Questions about adding sub panel to attached garage

[electricus]

Here is the scope of the project: Using 2-2-2-4 Aluminum XHHW SE-R cable for sub panel board in dwelling unit attached garage feeder from house main sub panel board. The cable will be routed through framing members of the basement ceiling, garage walls and "attic" area and through insulation.

100 Definitions - Feeder

• By definition this is what the 2-2-2-4 from the main panel board in the house to the sub panel board is the garage would be.

Table 310.15(B)(7)

• States can normally be used to size dwelling unit feeders. Here it states for this purpose #2 aluminum is rated at 100 amps. (XHHW is listed, planning to use SE-R cable).

338.10(B)

• States SE-R cable can be used as a feeder

338.10(4)(a)

• States when SE cable is used for interior wiring part II of article 334 (NM cable) excluding 334.80 must be used. (Will be used for interior wiring along complete run).
  • 334 - NM cables
  • 334.30 - Securing and Supporting. Within 12 in. of every box.
  • 334.30(a) - Horizontal runs through holes and notches. Support every 4.5'
• Service Entrance Cable. Interior installations - States where installed through thermal insultaion the 60?C conductor temperature rating must be used. (For this installation it will be in insulation).

Table 31015(B)(16)

• States #2 aluminum is rated for 75 amps @ 60?C

240.4(B)

• States the next highest standard breaker size can be used.

240.6(A)

• 75 amps is not a standard size, 80 amps is the next standard size.

In conclusion for this installation the SE-R cable needs to be installed like NM cable using all the same rules. Because it is installed though insulation it has to be protected by an 80 amp breaker.

Please correct or comment on any area that needs it.

Another question: How do most of you feel about using aluminum wire? I have personally not used much to date. It seems SE-R is mainly sold in aluminum and according to some of the information on the manufactures websites this is not the same aluminum from the 1950s that gave aluminum wire a bad name.

 

[Dennis Alwon]

You are correct in your assessment and I would not worry about the aluminum wire as a feeder as it is not the same alloy from the 70's.

 

[augie47]

Like Dennis, I see no problem, but I will note that along with the "next size up" breaker, you need to assure the calculated load does not exceed the wire ampacity. I am confident that in this situation that is not a problem, but I did not want those reading to assume you can automatically just go to the next size breaker. There are conditions.

 

[electricus]

augie47

Like Dennis, I see no problem, but I will note that along with the "next size up" breaker, you need to assure the calculated load does not exceed the wire ampacity. I am confident that in this situation that is not a problem, but I did not want those reading to assume you can automatically just go to the next size breaker. There are conditions.​

Could you please elaborate, was under impression that the 80 amp breaker is protecting the wire. I figured the "next size up" could be used most of the time.

By the way which temperature derating table does one use? What is the difference between Table 310.15(B)(a) and Table 310.15(B)(2)(b)? Using Table 310(B)(2)(a) Ambient Temperature Correction Factors Based on 30?C (86?F) the XHHW is good for 90?C and can be used for derating purposes. According to the table at 122?F attic or wall ambient temperatures the wire is good for 82 amps. Any hotter than this and a smaller breaker would have to be used. I have no idea what the temperatures will be in a wall or attic in the North East in the summer.

How do you do a load calculation for a garage, I don't see anything in the code for this, a dwelling unit yes, garage no. For the square footage with one or two people working in the garage I added up all the likely loads that could be drawing current at the same time.



Possible loads figured at 80% of breaker rating. The lighting would be the only continuous load. Figured the rest at 80% because I don't know what the future holds for loads.

Small appliance (radio, fans, solder iron?, work light, etc.) 20 amp x 80% = 16A x 120V = 1920 VA

Lighting 15 amp x 80% = 12A x 120V = 1440 VA

(480 SQ. FT. x 3VA = 1440 VA) is all that is really needed for the two above, but figuring on the heavy side


Welding receptacle 30 amp x 80% = 24A x 240V = 5760 VA

Compressor 20 amp x 80% = 16A x 240V = 3840 VA

(Heating 15 amp x 80% = 12A x 120V = 1440 VA) or Cooling 20 amp x 80% = 16A x 120V = 1920 VA

Garage door opener 15 amp x 80% = 12A x 120V = 1440 VA


Total 16320 VA / 240V = 68 amps.

80 amp main breaker x 80% = 64 amps (continuous load)


Even though the total is 4 amps higher than the continuous load rating, the chance that all this would be running at the same time is low. I think figuring it this way is very conservative. I don't see the need in added cost to use larger wire to get the main breaker rating up to 100 amps. Considering 2-2-2-4 SE-R cable is common and cheapest available from a big box store.

Your thoughts please.

 

[Smart $]

...
Table 310.15(B)(7)

• States can normally be used to size dwelling unit feeders. Here it states for this purpose #2 aluminum is rated at 100 amps. (XHHW is listed, planning to use SE-R cable).

...

Please correct or comment on any area that needs it.

...

While your the end result of your assessment is correct, 310.15(B)(7) does not apply to the subpanel's feeder.

 

[Smart $]

Could you please elaborate, was under impression that the 80 amp breaker is protecting the wire. I figured the "next size up" could be used most of the time.

Under 800A it can be... but as Gus points out, the next size up rule is conditional. The calculated load cannot exceed the ampacity of the conductor when the ampacity is less than the breaker rating.

By the way which temperature derating table does one use? What is the difference between Table 310.15(B)(a) and Table 310.15(B)(2)(b)?

At the end of an ampacity table header it will say whether the table is based on 30?C or 40?C ambient...

Using Table 310(B)(2)(a) Ambient Temperature Correction Factors Based on 30?C (86?F) the XHHW is good for 90?C and can be used for derating purposes. According to the table at 122?F attic or wall ambient temperatures the wire is good for 82 amps. Any hotter than this and a smaller breaker would have to be used. I have no idea what the temperatures will be in a wall or attic in the North East in the summer.

As you've already pointed out, you must use the 60?C ampacity value because installed in thermal insulation [this coincides with 110.14(C) requirements]. For #2 AL, the allowable ampacity value is 75A. So if your adjusted and corrected ampacity value is greater than 75A, you are limited to the 75A value for the "coordinated" ampacity. If it is less than the 75A value, then this lower value is the "coordinated" ampacity. With an 80A breaker, you can have a "coordinated" ampacity as low as 71A and still meet the rules (provided the calculated load, not including an extra 25% for continuous load, is not greater than the "coordinated" ampacity). Rather complex for something which seems simple on the surface, huh?

How do you do a load calculation for a garage, I don't see anything in the code for this, a dwelling unit yes, garage no.

Good question. Perhaps someone else will touch on this... or I'll get back to it later

 

[electricus]

While your the end result of your assessment is correct, 310.15(B)(7) does not apply to the subpanel's feeder.

Agreed, I should have pointed out I realized 310.15(B)(7) is only for a "main" feeder. I was pointing this out because if you go into a big box store, I won't say the name, but they have an orange color theme. The sign they have posted for wire ampacities is a little misleading depending on the exact application. In addition the "Electrician" on staff may make assumptions when they are talking with you about what can and can't be done. They may know the code, but if they don't know they exact scope of the project they can and will provide incorrect information. This is why I looked it all up in great length in the first place.

As you've already pointed out, you must use the 60?C ampacity value because installed in thermal insulation [this coincides with 110.14(C) requirements]. For #2 AL, the allowable ampacity value is 75A. So if your adjusted and corrected ampacity value is greater than 75A, you are limited to the 75A value for the "coordinated" ampacity. If it is less than the 75A value, then this lower value is the "coordinated" ampacity. With an 80A breaker, you can have a "coordinated" ampacity as low as 71A and still meet the rules (provided the calculated load, not including an extra 25% for continuous load, is not greater than the "coordinated" ampacity). Rather complex for something which seems simple on the surface, huh?

You are right it seems like something that should be simple, but there is more to it, once the digging of information starts it does get complex. So I follow along with what you are saying and read 110.14(C), I never knew the exact code, that was one of the "common knowledge" ones for me. I get what you are saying with the coordinated ampacity. You loose me when you say that a coordinated ampacity low as 71A is possible. How did you get 71A?

Side note: How do you get the persons screen name to appear when putting what the typed in a quote bubble?

 

[GoldDigger]

Side note: How do you get the persons screen name to appear when putting what the typed in a quote bubble?

If you do a "Reply With Quote" it is automatic. If you need to do multiple separate quotes from the same person, just copy the extra information inside the first

tag to the others.
To quote multiple people in one post, you have to figure out what to add to tag for them. The number is what allows the little bug after the name which lets you to browse to the original post that you replied to.

 

[Smart $]

... You loose me when you say that a coordinated ampacity low as 71A is possible. How did you get 71A?

240.4(B), the next size up rule. Next lower standard size per 240.6 is 70A. If your "coordinated" ampacity gets as low as 70A, you'd have to use a 70A breaker. 71A is the lowest "coordinated" ampacity permitted on an 80A-rated breaker.

 

[Smart $]

If you do a "Reply With Quote" it is automatic. If you need to do multiple separate quotes from the same person, just copy the extra information inside the first ]QUOTE[ tag to the others.
To quote multiple people in one post, you have to figure out what to add to tag for them. The number is what allows the little bug after the name which lets you to browse to the original post that you replied to.

There's also the Multi-quote button to the right of the "Reply with Quote" button. This adds each enabled post into the "Reply with Quote" reply.

The number in the opening tag is not required if you want to do manual edits (as I did with yours) or additions.

 

[david luchini]

How do you do a load calculation for a garage, I don't see anything in the code for this, a dwelling unit yes, garage no.

You calculate the branch circuit loads per Section II of Art 220. You'd maybe have luminaires, receptacles, motors, appliances, and specialized applications such as your welders, etc.

Then per Section III or Art 220, you sum the branch circuit loads and apply any applicable demand factors to determine the feeder load. It's no different than any other load calculation.

 

[electricus]

240.4(B), the next size up rule. Next lower standard size per 240.6 is 70A. If your "coordinated" ampacity gets as low as 70A, you'd have to use a 70A breaker. 71A is the lowest "coordinated" ampacity permitted on an 80A-rated breaker.

I understand what you are saying now, it make perfect sense.

There's also the Multi-quote button to the right of the "Reply with Quote" button. This adds each enabled post into the "Reply with Quote" reply.

The number in the opening tag is not required if you want to do manual edits (as I did with yours) or additions.

Thank you for the help with the quotes, I never noticed those buttons on the side, I used the mulit/enable for this post.

 

[kwired]

When using "next size up" overcurrent device rules you still have to remember you have more than one calculation involved.

First thing is determining minimum ampacity of conductor that is needed. Whether you are sizing a branch circuit, a feeder, or a service conductor the applicable sections all say something to the effect of - the conductor must have ampacity equal or greater than the load.

Second thing is overcurrent protection.

So if you have a 77 amp load then a 75 amp conductor is not large enough even though you are allowed to protect it at 80 amps.

 

[Smart $]

You calculate the branch circuit loads per Section II of Art 220. You'd maybe have luminaires, receptacles, motors, appliances, and specialized applications such as your welders, etc.

Then per Section III or Art 220, you sum the branch circuit loads and apply any applicable demand factors to determine the feeder load. It's no different than any other load calculation.

But it is different regarding luminaires and general-use receptacles.

220.12 ... For dwelling units, the calculated floor area shall not include open porches, garages, or unused or unfinished spaces not adaptable for future use.

220.14(J) Dwelling Occupancies. In one-family, two-family, and multifamily dwellings and in guest rooms or guest suites of hotels and motels, the outlets specified in (J)(1), (J)(2), and (J)(3) are included in the general lighting load calculations of 220.12. No additional load calculations shall be required for such outlets.

(1) All general-use receptacle outlets of 20-ampere rating or less, including receptacles connected to the circuits in 210.11(C)(3)

 

[david luchini]

But it is different regarding luminaires and general-use receptacles.

What's different about it? You apply the branch circuit calculations per Part II of Art 210. You would do the same for a garage as for a dwelling unit as for an office building as for a hospital, etc.

There is no minimum lighting load for a garage in Table 220.12, so luminaires would be calculated per 220.14(D). Receptacles would be calculated per 220.14(I).

 

[kwired]

What's different about it? You apply the branch circuit calculations per Part II of Art 210. You would do the same for a garage as for a dwelling unit as for an office building as for a hospital, etc.

There is no minimum lighting load for a garage in Table 220.12, so luminaires would be calculated per 220.14(D). Receptacles would be calculated per 220.14(I).

Gets kind of tricky doesn't it? Garage in many cases where people want to feed with a 60 amp feeder, could often be fed with a single 15 or 20 amp branch circuit and be just fine as far as art 220 goes.

Sometimes you kind of have to consider the reality of what portable loads may be present in such a location instead of just going off of 180 VA per outlet, to come up with a supply that is deemed reliable to the user, but this is design that goes beyond code minimums.

 

[Smart $]

What's different about it? You apply the branch circuit calculations per Part II of Art 210. You would do the same for a garage as for a dwelling unit as for an office building as for a hospital, etc.

There is no minimum lighting load for a garage in Table 220.12, so luminaires would be calculated per 220.14(D). Receptacles would be calculated per 220.14(I).

No you don't do the same for a garage as for a dwelling unit. A dwelling unit is figured [typically] per 220.12 and 220.14(J). I'm of the impression when most do a service calculation, they do not include any extra for "open porches, [attached] garages, or unused or unfinished spaces not adaptable for future use". So if no extra for these areas was included in the service cal', the only way adding a subpanel for the garage would be permitted is if the service was oversized to begin with. You'd have to do a new service cal', including the extra garage loads, and compare with the existing service rating to make sure it isn't undersized.

 

[david luchini]

No you don't do the same for a garage as for a dwelling unit. A dwelling unit is figured [typically] per 220.12 and 220.14(J). I'm of the impression when most do a service calculation, they do not include any extra for "open porches, [attached] garages, or unused or unfinished spaces not adaptable for future use". So if no extra for these areas was included in the service cal', the only way adding a subpanel for the garage would be permitted is if the service was oversized to begin with. You'd have to do a new service cal', including the extra garage loads, and compare with the existing service rating to make sure it isn't undersized.

Let me try again.

220.1 This Article provides the requirements for calculating branch-circuit, feeder and service loads.

220.10 Branch-Circuit loads shall be calculated as shown in 220.12, 220.14 and 220.16

220.40 The calculated load of a feeder or service shall not be less than the sum of the loads on the branch circuits supplied, as determined by Part II of this Article, after any applicable demand factors permitted by Part III or IV or required by Part V have been applied.

Question:

How do you do a load calculation for a garage, I don't see anything in the code for this, a dwelling unit yes, garage no.

Answer: You calculate the load for a garage in accordance with Article 220 - Branch-Circuit, Feeder and Service Calculations. You also calculate the loads for a dwelling unit, and a hospital, and a school, and an office building, etc., in accordance with Article 220. It's really that simple (unless you know of some other Code section that provides the requirements for calculating loads for garages.)

 

[mjf]

The OP states ATTACHED garage....it is, in this case, part of the dwelling unit anyway.....right?

 

[david luchini]

The OP states ATTACHED garage....it is, in this case, part of the dwelling unit anyway.....right?

No, not in terms of load calculations. See Smart$'s post #14 (specifically... 220.12: For dwelling units, the calculated floor area shall not include open porches, garages, or unused or unfinished spaces not adaptable for future use.)