*C and a finishing temperature of 150*C- the maximum time withstand of a #6 copper conductor is 1.34 seconds.
Assuming a more realistic 40*C starting and 150*C finishing, the maximum time limit is 2.08 seconds.
If we assume the conductor is subjected to the full 5 seconds of fault current with a starting at a temp 40*C, at 5 seconds in the conductor will heat to 390*C.
At 390*C not only will the insulation compromise, but the copper will anneal loosening under the lug resulting in breaking of the EGC path once the copper cools back down.
In reality trip time will be a bit longer as the copper heats up and R goes up further reducing current and in turn breaker opening time.
Now, if we assume a #1 copper EGC, the same equation gives us 21 seconds in order to reach 150*C starting from 40*C. 13.6 seconds from 75*C to 150*C.
As can be seen, with a half size EGC 5 seconds is well before the EGC has even a remote capability to overheat.
mbrooke
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Now for Aluminum-
Using a published time current curve from an Eaton Type EDB breaker:
We obtain 1,500 amps at 10 seconds under maximum tolerance.
To raise #4 AL from 40*C to 150*C with 1500 amps, it takes only 1.45 seconds.
To raise #6 CU from 40*C to 150*C with 1500 amps it takes only 1.33 seconds.
Now, if both are subjected to 1,500 amp for 10 seconds:
#4AL will heat to 2,700*C
#6 CU will heat to 3,200*C
In both cases the EGC will melt, whereby the metal will phase change into a liquid state of matter.
Now, half size EGCs subjected to 1,500 amps of current for 10 seconds:
# 2/0 AL will heat to 111*C
#1 CU will heat to 118 *C.
As can be seen from the equation in post #19, a half size EGC will not approach 150*C under worse case conditions using official time current curves, where as an EGC sized to table 250.122 will melt open before the breaker trips.
Typically, 1080*C is the established melting point for copper. Above 250*C is annealing, above 150*C is considered insulation damage.
As long as the conductor does not exceed 150*C, no harm is done.
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