Size of Sub Panel

[wrobotronic]

Hi all,
Would someone direct me to the code section for sizing of a subpanel. I was always "taught" that a sub should be 50% of the rating of the main. IE a 150A main could only take a 70A sub. Therefore a 100A sub was not allowed.

I do realize that how I was "taught" and what is the proper application can be two very different things. I would like to know for my own sake how one determines the proper size of a subpanel.

I believe I may be overthinking the whole scenario, but again any references to codes would be much appreciated.

TYIA,
Wrobo

 

[GoldDigger]

Hi all,
Would someone direct me to the code section for sizing of a subpanel. I was always "taught" that a sub should be 50% of the rating of the main. IE a 150A main could only take a 70A sub. Therefore a 100A sub was not allowed.

I do realize that how I was "taught" and what is the proper application can be two very different things. I would like to know for my own sake how one determines the proper size of a subpanel.

I believe I may be overthinking the whole scenario, but again any references to codes would be much appreciated.

TYIA,
Wrobo

You are overthinking.
The code does not put any limits on the designated size of the panel, just on the load it can feed and the breaker which supplies it.
The relationship between the main breaker and the feeder breakers in a single panel is determined entirely by the mechanical and electrical design of that panel, so you need to look to the manufacturer's literature and labels, not the NEC.

 

[macmikeman]

There is nothing in the NEC stating that a panel can only be rated 50% of its source panel board. Thanks for bringing the concept up at this forum however , that way it will soon come to pass as there will be a sudden race of submitted code change proposals from the ''regulars'' here who tie their status to code changes they authored. Hip Hip Horray!

 

[jaggedben]

Article 220 covers load calculations for feeders, and thus, by logical extension, to a subpanel fed by a feeder. The only generally implied restriction is that a panel feeding a subpanel must have at least the same rating as the subpanel, since the same load calculation at minimum applies to the panel feeding it. However there are even some situations under the optional load calculations and Article 230 where the upstream panel is technically allowed to be rated less.

 

[GoldDigger]

Article 220 covers load calculations for feeders, and thus, by logical extension, to a subpanel fed by a feeder. The only generally implied restriction is that a panel feeding a subpanel must have at least the same rating as the subpanel, since the same load calculation at minimum applies to the panel feeding it. However there are even some situations under the optional load calculations and Article 230 where the upstream panel is technically allowed to be rated less.

And there is absolutely no restriction in the case that the subpanel is rated for far more than the calculated load.
If I choose to put a 200A subpanel in a system fed by a 60A service the NEC does not care.


However, some POCOs (including LADWP) will not allow a service panel rating to be greater than the nominal service size, even if a smaller main breaker is installed. (Makes meeting the 120% rule difficult in their territory sometimes.)

 

[wrobotronic]

Thank you all very much.

 

[ecohouse]

Subpanel minimum for out building

Subpanel minimum for out building

Is there a minimum besides 60A (225.39(D)) for an outbuilding? My scenario is a new barn near a customer's house that he would like wired. Another electrician said it needs to be 100A, but we don't need that large of a panel. If we can do a 60A subpanel at the barn, it would be much more cost efficient. Is this ok?
Thank you

 

[augie47]

Is there a minimum besides 60A (225.39(D)) for an outbuilding? My scenario is a new barn near a customer's house that he would like wired. Another electrician said it needs to be 100A, but we don't need that large of a panel. If we can do a 60A subpanel at the barn, it would be much more cost efficient. Is this ok?
Thank you

Depends on the # of circuits...

Take a look at 225.39 & it will give you minimums.

(It has been argued that 225.39 only addresses size of the disconnect and not the feeder itself..slippery slope and probably an AHJ call )

 

[GeorgeB]

Recognize that the "big box" stores sell packages at far lower prices than individual pieces ... but be sure they are NOT "suitable for service equipment ONLY". Neutral and ground must be separate ... or separable.

 

[electrofelon]

Is there a minimum besides 60A (225.39(D)) for an outbuilding? My scenario is a new barn near a customer's house that he would like wired. Another electrician said it needs to be 100A, but we don't need that large of a panel. If we can do a 60A subpanel at the barn, it would be much more cost efficient. Is this ok?
Thank you

I often go with a 90A feeder (#2 AL) and a 100 A panel just because the cost over anything smaller is quite minimal (that considering AL wire. Go to something like UF and the wire can be a big cost). Even if something smaller would suffice, there is voltage drop to consider once you get into the hundreds of feet. If its definitely a minimal usage scanario, I might go #6 AL for 50A.

 

[ecohouse]

More info

More info

In this scenario, Voltage drop and distance is the main factor driving the cost train. The barn is about 300' from the house, so doing a 60A subpanel instead of 100 would make a significant difference I think. Back to the original question, there is no code that says a subpanel to an out building such as a barn is required to be at least 100A?? Thanks.

 

[ActionDave]

In this scenario, Voltage drop and distance is the main factor driving the cost train. The barn is about 300' from the house, so doing a 60A subpanel instead of 100 would make a significant difference I think. Back to the original question, there is no code that says a subpanel to an out building such as a barn is required to be at least 100A?? Thanks.

I would run 2-2-4-6 off a 90A breaker to a 100A sub panel at an out building in this scenario almost every time. Overall it's the most cost effective way to go.

 

[electrofelon]

I would run 2-2-4-6 off a 90A breaker to a 100A sub panel at an out building in this scenario almost every time. Overall it's the most cost effective way to go.

+1. Going off memory, #2 Al is .28 a foot, #6 is .18 a foot. Say either case gets the #6 ground. So for 300 feet, you are at $306 vs $216. Around here, I would probably use a "4 wire URD" or a "mobile home feeder" instead of individual conductors.

 

[ActionDave]

+1. Going off memory, #2 Al is .28 a foot, #6 is .18 a foot. Say either case gets the #6 ground. So for 300 feet, you are at $306 vs $216. Around here, I would probably use a "4 wire URD" or a "mobile home feeder" instead of individual conductors.

Yes, 2-2-4-6 mobile home feeder on up to 250MCM is about the best bang for the buck for underground.

 

[Chamuit]

Farm Panel

Farm Panel

I used to install a 200A farm panel (GE TM820RCUFL) on alley garages and then feed a 200A load center on the house from there. You have the full capability of the system at both locations for what ever may arise.

 

[ecohouse]

URD wire

URD wire

With the mention of URD wire I've been doing some research today. I've never heard of that type. Upon further investigation, I see that if it doesn't have the proper ratings, then URD doesn't even fall under the NEC.
Again, I appreciate all of the opinions on what you all have done in the past, but my question is can I legally install a 60A subpanel at the barn. I'm still going to assume using UF wire which is what we've done in the past because it's rated properly.
The whole scenario goes like this: The wire run will be 275' to the barn. The owner literally needs 2 120v plugs and 2 4' LED lights. Go!

 

[peter d]

With the mention of URD wire I've been doing some research today. I've never heard of that type. Upon further investigation, I see that if it doesn't have the proper ratings, then URD doesn't even fall under the NEC.
Again, I appreciate all of the opinions on what you all have done in the past, but my question is can I legally install a 60A subpanel at the barn. I'm still going to assume using UF wire which is what we've done in the past because it's rated properly.
The whole scenario goes like this: The wire run will be 275' to the barn. The owner literally needs 2 120v plugs and 2 4' LED lights. Go!

There is URD that is marked and listed according to NEC requirements. I use it on occasion and it's readily available at my suppliers. There is also utility spec URD that is not allowed to be used under NEC applications.

 

[electrofelon]

With the mention of URD wire I've been doing some research today. I've never heard of that type. Upon further investigation, I see that if it doesn't have the proper ratings, then URD doesn't even fall under the NEC.
Again, I appreciate all of the opinions on what you all have done in the past, but my question is can I legally install a 60A subpanel at the barn. I'm still going to assume using UF wire which is what we've done in the past because it's rated properly.
The whole scenario goes like this: The wire run will be 275' to the barn. The owner literally needs 2 120v plugs and 2 4' LED lights. Go!

URD and MHF are just several conductors twisted together. They are almost always aluminum. I have never seen URD that is not an NEC type, but I hear it exists. The main thing you want to be careful of is if you are using it for indoor or above ground wiring that it is marked more than USE-2. I sometimes see it without the RHH marking.

I see nothing wrong with you plan, but Personally, I wouldnt use UF. Its expensive. I would go with #6, #4, or # 2 AL.

 

[ActionDave]

... my question is can I legally install a 60A subpanel at the barn.

Yes.

 

[kwired]

With the mention of URD wire I've been doing some research today. I've never heard of that type. Upon further investigation, I see that if it doesn't have the proper ratings, then URD doesn't even fall under the NEC.
Again, I appreciate all of the opinions on what you all have done in the past, but my question is can I legally install a 60A subpanel at the barn. I'm still going to assume using UF wire which is what we've done in the past because it's rated properly.
The whole scenario goes like this: The wire run will be 275' to the barn. The owner literally needs 2 120v plugs and 2 4' LED lights. Go!

Yes, see 225.39. For just two two wire circuits you could even install a 30 amp feeder if you wanted, if you put it all on one circuit it can be a 15 amp circuit.