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Old 09-14-2008, 09:46 AM   #1
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Noob needs help understanding sub-panels, please!


I just completed a rather expensive upgrade from 100 amp overhead service to 200 amp underground service and now I'm ready to tackle those long-envisioned projects......and there are enough of them that I think maybe a sub-panel or two are in order.

Here's the issue--I had originally envisioned a 30 amp/120V circuit that would have been used to power multiple audio power amplifiers, those things can be very power hungry. I even went so far as to pre-wire three of these circuits into the house with 10/2 with ground wiring, on 30 amp 120V breakers, only to find out from a different thread that I really should not do that. Since I already had the 10/2 w/grd wiring in place, I left the wiring there and changed the breakers to 20 amp in order to avoid creating a dangerous situation. Now I plan to run multiple 20 amp circuits to the same location, but hold the phone, Mabel, maybe there's a better way ?

How about a sub-panel to this location? I have plenty of empty slots in the new panel box, could easily install a 240V double pole breaker of the required rating and run ONE larger wire to a sub panel in the target area, branching off with multiple short runs of 12/2 with ground wiring instead of the longer "multiple" 20 amp circuits I was planning on running from the existing service panel to the location.

How about this, folks? If I were to install a 60 amp double pole 220V breaker and run 6 gauge wire to a subpanel in the target area, would that support three 20 amp 120V circuits, more than three, less than three--as you can see I am confused about how the transitiion from 240V to 120V circuits impacts the distribution of power as well as whether or not 3 X 20 amps actually = 60 amps.

Is this a safer, better alternative to running multiple 20 amp circuits off the current 40-slot breaker panel?

Not especially worried about the cost factor, I already have adequate 12/2 with ground wiring to run all the envisioned new circuits from the existing 40 slot panel, just want to avoid the spaghetti bowl effect of so many circuits.

Thanks to all who can help for assisting this DIYer in understanding the issue of sub-panels. It seems to me that this is a very workable way to expand an electrical system...but I'm not sure, that's why I'm seeking advice from those of you who are more knowledgable than I am!

Dugly

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Old 09-14-2008, 09:55 AM   #2
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Noob needs help understanding sub-panels, please!


Think in terms of watts.

A 60A sub panel is a 120/240v feeder. So 60A @ 240v = 14,400 watts.
A 20A-120v circuit provides 2,400 watts.
See how this works.

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Answers based on the 2008 & 2011 NEC.
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Old 09-14-2008, 09:59 AM   #3
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Noob needs help understanding sub-panels, please!


You need to calculate the current draw of ALL of the equipment that will powered from this sub-panel and size it from that. As long as you don't overload the breaker protecting this subpanel you could put as many circuits in the sub-panel as you need.
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Old 09-14-2008, 10:14 AM   #4
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Noob needs help understanding sub-panels, please!


Quote:
Originally Posted by YerDugliness View Post
I just completed a rather expensive upgrade Not especially worried about the cost factor, I already have adequate 12/2 with ground wiring to run all the envisioned new circuits from the existing 40 slot panel, just want to avoid the spaghetti bowl effect of so many circuits.

Dugly
As far as I know there are no regular 120V outlets that support larger than 12gage 20A service.

I suspect that you could junction the 10 gage wire with some 12 gage and then run the 12 gage to the outlets, but you would still be limited to 20A. Don't do this until you check and make sure it is ok to do, but I think it is.

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Old 09-14-2008, 10:37 AM   #5
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Noob needs help understanding sub-panels, please!


Ok....

A regular 240 volt home electrical service is a "balanced load". There are two "hot" legs and a "neutral. Hot to hot is 240V and either of the hots to neutral is 120V.

Breaker panels typically alternate hot legs as you go from top to bottom.

If you install a single breaker in any slot and connect the black wire to the breaker, then the white wire to the "neutral bar" (separate from the ground bar in a subpanel), then you get 120V.

If you install a double "tie-bar" breaker, connect the black wire to one of these breakers, and connect the white wire to the other breaker, you get 240V (wrap black electrical tape around the white wire at both ends to indicate it is also a hot wire and a 240V circuit).

What happens is that some of the load from the house is on one hot leg and some of the load from the house is on the other hot leg. This balances everything out so not much current may be flowing on the neutral wire at the service drop.

So some 120V things on this leg, some 120V things on that leg, some 240V things on both legs.

Then another thing is that the total value of all the breakers in a panel can add up to be MORE than the main breaker value! This is because the job of each breaker is only to protect the wiring for the circuit it is attached to.

Typically not *everything* in the house will be turned on at the same time.

So "in theory", you could have a 20 amp panel* with a 20 amp main breaker. And have four 20 amp breakers in this hypothetical panel going to 4 different 20 amp circuits. And if you only used one of these 20 amp circuits at a time, then the main 20 amp breaker would never trip. (*I have never seen a 20 amp panel, just made that up to 'splain things.)

For your needs, I would start by working "backwards". Add up the total wattage of everything you plan to plug in in that area (to determine total amperage of subpanel). Then figure out how many 20 amp circuits you will need (how many slots needed in subpanel).

Then how many of these things will you *ever* have powered on at the same time? What might you buy in the future and how much power will that require? Then get a panel with more amperage than you will ever need and more slots than you will ever need. (Always need more slots I say!)

Convert watts to amps calculator...
(Use the box under "Single Phase")
http://www.jobsite-generators.com/po...lculators.html

More reading...
http://en.wikipedia.org/wiki/Split_phase
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Old 09-14-2008, 03:13 PM   #6
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Ok....

A regular 240 volt home electrical service is a "balanced load". There are two "hot" legs and a "neutral. Hot to hot is 240V and either of the hots to neutral is 120V.

What happens is that some of the load from the house is on one hot leg and some of the load from the house is on the other hot leg. This balances everything out so not much current may be flowing on the neutral wire at the service drop.

Then another thing is that the total value of all the breakers in a panel can add up to be MORE than the main breaker value! This is because the job of each breaker is only to protect the wiring for the circuit it is attached to.
Thanks for the help so far, guys--I'm starting to grasp this concept, please humor me while I ask a few more questions:

As I add more circuits, do I need to calculate the relative total load on each of the two legs and attempt to balance those values? Right now, while I still have 15 open slots, that would be fairly easy, later on.......well....

Also, so back to my original question and how it relates to the last part of your reply above, I realize now that if I use a 60 amp breaker to protect a sub-panel with three 20 amp circuits, that 60 amp breaker will most likely never trip, whereas any or all of the three 20 amp circuits would. Add a fourth 20 amp circuit and now we're into more difficult territory, it's possible to load those 4 circuits to the point that none of the four 20 amp sub-panel breakers would trip, but the 60 amp breaker in the main panel would.

On second thought, there should not be a 60 amp breaker in the main panel if there are four 20 amp circuits in the sub-panel, it should be an 80 amp breaker and correspondingly larger line to the sub-panel.......or is it? This is the part that is confusing, since the total of the load ratings in a main panel can easily exceed the total amperage available, does the same concept apply to sub-panels? Could I safely operate four 20 amp circuits from a sub-panel that was protected at the main service panel by a 60 amp double-pole breaker (also, would I be limited to 30 amps a "leg"?)?

So, based on the knowledge I can gain regarding the power requirements of each unit (usually in the manuals), I can calculate how many would be reasonably expected to not overload a 20 amp circuit. Add an extra couple of circuits, I'm probably good to go for quite a while.

It's audio gear, including some old tube-type amplifiers, some solid state, a LOT of it is not power hungry at all, things like CD players, tuners, pre-amplifiers, equalizers, recorders, dynamic range processors, etc. I could probably run all of those sort of things on one 20 amp circuit and then have two other 20 amp circuits between which I could split the load from the many power amplifiers.

Then again, I could be so far off-base on what I think I understand here that it's not even ..........

Dugly
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Old 09-14-2008, 05:29 PM   #7
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Noob needs help understanding sub-panels, please!


You guys on this forum are pretty sharp! You're getting all this quickly.

So far as balancing, don't worry about it. The wiring will handle everything just fine. Just don't stick all the breakers on one leg. I install them one after the other going from the top down, using both sides. Empty slots at the bottom of the panel.

Now so far as a 60 amp breaker for a sub panel...

I could install a 60 amp subpanel and have it cover 6 bedrooms, each with their own 20 amp circuit. And I might vacuum the bedrooms, but not all at once! I would vacuum one bedroom at a time. Unlikely I would ever be using 20 amps in each bedroom all at the same time to total 120 amps.

But if that same 60 amp subpanel was installed for just the kitchen, I could be in trouble when Thanksgiving came around! I might have the refrigerator going, all range cooktops going, oven, dishwasher, microwave, bread maker, hot plate, etc. Then I turn on the kitchenaid mixer, and POP goes the main breaker in the subpanel. Dinner is ruined and we have to got to McDonalds for dinner!

So it all depends on what all you will ever have turned on at the same time.

So far as amperage, breakers, and wire size goes... Basically the breaker protects the wire from overheating and causing a fire. It is a protective device. The smaller the wire, the smaller the amperage of breaker you can use. The larger the wire, the larger the amperage of breaker you can use.

So if you want more amperage, then in general you would need a larger size wire. To further complicate matters, larger wire gets smaller in number. So wire size 14 is smaller than wire size 12!

Another thing is you don't want to load up each circuit with 20 amps. (Don't push it!) 80% of circuit amperage is more reasonable. So max of 16 amps per 20 amp circuit.
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Old 09-14-2008, 06:45 PM   #8
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Noob needs help understanding sub-panels, please!


If access and breaker space are not a problem, I would just run multiple 12/2's from the panel to the equipment outlets. No sense in complicating things.
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Old 09-14-2008, 08:17 PM   #9
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Noob needs help understanding sub-panels, please!


Quote:
If access and breaker space are not a problem, I would just run multiple 12/2's from the panel to the equipment outlets. No sense in complicating things.
I was wondering the same thing.....


What is the reasoning for the sub-panel?
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Old 09-14-2008, 08:31 PM   #10
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Another thing is you don't want to load up each circuit with 20 amps. (Don't push it!) 80% of circuit amperage is more reasonable. So max of 16 amps per 20 amp circuit.
Someone please explain to me why so many people believe this?

This is only true if supplying continuous loads.
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Old 09-14-2008, 08:50 PM   #11
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Someone please explain to me why so many people believe this?

This is only true if supplying continuous loads.

Got me, I gave up on replying to this nonsense.
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Old 09-14-2008, 09:53 PM   #12
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What I'm wondering....

My *whole* home entertainment center works just fine off of one 20 amp outlet. I'm sure 15 amps would do fine as well. But I have never had my stereo at full volume, it would be way too loud!

So what I am wondering is just how loud all these amplifiers get?
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Old 09-14-2008, 10:15 PM   #13
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My volume go to 11 so I need a 30 amp circuit.
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Old 09-14-2008, 10:19 PM   #14
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Noob needs help understanding sub-panels, please!


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What I'm wondering....

My *whole* home entertainment center works just fine off of one 20 amp outlet. I'm sure 15 amps would do fine as well. But I have never had my stereo at full volume, it would be way too loud!

So what I am wondering is just how loud all these amplifiers get?
They could get louder, I'm sure, but that's not what it is about. Each of your speaker cabinets has an internal network that divides the already amplified sound spectrum into multiple bands (probably three), then diverts those sounds to the speakers that can best produce them. Sounds good, but the electronics involved in doing all that dividing add distortion.

I'll be dividing the sound spectrum electronically before the sound is even amplified to eliminate that source of distortion, in actuality each of the three speakers in each cabinet will have a separate amplifier for it. It allows the speakers to be matched to the amplifiers for better quality sound.

My ears already ring when things get loud, that's why I do this--better quality sound equates to less need for dB's for me .

I count 6 amps for the system, possibly eight, with a total wattage of 300+ watts per channel, and a powered subwoofer--all this for a stereo system...but it is one fine quality stereo system!

Dugly
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Old 09-14-2008, 10:35 PM   #15
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Well that takes the cake!

A stereo with its own subwoofer *and* subpanel!

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