Sub panel install

Quick question: If I'm installing a sub panel of my main box(100 amp service) using a

100 amp breaker as feed can I get away with 4/3 romex or does it have to me something bigger?

Thanks in advance for all your help.

p.s. It's an 8 space 16circuit Square D 100a main lugs panel. (I bought the extra grounding bus to add). The chances of the sub panel getting full is about 80% mostly with 15 and 20amps.

Reply to
Nitram
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#4 Romex is only good for 70a. Either use a smaller breaker or bigger wire. #2 copper SER would be a better choice for 100a if you are using a cable.

Reply to
Greg

Romex is really not rated for service cables. SER is, good call Greg

Do a load calculation and see if you really need 100 amps in the new box. I bet you do not, you could same some bucks. That 100 amp 2 pole breaker is going to be spendy.

Reply to
SQLit

crikey ! - things must be very different your side of the pond UK 100A breakers (neutral make first) are $20

Reply to
Chris Oates

The branch circuit rated breakers are all about the same price between 40-100. It is the "service rated" breaker that gets spendy. The difference is the amount of fault current it will handle.

Reply to
Greg

This is a subpanel fed from a breaker, so this is a feeder and not a service conductor. Therefore, NM cable (romex) is allowed.

A 100A breaker is usually around $30. Cost is much less if you buy it with a panel, as I bought a CH panel w/100A breaker for $69 whereas the MLO panel was $60. This breaker is a normal branch circuit type with a hold down clamp and not a bolt in square one.

-- Mark Kent, WA

Reply to
Mark or Sue

The only problem is the largest Romex made is #2 and that is only good for 95 amps. We were talking about 100a feeders.

Reply to
Greg

Help me out here. The code says that #4 can be protected at 100 amps if it is being used for (residential) service entrance conductors or feeders. It also says that feeders do not need to be larger than service conductors.

So, if you have a 100 amp (residential) service with #4 entrance conductors and you want to run a 100 amp subfeed to another panel, what size conductors would you use for the subfeed?

Side point: If #2 is rated at 95 amps, isn't it legal to use the "next larger" disconnect (100 amps) in that case? (obviously 95 amps is not a standard size)

Reply to
User 1.nospam

Read 310-16(B)(6) again. You acn't just look at a table without reading the application. "... feeder conductors that serve as the main power feeder to a dwwelling...".

This assumes "diversity". You can't guarantee diversity on a feeder to a sub panel. I agree it will be OK 99% of the time, depending on the application but it isn't "code". That also assumes you are using something other than type NM (Romex). NM is always limited by the 60 degree column of 310-16 (344-80)

Reply to
Greg

--snip--

Hmmm, what is 310-16 (344-80) of the NEC? Here in Canada I'm pretty sure NMD90 (Romex) is rated for 90 degrees. what do you use to feed light fixtures, which require 90 degree wire?

Romy

Reply to
Romy Singh

310-16 is the table that determines ampacity. 334.80 (my previous was a typo) is the section describing the ampacity of type NM in particular. Type NM, AC and UF have an artificially lowered ampacity requiring that they only be used at the 60c rating for the base ampacity. You can use the 90c for derating purposes. I suspect this is because these are the main cables used in residential wiring and residential sparky's are not as likely to understand what derating is all about. In a hot attic you could easily be approaching the point where the ampacity limits would cross. This is actually moot in 14/12/10 ga anyway because there is also the 15/20/30a limitation in 240.4(D) It really only becomes a factor in #8 and higher.
Reply to
Greg

There's a lot of words in 310.15(B)(6) spread between the text and the table. In his example of a

100A service, then a #4 cu feeder to a 100A subpanel would be allowed because you never need a feeder to be larger than the service entrance conductors. But I would question how you get a 100A sub on a 100A service, as there are probably other loads than the subpanel.

I also question why you don't get diversity on a sub panel. If you get diversity in the calculations, it should apply to loads regardless of how they are segmented. If you had a 200A service that was split between two 100A panels, you still have the same house loads spread across those two panels. The diversity in each panel will be less, but the calculations take this into account assuming you calculate each panel as if the other does not exist. I have a similar problem with my 400A service. As a whole, I have plenty of service (calculates to 300 to

340 amps depending on the method). When I split this between two panels, each is very near its 200A limit (mostly because I didn't reach the 4 or more fixed appliances in either panel, you have a full hit for the first 3 KVA in each panel, and the two kitchen ranges are not in the same panel). I could feed those panels per 310.15(B)(6) without question, although I used 3/0 copper anyway.

And you're right, you can't use NM when using 310.15(B)(6), but #4 copper SER would work.

-- Mark Kent, WA

Reply to
Mark or Sue

It all depends on what the sub panel is feeding. If this is a sub for a spa or an array of HVAC equipment, you could quite easily be using it at a significant portion of it's ampacity. My spa pulls 70a. by itself. When I also dump the pool motor and some lights on that it would seriously overload a #4.

As for putting the 100a sub on a 100a service, I would suggest that the service was probably overloaded. Bear in mind, when they allow undersized service conductors they assume you will not be using a service at the panel breaker size based on the original load calc. It is quite easy to keep adding loads and exceed the original design load still never exceeding the trip value of the main. At that point you will have some pretty hot service conductors if you used the residential exception. Simply because this is a residence, it doesn't increase the ampacity of the wire.

Reply to
Greg

I was thinking the same thing, but I have trolled thru my code book, front to back and cannot find a table for SER. I know it must be there.....can I have a citation please?

Reply to
User 1.nospam

SER is just 3 wire with ground SE. (SE Round) Article 338 NEC2002

It is listed for 75c if the terminations will handle that.

Reply to
Greg

I think it is difficult to use 100A for very long in a house. In the example you cite, a #4 feeder could normally be protected with a 90A breaker (unless using NM cable) since most breakers and panels today are rated at 75C. I don't see 90A as a serious overload. If you can manage 100A, then I'd agree that cable is slightly overloaded per 310.16.

Does your spa really pull 70A, or does it have a 70A circuit breaker? To properly load your subpanel, there is usually some device with some margin in its calculation, such as your largest motor. How long will that spa heater run before it begins to cycle on and off? I'd also think about applying a continuous load burden to pool heaters that could run for many hours.

Perhaps we'd feel better if 310.15(B)(6) began at 125A instead of 100A. At some point, it just gets harder and harder to pull that much current for more than 10 minutes or so, especially in a residence. But I'm sure you could always find an example where 310.15(B)(6) is not a good idea, but those are just not common at all in a residence once you get in the over 100A range.

-- Mark Kent, WA

Reply to
Mark or Sue

It has a 2HP jet motor, a 3/4HP circulation motor and 11kw of heat. do the math. It cruises around 66 with the clamp on when the heater is on. The same panel is also for a 1HP pool pump, lights and the service outlets.

310.15(B)(6) is just fine the way it is, you just have to read the whole thing. If you don't like the wording write a proposal to change it. That way you can get a free copy of the ROP. The 2005 code is closed but proposals for the 2008 code will open up in the summer of 2004.
Reply to
Greg

Use a 70a breaker to feed the sub panel. (and save $20) If the 70a feeding the sub panel trips, you most likely need to upgrade your 100a service.

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Reply to
Bill Coffel

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