Size Mobile Home Cable?

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  #1  
Old 08-10-17, 02:00 PM
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Size Mobile Home Cable?

When suggesting MH cable here for 100 amp subpanels what to suggest for 100 amp subpanels. Size jumps from 2-2-4-6 to 2/0-2/0-1-4. Of course the second size is good for 100+ amps but there seems to be no between size and it has been written #2 between panel boards is only good for 90 amps yet it seems it is being sold for mobile homes to supply 100 amps between the first OCPD on the pole to the panel in the MH. (Yes I know it would be rated 100. meter to first OCPD but I'm talking subpanels.)

Wondering what you pros recommend main panel to sub with a 100 amp breaker.

Example of sizes available I see on line. Quadruplex Aluminum Cable Type Mobile Home Feeder
 
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Old 08-10-17, 03:50 PM
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Providing there are no voltage drop issues, for 100A subpanel feed I recommend #1 Al or #3 Cu for the current carrying conductors and a #6 Al or #8 Cu for the ground. If using preassembled cable such as MHF or URD you are limited to a few sizes. SE comes in more sizes.
 
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Old 08-10-17, 05:14 PM
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SE comes in more sizes.
But in many case here we are talking about buried cables to detached structurers. Consensus on the net seems to be SE is not suitable for burial even in conduit so not really an option in many of the threads here.
 
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Old 08-10-17, 05:21 PM
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True. When I added SE I was just thinking subpanels in same building. Detached building using direct bury for a 100A I'd use RHH/RHW/USE in the sizes I stated. That's the same wire used in MHF. If using conduit I'd go with XHHW or THWN.
 
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Old 08-10-17, 06:30 PM
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But if USE isn't dual rated you have to transition to another type of wire where it enters the building. I'm really just basically asking isn't MH cable the easiest choice for subpanels in detached building so what size? Use the smaller available size and down size the breaker or go to next size up which is over size. It is almost as if the smaller size is being sold for 100 amp service to MH. In fact my link lists it as 100 amp with no qualifier only between meter and first OCPD. Just wondering what is commonly done. Are we being too strict requiring a 90 amp breaker.
 

Last edited by ray2047; 08-10-17 at 06:47 PM.
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Old 08-10-17, 07:02 PM
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When you say are we being too strict, I can only cite the NEC and the #2 is limited to 90A as a sub feeder. If the full 100A is needed then the next available size up in MHF is 2/0 which is good up to 135A. It's one of those things with the NEC that makes no sense where the #2 Al is okay when supplying the full service load to a dwelling and can be used for 100A, but as a branch feeder it's 90A. That's why I said I'd just use (RHH/RHW/USE) in #1 for 100A instead of buying the preassembled 2/0 MHF to get 100A. I've known inspectors here where I live to let #2 Al as a branch feeder to be protected at 100A.

To say MHF is the easiest choice for buried sub feeds is subjective and may be the easiest for one case and not for another.
 
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Old 08-10-17, 07:12 PM
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Thank you for the input. .
 

Last edited by ray2047; 08-11-17 at 11:25 AM.
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Old 08-11-17, 10:07 AM
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I've known inspectors here where I live to let #2 Al as a branch feeder to be protected at 100A.
Same here. A branch feeder is a branch circuit and something different than a dwelling feeder.


As far as a dwelling feeder, consider Article 310.15. If we have a 100 amp service at a meter pole with #2 aluminum service entrance conductors to a 100 amp disconnect, the 100 amp feeder to the mobile home isn't required to be larger than the service entrance conductors so using 2-2-2-4 MHF would be acceptable. But, if we run #2 aluminum branch feeder to a machine, the machine circuit would need to be protected at 90 amps.

It's all in the interpretation.
 
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Old 08-11-17, 11:28 AM
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Thread is reopened due to new post.

Thank you, Joe, that answers my question why they seem to be selling MH cable for 100 amp use to subpanels in mobile homes.
 
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Old 08-11-17, 04:17 PM
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A feeder in a dwelling you can use #2 under table 310.15(B)(7) (which has changed in 2014 code and newer) A branch circuit you need to follow 310.15(B)(16).
 
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Old 08-11-17, 07:11 PM
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A feeder in a dwelling you can use #2 under table 310.15(B)(7) (which has changed in 2014 code and newer) A branch circuit you need to follow 310.15(B)(16).
I don't have a copy of the 2014 code. What does it say if the service and panel at the pole is 200 amps and a 100 amp breaker is used to feed the subpanel in a mobile home? Can the 2-2-2-4 be used as the feeder to the subpanel?
 
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Old 08-11-17, 08:05 PM
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2014 changed it from an easy to use chart to a calculation, likely due to differences in wire types and termination ratings. The code now reads:

For one family and the individual dwelling units of two family and multifamily dwellings, service and feeder conductors supplied by a single phase, 120/240 volt system shall be permitted to be sized in accordance with 310.15(B)(7)(1) through (4)

(1) For a service rated 100 through 400amps, the service conductors supplying the entire load associated with a one family dwelling, or the service conductors supplying the entire load associated with an individual dwelling unit in a two family or multifamily dwelling, shall be permitted to have an ampacity not less then 83 percent of the service rating.

(2) For a feeder rated 100 through 400amps, the feeder conductors supplying the entire load associated with a one family dwelling, or the feeder conductors supplying the entire load associated with an individual dwelling unit in a two family or multifamily dwelling, shall be permitted to have an ampacity not less then 83 percent of the service rating.

(3) In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater then that specified in 310.15(B)(7)(1) or (2).
(4) Just talks about the grounded conductor can be smaller then the ungrounded conductors.

So, 83% of 100 amps is 83 amps. #2 aluminum is rated at 90 amps under the 75 degree column. I am not sure what MHF wire is (it should be marked) but I would be willing to guess it is USE-2.
 
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Old 08-11-17, 08:14 PM
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MHF is made up multiple rated conductors. The ratings are RHH/RHW-2/USE-2
 
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Old 08-11-17, 08:40 PM
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but I would be willing to guess it is USE-2.
I guess my guess was correct then.
 
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Old 08-12-17, 12:03 AM
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Great info guys. So should we recommend #2 MH cable for 100a subpanels here on the board or just say use a 90 amp breaker just to be on the safe side and refer them to local AHJ if they want to use a 100 amp breaker?
 
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Old 08-12-17, 06:04 AM
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For a feeder or service, I would be comfortable recommending 100 amps on #2 MHF. 90 amp breakers are also not as readily available at the home stores, so it makes it more difficult for a DIYer to get. Of course all this will depend if they are on 2014 code or newer. 2011 code just has the table showing #2 aluminum OK for 100 amps on a service or feeder to a dwelling.
 

Last edited by Tolyn Ironhand; 08-12-17 at 06:23 AM.
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Old 08-12-17, 04:56 PM
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I guess my guess was correct then.
Partially. If MHF was just USE-2 it could not be installed inside the structure.

I'm sticking to 90A for #2 Al as a branch feeder.
 
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Old 08-14-17, 07:55 AM
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Inspections around here now hold tight on 90A for #2 aluminum subpanel feeders. A 100A feeder requires #1 or larger aluminum. My usual solution is just to use a 90A breaker. How common is it for a subpanel load calculation to actually land between 90A and 100A? Almost never, and in that case I would recommend upgrade to 150A to allow for future expansion anyway. No reason to pick one arbitrary service size (100A) over another (90A) in the absence of a firm calculation.
 
 

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