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post #1 of 22 Old 09-24-2002, 07:18 PM - Thread Starter
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Fellow HT-heads,

I've searched past threads for advice on this, but I can't seem to find a definitive answer. Can anyone help with the following:

I want to install a 100-amp 2-phase subpanel (with no main breaker in it) in my basement with the following circuits running from it:

(4) 20-amp circuits
(4) 15-amp circuits
Total = 140 amps

I've done some figuring and I conservatively estimate that at no time will more than 70 amps be drawn from the subpanel.

However, to be safe I would like to provide an 80-amp feed from my main panel using a double-pole breaker. My questions are 1) To provide 80 amps total to my subpanel using a double-pole breaker, should the breaker be rated at 80 amps or 40, and 2) What size wire should be used for the subpanel run, assuming 60 C rating.

Thanks in advance for any and all help!

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post #2 of 22 Old 09-25-2002, 09:28 AM
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I believe you need to install an 80 amp breaker in the main panel and run #4 AWG to the mains in the sub (#4 AWG has 85A rating per NEC 2002, Table 310.16 assuming 75 C wire, if you stick with 60 C wire you need to move up to #3AWG or larger).

If in doubt, consult your local code official.
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post #3 of 22 Old 09-25-2002, 02:23 PM
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Each pole of a 40A double pole breaker will provide 40A at 120VAC. So the total amperage available at the panel would be 80A. Use a double pole 40A breaker. And use 3/C #8 AWG (w/ground) (Two hots, one neutral, and one ground).
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post #4 of 22 Old 09-25-2002, 03:51 PM
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Maybe a simpler way to look at this is on a KW basis. If you require 9600 watts or less, go with the 40A breaker, if you require up to 19,200 watts go with the 80A breaker. Using KW is the easiest way to keep things straight.
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post #5 of 22 Old 09-25-2002, 08:16 PM
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filmnut

First of all I would not load breaker or wire over 80% of rated ampacity.
The reason for this is to avoid nuisance tripping.

I plan on running 3c#6 copper to my sub panel in my theater, in combination with a 2 pole 60 amp breaker. This gives you almost 50 amps per leg @ 80%.

copper
#8-- 45 amps
#6-- 65 amps
#4-- 85 amps
#3--105 amps

Typical household you are allowed 12 convenience outlets on one 15 amp circuit.

At least that's the way its done in frigid north.
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post #6 of 22 Old 09-26-2002, 05:48 AM
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Why do you want to use a 100 amp panel without a main breaker in it? Why not use a 100 panel with a main breaker? It much simpler. Then you don't have to add breakers to your other main panel. In this case you would just use wire rated for a 100 amp service.

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post #7 of 22 Old 09-26-2002, 07:05 AM - Thread Starter
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I appreciate all the help guys.

Aviman33, the reason I went with a subpanel without a main breaker is so I could use smaller gauge wire. I have plenty of space in my main panel for additional circuits, so at this point I'm leaning toward using a 40-amp double pole breaker and #8 wire (which is cheaper and easier to work with than #3).

I did some more figuring last night, and 99% of the time I should be pulling no more than about 20-25 amps per leg off the subpanel. With 100% lights, equipment, HVAC, etc. pulling at the same time, it should be no more than 35 amps per leg. But I can't imagine ever having that scenario.

The subpanel and home run will be located in an unfinished area of the basement, so if I ever need to upgrade the ampacity, it can be easily done.

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post #8 of 22 Old 09-26-2002, 12:59 PM
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Why not use a 100 panel with a main breaker? It much simpler. Then you don't have to add breakers to your other main panel. In this case you would just use wire rated for a 100 amp service.
A clarification to this quotation: If the subpanel is located within 10 feet of the main panel, the NEC tap rule may apply and the main panel breaker may be omitted. However, in the general case, a main breaker is required. See NEC code section 240-21.

You have to have a breaker anyway, so why not put it in the safest place: at the source (in the main panel).
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post #9 of 22 Old 09-26-2002, 02:32 PM
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You have lots of input here, but if you are concerned with wiring this sub-panel up in accordance to the NEC (National Electrical Code), you probably need some better direction than these replies (no disrespect intended to the posters, but the NEC rules can be somewhat tricky).

Here are some things to consider:
1) Do you really need this sub-panel? If you have plenty of available breaker spaces in your existing panel, the only advantage in a sub-panel is to minimize wire runs for your branch circuits.

2) What types of loads will this panel service? Will your new loads overload your main service panel's capacity? If you are only adding general use lighting and receptacle loads (even HT equipment is generally considered general use), probably not, but you mentioned HVAC. To answer these questions you will need to do a load calculation for your service, which is tricky to do. If you are so inclined, a helpful website for doing this according to the 2002 NEC can be found here: http://homewiringandmore.com/hom...DmdCalc02.html

3) You do not need a main breaker in your sub-panel if it derives power from your main service panel. Sub-panels are not service rated panels and therefore do not fall subject to a main disconnect or "6 swipe" disconnect rule. The term sub-panel implies that this is in fact a non-service rated panel, which means the sub-panel can be disconnected via the main service panel or main disconnect. If the "sub-panel" is being directly connected to a power meter, then it is not a sub-panel but a service rated panel, and many more rules apply to it's location, wire sizing, etc.

4) Are you planning on running any 240V loads, or strictly 120V?

You mentioned that you want to add (4) 20-Amp circuits and (4) 15-Amp circuits to this sub-panel. Note that if you are powering the panel with 3 current-carrying conductors (neutral and 2 wires making up the 240V), your total number of amps drawn by the panel is only HALF the sum of your total 120V loads. This is because your sub-panel will running 240V, so half the loads will be from one 120V leg, and half will be off the other (approximately). If you are certain these 8 circuits are all you will ever use from the sub-panel, you would be safe wiring the sub-panel to handle 70A, maybe even less. But if you wanted to allow for future expansion, you should wire it to handle more.

Note that wiring the sub-panel allows lots of freedom, but once you pick your wire size, you are limited in the breaker size protecting it in the main panel. If you use 8 AWG wire to the sub-panel, you cannot simply upgrade the sub-panel's capcity by upgrading the breaker in the main box from a 40A to a 50A; you would have to change the wire too.

In general, you will be better off running as large a wire as you can - better to be too big a wire than too small. Also keep in mind that certain circumstances require you to "de-rate" your wire's ampacity (temperature, number of conductors in the conduit, length of run, etc). So sizing the wire and breaker may be a little tougher than simply looking up the value in a table.

By the way, there is no hard limit to the number of receptacles you can put on a circuit. The limit is calculated based your general lighting requirement (sq footage of the finsihed area) and how many lighting branch circuits you have for that area. Local code may have a limit, however.

If you really don't want to protect yourself against having to upgrade in the future, have no dedicated loads from the sub-panel (HVAC, pumps, etc), you can get away with a 30A double-pole breaker (2 @ 30A each) and 10 AWG wire, assuming you don't have any de-rating issues.

Note that you cannot generally run NM cable against concrete basement walls and have to protect the cable's sheathing with conduit. If you keep the cable in the floor joists and in walls, you can probably run NM cable. Of course, you should check with your local inspector to be sure.

If you have to use conduit, you might want to upgrade the service to be one size bigger than needed. If you have to go through the hassle of pulling cable through conduit anyways, you might as well up-size the wire and conduit to have some room to grow!
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post #10 of 22 Old 09-26-2002, 03:53 PM
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Quote:
you probably need some better direction than these replies
Good job ssabin, your reply is much, much better.
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post #11 of 22 Old 09-26-2002, 04:31 PM - Thread Starter
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Quote:
Originally posted by ssabin
[b]Note that you cannot generally run NM cable against concrete basement walls and have to protect the cable's sheathing with conduit. If you keep the cable in the floor joists and in walls, you can probably run NM cable./B]
Does NM cable have to be in conduit even in the interior? My main panel is located on the exterior of the house, and my subpanel will be about 30 feet away, mounted on a sheetrock wall in the basement interior. The feeder would run about 10 feet along the exterior of the house, then through the wall into an elevated crawlspace, continuing into the basement utility room where the subpanel will be. The plan is for NM conduit on the exterior portion of the run, but once it's inside do I still need the conduit all the way to the subpanel?

ssabin, your advice was extremely useful. some of it I already knew but I also learned a few things. Thanks! BTW, when I said my main panel had plenty of space, I may have overstated it. It has 4 breaker spaces left, which is good for a double-pole feeder plus 2 tandems. My main reason for wanting a sub is the distance to the loads and the number of new circuits.

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post #12 of 22 Old 09-27-2002, 07:56 AM
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In general, the NM cable has to be protected from physical abuse. This usually allows an NM cable to protrude through a house to the outside, as long as it is terminated within a reasonable and short distance. I would think 10 ft of exposed NM cable would have the possibility of rubbing against the house (especially if brick sided), or being stepped on, so most likely it will need to be protected with some sort of conduit. Normally, TH/TW types of wires in conduit are used in outdoor runs, TW where exposed to wet locations.

If I had to guess, I would say that your inspector would probably not allow a 10 ft run of NM cable along the exterior of your house. But, the enforecement of what is considered physical abuse prevention is really inspector dependent. You should ask your inspector to be sure.

As a final note, if the inspector's only concern is cable protection from abuse, then you can use conduit without the conduit fill limitations applying (your NM cable can exceed the fill diameter of the conduit allowed by code). This will still be a bigger size conduit than you would have to use with TH/TW wire, however, so the only benfit is that it might allow you to eliminate having to tie TH/TW wires to NM cable wires near the entry point into the house.

Regarding NM cable in the interior, no, you aren't allowed to run NM cable in an exposed area. Between studs and joists is not considered exposed, but below joists and on top of sheetrock is. And most inspectors will require some protection if run along an unfinished cement basement wall.

My local inspector allowed my A/C installers to run the wire to the compressor to run underneath (but stapled to) the bottom of the floor joists in the basement, so even though this is against the rules, some things may slide if you ask your inspector ahead of time.

The most popular way of mounting a panel on a wall with sheetrock is to mount the panel between the studs to make the cover flush with the finished wall. It's not a pleasant thought, but you should consider opening up this wall and running your wire (NM would be perfectly fine) in the wall studs to your sub-panel. If you don't want to do this, you will need conduit, and you should ask your local inspector what type of conduit is allowed for this application.
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post #13 of 22 Old 09-27-2002, 08:13 AM
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I am not an electrician, but from my previous research on one of the Electrical forums and review of NEC codes, if the wiring is inside the house, you must use the 60 deg rating. I am installing a 120A subpanel (no breaker) connected to a 70A double-breaker in the main panel, wired with 3 conductor w/ground #4 Romex. This was the most cost effective way for me, which still met code. Going to an 80A double-breaker would have required #2, which is usually only available in single conductors, so you would have to purchase 3 separate #2, plus ground, and run them in conduit.

To relieve the load on the subpanel, I ran as many 20A circuits as I could to the main panel first, and am using the subpanel for the lighter load circuits and future expansion of the remainder of the basement and house. Sure, it required longer #12 runs for circuits, but I am going to a "bigger pipe" 200A vs the more restricted 70A pipe.

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post #14 of 22 Old 09-27-2002, 06:21 PM
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Geordon,
Your "bigger pipe" analogy is not true. All of the load thru your sub-panel also comes through your main panel. A sub-panel does not add any overall electrical capacity to your house.

Buying single conductor wires is very similar in cost to buying the same size NM cables. In fact, they are essentially the same wire. The relevant difference is the ease of installation.
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post #15 of 22 Old 09-27-2002, 06:42 PM
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ssabin,

If you think I am wrong, you misunderstood what I meant. I know the total amperage is still 200A, but what I was saying what that if filmnut is adding 8 circuits for a potential of 140A, he should not expect to get use of anything over 70A through the feeder, whereas, if he could add all 8 circuits, which probably isn't possible, but for sake of argument, is possible, then he would have access to up to 200A minus the draw from the rest of the house.

In other words, the best filmnut can hope for on the subpanel is 70A to distribute among 140A of circuits vs the possibility of getting to use all 140A from the main panel.

Does this make sense to you now?

As for cost of #4 Romex vs 4 separate conductors in conduit, we must be in different markets, as I know I can't buy 4 separate conductors plus conduit for the same price as equivalent Romex.

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post #16 of 22 Old 09-27-2002, 10:39 PM
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I don't have the code book at my side so be gentle, but to get 200amp 2ph at the remote/sub panel its something like #0 or #00 copper in free air between the two. In conduit residential I believe you up 1 size ? Aluminum the size increases again.

So the sub-panel needs to be on a supply breaker.

I think filmnut would be OK with 80amp 2ph. That puts his circuit load equal to the supply without relying on 80% circuit loading. Although check local codes for load rating sub panels.

Than again like many advanced members say you want all your equipment on the same phase!! Oh boy!!

AJF

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post #17 of 22 Old 09-28-2002, 09:35 AM
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Do you know my brother....yardlong! :D (really)

What size service do you have in your house? This should be the starting point for your sub panel sizing.

THE FOLLOWING PARAGRAPH IS FYI ONLY!

You have calculated your loads based on the ratings on your equipment. These ratings are based on the maximum the equipment will draw under extreme conditions. I doubt you would ever get any of your equipment to draw the name plate rating on it. Motors would be an exception. On start up they can draw 5 & 6 times there name plate rating for a second or two. This is why you want to avoid having these in your sub panel, or your house for that matter.;)
As well, they send all kinds of interference down the line. Ever have your neighbor start up his table saw while your watching TV. Watch your lights closely when your wife starts up that big honkin hair dryer.
Put your mind at ease a little, and take a clip on amp meter and check various electrical equipment and compare the name plate to your readings.

As stated above, that was just to put your mind at ease a little. However bigger is always better .....right Huge Member.....The larger the wire, the less resistance,( sometimes.... :p sorry, couldn't pass it up) the less resistance, the easier it is on all the equipment.

Now when wiring your sub panel, avoid putting motor loads into that panel along with fluorescent lighting. Keep your wiring away from other wiring feeding motor loads, fluorescent, and video and sound equipment. When installing breaker in main panel that feeds sub panel, avoid placing beside breakers with motor loads connected. If running metal conduit outside of house, as suggested above, make sure to bond it to your panel with #6 bare (mechanical protection) or green to act as a drain for any outside interference (air conditioners etc). Also NEVER COIL cables of any kind, make long snake like runs with space between if you have extra.
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post #18 of 22 Old 09-28-2002, 01:44 PM - Thread Starter
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Well, it's about time somebody made a comment.:p The good news is that the subpanel will serve no motor loads. Not sure about the main panel - is a dryer or an A/C compressor a motor load?

The other good news is that of my 8 circuits wired to the subpanel, 4 are for equipment and 4 for lights and other stuff. I've wired the equipment circuits on one phase and everything else on the other. I planned the circuit ampacity partially so the loads would be balanced.

FYI, I believe it is a code violation to run NM-sheathed cable (like Romex) inside metallic conduit. My plan is for flexible NM conduit.

Tell Yardlong I said hello.

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post #19 of 22 Old 09-29-2002, 08:37 AM
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Yes. Dryers and A/C compressors are motor loads.

Also, make sure that even in the main panel, you keep the phases straight. Keep your theater equipment on a phase with no motors or flourescent lighting ( no mechanical dimmers if you can help it) on that phase in EITHER panel. This may require moving some circuits in the main box.

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post #20 of 22 Old 09-29-2002, 12:02 PM
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filmnut

You can try to get all motor loads on the same phase. I wish you luck on your endeavor. You may find, unless you have a gas dryer that this will be impossible. If you end up putting both legs on the same phase they will not work, as you will have a potential of 0 volts between the two legs even if you have 120 volts to ground.

You still however should keep the motor loads out of the sub panel. The reason for this is that the wiring from your service will be larger than that from your sub panel and can therefore handle the in-rush on start up better and lessen the effects on your theater equipment.
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post #21 of 22 Old 09-29-2002, 06:39 PM
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Jim is right. It's impossible to keep one of your power legs "clean" for use in your HT. If you have any 220V circuits whatsoever (electric range, electric dryer, A/C, hot tub, well pump, electric heat, etc), you have a motor load on BOTH legs.

People get very into tweaking all kinds of things with HT equipment, and the power wiring is another area. Do the things that you can which make sense, but don't stress over it. Chances are your biggest problem will be caused by that damn cable TV ground or lack of it! Each piece of equipment we use in our theaters has a power supply that is designed to be robust to noisy AC.

For what it's worth, Romex can be used in all types of conduit. However, in an installation where conduit is explicitly required, you calculate the conduit fill using the widest dimension of the NM cable as the conductor diameter. This results in needing very large conduit. Aside from the bigger size costing more, and perhaps its difficulty in being hidden, it's just as easy to work with. If you only need conduit to protect the NM cable from a masonry wall, then this fill requirement does not apply and you can use whatever size you wish.

As a final note, you might again want to check with your local inspector regarding the flexible conduit. Some locales expressly prohibit its use, and it has more limitations in the NEC as well.
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post #22 of 22 Old 09-30-2002, 07:42 PM
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I'm not an electrician, but I did stay at a Holiday Inn last night!

Actually, I am an electrician. This is an interesting thread. Most posts were accurate, but I'd like to add my two cents' worth:

First of all, except for specific single loads, breakers and fuses are there for one thing: To protect the wire. They don't care about your appliances or other loads. If you overload a conductor, the heat damages the insulation, resulting in increased fire hazard.

When wiring a sub-panel, the breaker goes into the main panel, not the sub-panel. The conductors between the two panels must be protected, and this is done at the source end. The panel rating defines the largest total load that the panel should carry (with an 80% allowance for continuous loads), so the 70 amp panel should never be subject to greater than (70 X 0.8 =) 56 amps. This is per phase. This means that the predicted load will be fine.

The breaker for the main panel has to be limited to 70 amps to suit the sub-panel and the feeder size, whichever is lower. If you believe that the total load will never exceed 80 amps at 120 V., which is the same as 40 at 240 V., then #8 cu or #6 al could be used, as long as the breaker in the main panel is sized accordingly to protect this wire size. Therefore, a 40 A./2-pole brteaker would be suitable. For the purpose of reduced voltage drop, it is certainly permissible to use a larger wire size, but a 30' run is not long enough to require this.

The conduit needed for physical protection needs to be run unbroken from panel to panel if individual conductors are used, but only needs to be used where the run is exposed on the outside of the building, and where exposed to physical damage inside if a cable, such as NM or SE is used. If the conduit is full-run, and metal, the lock-nut connections to the panels are sufficient fro grounding. If it's only used in short section for sleeving, grounding is not required. The crawspace is exempt, as are any in-wall locations, from exposed status.

The one point not explained by anyone is an important one: Beyond the service equipment (the main disconnect, whether in the main panel or before), the neutral (grounded conductor) and ground (grounding conductor) are to be kept separate. This means that the wiring run to the sub-panel must contain a total of 4 conductors (unless the aforementioned metal conduit connects the two panels electrically), the two hots, the neutral (white), and the bare(or green) grounding conductor.

Even though they are the same conductor ahead of the main disconnect, and may be connected together by the "main bonding jumper" in the main panel, they must be separate beyond that point. In the sub-panel, each circuit's neutral will connect to the neutral bus (strip), along with the white wire in the feeder, but DO NOT install the screw or strip that is supplied to join the neutral bus to the sub-panel's box. Instead, buy a separate ground bus for the sub-panel, and connect the bare wires to this strip, which will be screwed to the panel box, and will have the bare wire in the feeder attached to it.

I hope this was helpful, if not for filmnut, then for others at this stage.

Larry
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