100 Amp Subpanel Ampacity Sizing Question

[Wiz02]

There are many threads in this section of the forum asking about sizing 100 Amp subpanels, yet when I run the NERC calculator @ electrician2.com (NEC Table 310.15(B)(16) Conductor Size, OCPD, Voltage Drop, and Equipment Grounding Conductor Size Calculator), the calculated cable size (#1 Cu) is significantly larger than most recommendations on the forum.

Can anyone explain why the calculator is calling for #1 AWG cable, yet many electricians are recommending #2 or more typically #3/#4 cables for a 100 A subpanel? I know that you can replace the breaker with a 95 Amp and go down to #2, cable, but it seems like accepted practice is to run smaller gauge wire than what the calculator is calling for.

I've already run 2" conduit and I'm ready to go pull the permit. I usually work alone and pulling (3) #1 cables plus a #6 ground looks like quite a challenge, given the large number of junction boxes I have in the run. I suppose I could drop down to #2 and replace the 100 Amp breaker with a 95 Amp one, but that seems like I'm wimping out.

Thoughts?

 

[zmaxmotorsports]

How far are you running between panels and what are you feeding off of it,which table are you getting your numbers off of?

 

[Wiz02]

It's 150 feet between panels, voltage drop is minimal, I believe. The calculator states:

NEC Table 310.15(B)(16) Conductor Size, OCPD, Voltage Drop,
and Equipment Grounding Conductor Size Calculator

Check out the url: http://www.electrician2.com/calculators/wireocpd_ver_1.html

 

[DTE]

I am not an electrician but I pulled my own permit so I asked the inspectors what I needed to run for a full 100 amps going a 130 ft and they said I needed to use 1/0 al and a @ 6 ground .

 

[Wiz02]

Thanks, the calculator calls for 1/0 Al as well. I was a bit hesitant about asking, but I don't know why. I will definitely ask the code guys at the township.

 

[mdd1986]

What is the load on the panel?

 

[theoldwizard1]

Just because you bought a 100A panel, does NOT mean you need to wire it for its full capacity. If you use a smaller breaker in the main panel (90A or even 60A) you can save a lot of money on wire. The best "bang for the buck" is usually 4 conductor AL MHF.

I am not good with those calculators, but I'll bet that 60A @ 130' and 2-2-2-4 MHF you will have less than 5% voltage drop and a pretty big cost savings. 60A is plenty for a "one man" shop/detached garage even with a heat pump or A/C. How many of your "big" loads are you going to be running simultaneously ?

 

[pattenp]

The feeder size requirement for a whole house service load is different than the required feeder size for a sub-panel. The sub-panel is considered a branch feeder. #2 Al as a whole house service load feeder is approved for 100A, but as a branch feeder #2 Al is rated at 90A. Go figure, one of those weird NEC rules. A 100A branch feeder needs to be #1 Al. That size is with no size increase for voltage drop.




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[theoldwizard1]

The feeder size requirement for a whole house service load is different than the required feeder size for a sub-panel. The sub-panel is considered a branch feeder. #2 Al as a whole house service load feeder is approved for 100A, but as a branch feeder #2 Al is rated at 90A. Go figure, one of those weird NEC rules. A 100A branch feeder needs to be #1 Al. That size is with no size increase for voltage drop.

Okay ...

What if the OP wants to save a couple of bucks and use MHF 2-2-2-4 ? What size breaker is need to protect 130' of that ?

 

[mdd1986]

The feeder size requirement for a whole house service load is different than the required feeder size for a sub-panel. The sub-panel is considered a branch feeder. #2 Al as a whole house service load feeder is approved for 100A, but as a branch feeder #2 Al is rated at 90A. Go figure, one of those weird NEC rules. A 100A branch feeder needs to be #1 Al. That size is with no size increase for voltage drop.




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Article 310.15(B)(7) is what you are referring to. The table in that article allows a bit more amperage on service entrance feeders for dwelling units only. It makes sense because most residential applications have large demand factors and very little continuous load.

 

[mdd1986]

Okay ...

What if the OP wants to save a couple of bucks and use MHF 2-2-2-4 ? What size breaker is need to protect 130' of that ?

It would need to be a 90A breaker since it is not a dwelling unit service entrance conductor.

 

[pattenp]

Okay ...

What if the OP wants to save a couple of bucks and use MHF 2-2-2-4 ? What size breaker is need to protect 130' of that ?

#2 Al is rated at 90A for branch circuits, period. Regardless of distance. Sizing wire for VD is a process above just picking the wire size just based on amp capacity. The VD for #2 Al at full load of 90A at 130' is 3.2%. The branch feed should be sized for a combination of 100% noncontinuous and 125% of continuous loads . If a #2 Al feeder is used and is being pushed to the full load of 90A continuously then the feeder is undersized. So using a 90A amp feeder your load calculations should be under the 90A capacity. Bottom line is if the load calculations are below 90A then #2Al is fine at 130' on a 90A breaker. As stated on this forum multiple times is a typical one man shop is hard pressed to even max out a 60A feed much less than a 90A. But there's always an exception to that statement.

 

[unsinkable]

Technically, most services and subs are wired incorrectly.
Just because someone's using THHN or any other wire listed in the 90*c column of 310-15 (B) (16) doesn't mean that's the correct column to use.
Most (if not all) lugs and equipment are only rated at 75*c. This is actually the correct column to use.
The 90*c column is for DE-rating use only.
i'm sure the manufacturers ampacity of a given size has some "play" built into it.
The only time this rule seems to be enforced is with motors, where you MUST use 75*c column only.
So, back to OP's question, maybe that website is actually taking the VD into acct and using the correct column. I don't use those things so I cant say for sure.

 

[theoldwizard1]

#2 Al is rated at 90A for branch circuits, period. Regardless of distance. Sizing wire for VD is a process above just picking the wire size just based on amp capacity. The VD for #2 Al at full load of 90A at 130' is 3.2%. The branch feed should be sized for a combination of 100% noncontinuous and 125% of continuous loads . If a #2 Al feeder is used and is being pushed to the full load of 90A continuously then the feeder is undersized. So using a 90A amp feeder your load calculations should be under the 90A capacity. Bottom line is if the load calculations are below 90A then #2Al is fine at 130' on a 90A breaker.

You obviously know you stuff and please don't think I am trying to belittle you in any way, but ..,

For most non-industrial applications, how many customers really know what their load is, I mean within 50% ? The whole "The branch feed should be sized for a combination of 100% noncontinuous and 125% of continuous loads" seems illogical to me ! Or perhaps you accidental mixed those up (100% continuous and 125% non-continuous)

As stated on this forum multiple times is a typical one man shop is hard pressed to even max out a 60A feed much less than a 90A.

Real world experience triumphs !

But there's always an exception to that statement.

Yeah, well ...

 

[Wiz02]

Thanks everyone for all the information. I do agree that I could probably size the garage with much smaller feeder size like 60 amps, but I also know that over time we keep adding electrical loads. Homes used to have 60 amp services, now 200 amps is the typical minimum.

I am by no means a spendthrift, but spending a couple of hundred dollars extra on the feeder cable in order to future proof the garage service seems like good insurance to me.

Now to answer the question that I raised in this thread, I spoke to the local code enforcement gentleman yesterday and for a 100 amp service, the code used here in Southeastern PA requires #1 Cu or #1/0 Al, which is what the calculator that I referenced previously specified. I do believe that the reason for derating the ampacity of the cable is what "unsinkable" stated, the calculator returns values from the 75 deg C column of Table 310-15 (B) (16) .

I hope that this helps others out, as this question has been asked and answered numerous times, but it's difficult to get chapter and version references from the NEC tables as well as confirmation from the local AHJ in one post.

 

[mdd1986]

Technically, most services and subs are wired incorrectly.
Just because someone's using THHN or any other wire listed in the 90*c column of 310-15 (B) (16) doesn't mean that's the correct column to use.
Most (if not all) lugs and equipment are only rated at 75*c. This is actually the correct column to use.
The 90*c column is for DE-rating use only.
i'm sure the manufacturers ampacity of a given size has some "play" built into it.
The only time this rule seems to be enforced is with motors, where you MUST use 75*c column only.
So, back to OP's question, maybe that website is actually taking the VD into acct and using the correct column. I don't use those things so I cant say for sure.

They actually make 90 Degree terminations now. I have specified them on a few jobs I have done where I had an existing conduit I needed to utilize. Obviously its very rare so you are correct in saying we should be using the 75 degree column.

 

[mdd1986]

Thanks everyone for all the information. I do agree that I could probably size the garage with much smaller feeder size like 60 amps, but I also know that over time we keep adding electrical loads. Homes used to have 60 amp services, now 200 amps is the typical minimum.

I am by no means a spendthrift, but spending a couple of hundred dollars extra on the feeder cable in order to future proof the garage service seems like good insurance to me.

Now to answer the question that I raised in this thread, I spoke to the local code enforcement gentleman yesterday and for a 100 amp service, the code used here in Southeastern PA requires #1 Cu or #1/0 Al, which is what the calculator that I referenced previously specified. I do believe that the reason for derating the ampacity of the cable is what "unsinkable" stated, the calculator returns values from the 75 deg C column of Table 310-15 (B) (16) .

I hope that this helps others out, as this question has been asked and answered numerous times, but it's difficult to get chapter and version references from the NEC tables as well as confirmation from the local AHJ in one post.

75 degree #1Cu is good for 130A and #1/0 Al is good for 120A according to the NEC 310-15. My guess is they have a general rule of thumb to use ** size cable for ** amount of distance at 100A. I would challenge the inspector as to where in the code it says that or what code he is referring too. Honestly, I would just put in a 90A breaker and just use #2 AL. I highly doubt you will have a continuous load even close to 90A in a garage so voltage drop would be negligible (you could calculate it out if the inspector gives you a hard time).

 

[theoldwizard1]

Now to answer the question that I raised in this thread, I spoke to the local code enforcement gentleman yesterday ...

ALWAYS THE BEST THING TO DO !!

..and for a 100 amp service, the code used here in Southeastern PA requires #1 Cu or #1/0 Al, which is what the calculator that I referenced previously specified.

Ask the same question for 90A. I'll bet the answer is #2 AL.

Then go price out the difference between #1/0 AL and 2-2-2-4 MHF. ($2.74/ft versus $1.26/foot at HD online).