100a vs 90a help

[Spyider]

ok im going to use 2-2-2-4 MHF wire pulled inside 2" conduit about 100ft give or take a few...
Now my actual Q is this. i got a new box that is a main, 100a.. gona make it into my subpannel in the garage. so i bought a matching 100a breaker to put in my main.. i have been reading so many things about how that wire is only rated at 90a.. but then i go to a diffrent website/page and am told that 100a will be fine!! WTF. i need to know if my 100a breaker in the main box will be fine or if that will be overloading the wire and i need to get a 90a for the main.

still got the receipt so if i need to switch it its fine since i have to go get some larger ground lug connections also that i forgot about lol. just wana make one trip.

 

[Mustang51js]

If it's a service feeder then it can be 100 amps. But since your using to feed a subpanel you have to drop it down

 

[Norcal]

Ask the other website to justify their saying #2 AL is fine for 100A, in the 2011 NEC on back it is allowed for 100A residential services where it carries the ENTIRE load of a dwelling, no subpanels, no feeders to outbuildings, otherwise table 310.16 or table 310.15(B)(16) in the 2014 NEC allows 2 AWG AL as 90 ampere in the 75 degree column, the 100A main in the panel is fine, but a 90A breaker is required to supply it. Note: in the 2014 NEC, 2 AWG aluminum is still allowed for a dwelling just have to go about it differently.

 

[Norcal]

If it's a service feeder then it can be 100 amps. But since your using to feed a subpanel you have to drop it down

It has to carry the entire load of a dwelling.

 

[Spyider]

ok thanks. yes service coming into my home is 200a. i wanted 100a in the garage just because its a round number lol. so i guess ill go return that 100a breaker and get a 90a thanks for the help. cant imagine ill even notice the difference.

 

[Speedy Petey]

Right or wrong, many areas do allow a 100A breaker on a #2 feeder.

Other than the nuances of the code wording, I'll ask; what's the real difference in saying #2AL is OK for carrying the FULL load of a dwelling, but not for a sub-panel in a dwelling???

 

[theoldwizard1]

The reason you see 90A a lot is that it is down rated for most application. (I am not a code lawyer so I can't quote the section.) Better safe than sorry, swap that breaker for the panel in your house for 90A.

 

[Norcal]

Right or wrong, many areas do allow a 100A breaker on a #2 feeder.

Other than the nuances of the code wording, I'll ask; what's the real difference in saying #2AL is OK for carrying the FULL load of a dwelling, but not for a sub-panel in a dwelling???

It's one reason why it should just go away, but it is strange it's good for one thing but not OK for something similar.

 

[wyliesdiesels]

That part of the code has driven me nuts for years. It really needs to be clarified!

 

[Spyider]

ok so code aside.. the 100a in the main would be fine.. its not like it will burn the wire going to the sub..? since its allowed that in the home.. ide assume that the wire can physically take the 100a load its just not allowed by code? is that basically summed up?..

yes it can take the load.
no its not to code to do it that way..
yes/no?

 

[alfredeneuman]

Right or wrong, many areas do allow a 100A breaker on a #2 feeder.

I'm just curious. Do those same areas allow #4 Copper for 100 Amp use too?

 

[Speedy Petey]

I'm just curious. Do those same areas allow #4 Copper for 100 Amp use too?

I would assume.

 

[Charles (in GA)]

ok so code aside.. the 100a in the main would be fine.. its not like it will burn the wire going to the sub..? since its allowed that in the home.. ide assume that the wire can physically take the 100a load its just not allowed by code? is that basically summed up?..

yes it can take the load.
no its not to code to do it that way..
yes/no?

Based on the NEC tables for wire types and amperages, a full 100 amps would be an overload on the wire. I suspect that the NEC figured that loads change enough in a residence that a momentary overload would not be an issue. In a shop or outbuilding, people tend to skimp more, not put in any more than they have to. Put in the 90 amp breaker and keep it to code, it might make you in good steed with your insurance company if anything ever happens.

From Article 310 of the 2011 NEC

(7) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For individual dwelling units of onefamily, two-family, and multifamily dwellings, conductors, as listed in Table 310.15(B)(7), shall be permitted as 120/240-volt, 3-wire, single-phase service-entrance conductors, service-lateral conductors, and feeder conductors that serve as the main power feeder to each dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. For application of this section, the main power feeder shall be the feeder between the main disconnect and the panelboard that supplies, either by branch circuits or by feeders, or both, all loads that are part or associated with the dwelling unit. The feeder conductors to a dwelling unit shall not be required to have an allowable ampacity rating greater than their service-entrance conductors. The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided the requirements of 215.2, 220.61, and 230.42 are met.

The above paragraph describes the ONLY circumstances where the wire sizes listed in Table 310.15(B)(7) are allow to be installed for the purposes of carrying the amperages described in the table.

For all other installations, you would have to resort to the use of

"Table 310.15(B)(16) (formerly Table 310.16) Allowable Ampacities of Insulated Conductors Rated Up to and Including 2000 Volts, 60°C Through 90°C (140°F Through 194°F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30°C (86°F)"

OR

"Table 310.15(B)(17) (formerly Table 310.17) Allowable Ampacities of Single-Insulated Conductors Rated Up to and Including 2000 Volts in Free Air, Based on Ambient Temperature of 30°C (86°F)"

Table 310.15(B)(7) Conductor Types and Sizes for 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. Conductor Types RHH, RHW, RHW-2, THHN, THHW, THW, THW-2, THWN, THWN-2, XHHW, XHHW-2, SE, USE, USE-2

Conductor (AWG or kcmil)

Service or Feeder
Rating (Amperes)........Copper.........Aluminum or Copper-Clad Aluminum
100...........................4.................2
110...........................3.................1
125...........................2.................1/0
150...........................1.................2/0
175...........................1/0..............3/0
200...........................2/0..............4/0
225...........................3/0..............250
250...........................4/0..............300
300...........................250..............500
400...........................400..............600

 

[Norcal]

Please note the table quoted in the above post does not exist in the 2014 NEC.

 

[theoldwizard1]

ok so code aside.. the 100a in the main would be fine...

It is only FINE if the electrical inspector passes it. Call them and ask. They have the final say so.


FYI, you do know MHF must be in conduit if is not below the ground. Also use Schedule 80 PVC sweeps where the wire transitions from below the ground to above.

 

[Charles (in GA)]

Please note the table quoted in the above post does not exist in the 2014 NEC.

Is it there in some other fashion? I do need to break down and get a copy of the 2014 code, just don't want to receive all the **** in the mail I get from them. If I buy a new code, it will just get worse. I'm down to an item every couple of weeks now.

Charles

 

[Norcal]

It requires calculations to get the same results in the'14, buy a copy on eBay, or a supply house to avoid the spam emails from NFPA. 14 years ago, I needed a copy of the '96 NEC & ordered a copy from NFPA, not ordered since & still get emails from them.

 

[Mustang51js]

I don't have code book in front of me to see max amps of #2 al but I know #4 copper is rated for 95 amps,and then you can use the next size up breaker for a service,so maybe that's why they lower it down as a sub feeder. I always assumed it had something to with the weather and it being on the outside of home in free air and such.

 

[wyliesdiesels]

310.15(b)(17) applies to 'free air' wire aka wire hanging from poles such as service drops or overhead feeders feeding detached buildings!

I don't have code book in front of me to see max amps of #2 al but I know #4 copper is rated for 95 amps,and then you can use the next size up breaker for a service,so maybe that's why they lower it down as a sub feeder. I always assumed it had something to with the weather and it being on the outside of home in free air and such.

U don't need a code book at the ready. Just google the applicable tables. Ive downloaded them as PDFs to my phone.

 

[alfredeneuman]

2014 Code

"(7) 120/240 Volt, Single-Phase Dwelling Services and Feeders.
For service and feeder conductors of 120/240- volt, single-phase, individual dwelling unit one-family, two-family, and multifamily service ratings from 100 amperes through 400 amperes, an adjustment factor of 0.83 of the service ampere rating shall be permitted to be used to determine the size of the ungrounded conductors."

So basically the ampacity has to be 83% that of the service size.
You can also use 83% for the neutral, or you can do some calculations to further reduce the size of the neutral if it meets some requirements.

 

[Spyider]

yes i have conduit running into the home through a LB, schedule 40 in the basement all the way to my main box. then anything outside is all schedule 80. digging my trench right now and damn this is allot of work by hand lol. have to rent a concrete saw tomorrow to dig the rest.

ill just get a 90a to be to code, and safe.. thanks for the help.

 

[TractorJeff]

I have a 50 in my shop and have never tripped it even with the welder after I tightened the screws that hold the aluminum wire. Go with the 90 to keep it in code.