How much current to subpanel

[Aaron Fude]

Hi,

I have a subpanel that is in a very convenient space and it is so very temping to put a lot of stuff on it (kitchen /includes 30amp stove + garage + central A/C + etc), but my electrician told me that it's not proper to draw more than half the amperage from the main panel. Is that so and what's the rational for it.

The sub-panel is 18ft away from the main panel and is fed by a 2-2-2-2 wire. I found this table

WG gaug Maxamps for chassis Maximum amps for power transmission

2 181 94

which I guess is saying that the max amps is 94, but I'm hoping that that's a rating for 100ft and I can go to about 125 over 18ft.

So I guess there are two questions in one:

1. Can I draw 125 amps on the basis that it's more that 1/2 the total service (200amps)?
2. Can I draw 125 amps on the basis that it's 2-gauge wire?

Thanks

Aaron

[RBM]

I would recommend running your heavy loads off of the main panel, simply to prevent lights dimming due to the smaller feeder (#2 vs. 4/0)

If this is an existing sub panel, it will have a main breaker in the service panel, which will limit the current that can be pulled from it, which I can't determine, not knowing if it's feeder is aluminum or copper

[Aaron Fude]

The subpanel is fed by 2-2-2-2 copper.

[RBM]

[trader4]

I don't know the code specific answer to how many amps you can draw off a main panel, but I can tell you this. In a 4 year old house here, I just finished helping a friend add a new line for a new irrigation well. We hooked it to an 80A sub panel that was installed for the pool eqpt and run off one of the two 150A main panels. That sub panel had passed local inspection and is more that 50% of the main.

[RBM]

Your friend has a 300 amp service split between two "main" panels. The pool sub panel fused at 80 amps is not greater than half the size of the main panels. The OP has a 200 amp service, a 100 amp sub panel, and seems to be asking if he can pull 125 amps on it

[gfretwell]

The real answer will come from a load calculation. Nothing else matters. You can install a 200a sub panel on a 200a service if you want, the code is silent on it.

What are the loads on that current service?

2ga cu is good for 115a so you can put a 125a breaker on it using the round up rule.

240.4(B) Devices Rated 800 Amperes or Less. The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met: (1) The conductors being protected are not part of a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads. (2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments). (3) The next higher standard rating selected does not exceed 800 amperes.

[HeyBub]

I live in what was orginally a duplex. Each side was equipped with 200-Amp service.

By the simple expedient of adding a door to the common wall, I converted the building to a single-family residence. Bingo! I have 400-Amp service split between two main panels.

Is it your position I can now put a 175-Amp sub-panel off of one of the main panels because such would not exceed 50% of the total power delivered to the building?

[RBM]

[RBM]

[RBM]

Unless it's 2/3 Romex. It's not at all clear to me, what he's run to the sub. I know he said 2222 copper, but that doesn't make sense either.

[gfretwell]

I don't think they even make #2 RX and if this was SER the EGC would not be #2. I was assuming THHN/THWN in pipe.

[RBM]

I hear ya. I've used 2/3 Romex, but where I am, I can't get copper SER, or SEU for that matter, and of course the ECG would be smaller, which is why I question what the OP actually has. The OP says, " The sub-panel is 18ft away from the main panel and is fed by a 2-2-2-2 wire. Clearly, unclear

[Sam E]

[snip]

Wasn't it 2-2-2-2? 4 wires, all 2-gauge copper?

[gfretwell]

You are right; From the White book

NONMETALLIC-SHEATHED CABLE (PWVX)

"This category covers Types NM-B and NMC-B nonmetallic-sheathed cable, rated 600 V, intended for use in accordance with Article 334 of ANSI/NFPA 70, ??National Electrical Code?? (NEC), and Listed in copper sizes 14 to 2 AWG inclusive and aluminum or copper-clad aluminum sizes

12 to 2 AWG inclusive. This cable contains conductors rated 90°C; however, the ampacities of the cable are those of 60°C conductors as specified in Article 334 ..."

I have just never seen #2 RX. Guess I don't get out enough ;-)

[RBM]

Here is what he wrote: " The sub-panel is 18ft away from the main panel and is fed by a 2-2-2-2 wire.

Often, when people say wire, they mean cable. Since he's writing in the singular, "a 2-2-2-2 wire", my assumption is that he means cable, however, as gfretwell points out , there isn't any such cable. At this point, only the OP can clarify

[Tom Horne]

Aaron The term Sub Panel is not found in the National Electric Code. I don't know if your electrician is talking about what he/she believes to be good practice or what they believe is a code issue. The most accurate way to differentiate a "Sub Panel" from the Service Equipment for code purposes would be to call it a Feeder Supplied Lighting and Appliance Panel Board. Several manufacturers make feed through lug panels that are listed by a testing laboratory as "Suitable for use as Service Equipment. These panels have a main breaker and a set of main lugs, usually at opposite ends of the buss bars. They are designed to pass most of the current that comes through the main breaker into a feeder that is attached to the feed through lugs. One very common use for such panels is to provide a main breaker and a few load breakers on the outside of a home while moving the homes lighting and appliance panel board to a place within the home were the wire runs to the loads will be shorter.

As another poster has already opined much depends on the calculated load. If the calculated load is less than the ampacity of the feeder conductors then I cannot see were that would be wrong. Please remember that even though the code allows you to use a 125 ampere breaker to protect a number two copper feeder it would be dammed foolishness on your part to load it past it's rated ampacity of 115 amperes if it is #2 THHN in conduit or 95 Amperes if it is run in Non Metalic Sheathed Cable.

-- Tom Horne