Splitting legs to feed sub-panel...

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EdMann

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I'm planning on using a sub-panel for 110v circuits only. It's rated 240v 125amp. Can I feed each leg of this type of panel with a separate breaker? And, if so, could I use single pole breakers at 50 amps each? Or is this just a horrible idea to start with and I should be banned from this forum just for suggesting it? I would of course mark the sub-panel clearly to prevent future misuse.
 

infinity

Moderator
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Location
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So you want to use two 120 volt feeders to a panel instead of one 120/240 volt feeder?

Welcome to the Forum. :)
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
The idea strikes me as 'a horrible idea to start with' unless you only have 120V available at the source you are feeding the panel from. If that's the case, I see no reason why it would not be acceptable.

If you have 240V at the source then you save no significant wire or parts by not running a 240V feeder, and instead you'd create a potentially dangerous and unfamiliar situation.

And yes I would mark it if there's a good reason for not wiring it 240V. I'm not aware of any code requirement that requires it in the main part of the code but I would suggest using the language required for stand-alone inverters in article 690 and others.

WARNING.
SINGLE 120-VOLT SUPPLY.
DO NOT CONNECT.
MULTIWIRE BRANCH CIRCUITS!
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I'm planning on using a sub-panel for 110v circuits only. It's rated 240v 125amp. Can I feed each leg of this type of panel with a separate breaker? And, if so, could I use single pole breakers at 50 amps each? Or is this just a horrible idea to start with and I should be banned from this forum just for suggesting it? I would of course mark the sub-panel clearly to prevent future misuse.

so you want to use two 50A single pole circuits breakers instead of a single 50A 2 pole CB and feed it 120/240?

or do you want to feed it 2 legs of the same phase of 120?

I think either way is legal. If you use 2 breakers to feed 120/240 you have a multiwire circuit and the breakers need to be handle tied at least, so it would probably be easier to just use a 2 pole breaker unless you wanted to avoid the common trip feature for some reason.

Why you would want to feed 100A off a single phase is curious, if that is what you want to do.
 

EdMann

Member
I'm building two 'fish' rooms onto my shop. They will have only aquariums with pumps and heaters. All 110v. I will never branch off of the sub-panel in question. I just thought it would be cheaper to use two $5 50 amp single-pole breakers rather than one $40 100 amp double-pole breaker. I'm running less than 20 feet from my 200 amp 2-phase main panel. I know I would have to run two circuits but thought it still would be cheaper running 5 #8 AWG than a 3-3-3-5 combination. I live in the country so there is no code enforcement except for me. But I also am aware that the NEC rules are there for a reason. Just asking some of you guys with tons of experience what you thought. I'm never above asking for an expert opinion that may save my life or valuable 'LIVE' property.

I do have an add on question if any of you would indulge me. This sub-panel has room for 12 circuits. I know I'll use 4 circuits per room fed by 20 amp breakers. Of course each circuit won't use the full 20 amps but what is a rule to go by for the capacity of a small panel like this? It's rated at 125 amps but I'm only going to feed it using 100 amp total.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I'm building two 'fish' rooms onto my shop. They will have only aquariums with pumps and heaters. All 110v. I will never branch off of the sub-panel in question. I just thought it would be cheaper to use two $5 50 amp single-pole breakers rather than one $40 100 amp double-pole breaker. I'm running less than 20 feet from my 200 amp 2-phase main panel. I know I would have to run two circuits but thought it still would be cheaper running 5 #8 AWG than a 3-3-3-5 combination. I live in the country so there is no code enforcement except for me. But I also am aware that the NEC rules are there for a reason. Just asking some of you guys with tons of experience what you thought. I'm never above asking for an expert opinion that may save my life or valuable 'LIVE' property.

I do have an add on question if any of you would indulge me. This sub-panel has room for 12 circuits. I know I'll use 4 circuits per room fed by 20 amp breakers. Of course each circuit won't use the full 20 amps but what is a rule to go by for the capacity of a small panel like this? It's rated at 125 amps but I'm only going to feed it using 100 amp total.

Why not run 3-#8's with a #10 EGC and use a 2 pole 50 amp CB?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I'm building two 'fish' rooms onto my shop. They will have only aquariums with pumps and heaters. All 110v. I will never branch off of the sub-panel in question. I just thought it would be cheaper to use two $5 50 amp single-pole breakers rather than one $40 100 amp double-pole breaker. I'm running less than 20 feet from my 200 amp 2-phase main panel. I know I would have to run two circuits but thought it still would be cheaper running 5 #8 AWG than a 3-3-3-5 combination. I live in the country so there is no code enforcement except for me. But I also am aware that the NEC rules are there for a reason. Just asking some of you guys with tons of experience what you thought. I'm never above asking for an expert opinion that may save my life or valuable 'LIVE' property.

I do have an add on question if any of you would indulge me. This sub-panel has room for 12 circuits. I know I'll use 4 circuits per room fed by 20 amp breakers. Of course each circuit won't use the full 20 amps but what is a rule to go by for the capacity of a small panel like this? It's rated at 125 amps but I'm only going to feed it using 100 amp total.

What would make you think two 50 A single pole breakers would substitute for one 100 A 2 pole CB?
 

EdMann

Member
What would make you think two 50 A single pole breakers would substitute for one 100 A 2 pole CB?
That was part of my original question. Is that an equal trade. I assumed that 100 A would be for both legs so 50 A per leg? If that is not an equal trade off then it makes all of the other parts of my question inconsequential. I'll have to go with a 100 A 2 pole. If a double pole breaker trips at 100 amp per leg then I'm wasting 50 amps per leg doing it the wrong way. If my dear old dad were still alive (master electrician/electronics engineer), I'd just call him up and ask these questions and get a yes or no answer plus why I should/shouldn't do something. Man those were the good old days. Now I'm relying on the expertise and kindness of others to help me with my sometimes stupid questions. So thanks in advance for any help you can provide me! :)
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
That was part of my original question. Is that an equal trade. I assumed that 100 A would be for both legs so 50 A per leg? If that is not an equal trade off then it makes all of the other parts of my question inconsequential. I'll have to go with a 100 A 2 pole. If a double pole breaker trips at 100 amp per leg then I'm wasting 50 amps per leg doing it the wrong way. If my dear old dad were still alive (master electrician/electronics engineer), I'd just call him up and ask these questions and get a yes or no answer plus why I should/shouldn't do something. Man those were the good old days. Now I'm relying on the expertise and kindness of others to help me with my sometimes stupid questions. So thanks in advance for any help you can provide me! :)

If this is true that would seem to make this a DIY project, and we would not be allowed to help you with such a thing.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I'm sorry Ed but from the content of this thread it appears that you're over your head and need to consult a qualified electrician to help you. Forum rules prohibit DIY work therefore I am closing this thread.
 
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