Sub Panels

[transferguy]

posted by: jackp

Hi

I am running out of space on the main breaker panel (200 amp service but now lacking breaker space) and thinking rather than dealing with replacing the main panel to a larger one it would be far cheaper just to install a sub panel. Trying to soon renovate the basement including putting a small kitchen there, as well as a bathroom.

My question is to how to go about it...

1. is there a formula to determine the feed breaker size at the main panel that will feed the sub panel (sub panel will require both 120 and 240 voltages).

2. Grounding for the sub panel, is it ok just to ground to the main panel as the feed cable also has a ground wire in it, or does it also have to be separately grounded? Can it be grounded to a water pipe? (located in Canada)

3. what gage wire required to feed the sub panel?

Thanks
Jack

[transferguy]

posted by: suemarkp

The formula is the same one you use to calculate the load for the whole house, but it is reduced in scope to only cover the square footage and appliances served by the subpanel.

The subpanel will be grounded via the 4th (green or bare) wire in the feeder. Don't bond it to a water pipe.... It does not need nor require a ground rod unless it is in a separate building.

The wire size required goes hand-in-hand with the load calculation and circuit breaker chosen. If you need a 30A circuit, use 10-3 cable. For 40A, use 8-3. For 50A use 6-3. Above this you need to use the wire tables and know if you have 60C or 75C wire. With a subpanel mounted very close to the main panel, you could use a short conduit or offset nipple to connect the main panel to the sub. It would be use and inexpensive to then run 4 individual THHN wires through that nipple to have a 100A subpanel. Cable, up to 6-3, is usually cheap to run longer distances through 2x4 walls. Note that if the load calculation says you need 31 amps, then you need to run a circuit larger than that (so you'd run at least a 40A circuit).

If you're going to the effort of adding a subpanel, I'd go with at the 6-3 solution (best bang for the buck) and possibly larger if you think you may have need for a lot more power and the subpanel is close to the main panel. Also get a panel with more slots than you think you need (I'd put in a least a 12 slot panel and perhaps a 20 slot). You really can't have too many and if you must use AFCI breakers they require a full slot.

Mark
Kent, WA

[transferguy]

posted by: navyguy

In addition to what Mark mentions, I would consider a ?Service Entrance? rated panel with a main breaker. According to code you will not need that style of panel, but they are usually far less expensive than a ?loadcentre? style of panel. The size of the main breaker is not an issue as it will be protected by the breaker in the main panel.

Another option might be to consider ?twins? or ?minis?. If your existing panel can accommodate them, that may get you out of the bind if you only need a few more circuits.

I am not a fan of ?pony panels? or twins, but that is simply a personal choice. There are prudent times when a sub-panel should be installed such as a large addition to a home, new out building, etc.

The last comment is that I would consider using the same make of panel that already exists. Having said that, there are a number of common panels out there in Canada that are not made anymore. FPE as an example only makes breakers now. So this may be a consideration in deciding if you want to ?upgrade? to a larger panel at the service entrance.

Don?t forget, all of this needs to be inspected and you will require a ?permit?.

Cheers

[transferguy]

posted by: jackp

Hi Again

I think I need at least a 60 amp feed (as a small kitchen in the basement is part of this) but 100 amp feed would give even more headroom.

Navguy & suemarkp if I buy a new service panel, get a dual pole 240v breaker (60 to 100 amp) installed as a breaker in the existing panel, feed that from the existing main panel through conduit to the main service breaker secondary panel that would be ok? Now thinking again why would I want 2 breakers in line doing this one at the existing panel and one on the new service panel, that does not sound right to me?? The only way I could see logic in that is if the breaker feed from the existing panel was 100 amp and the main breaker at the secondary service panel was like 60 amps? Not sure if I read you right please clarify.

I know an electrician down the street but he is not going to help unless I pay him the big bucks, but may be able to get him to confirm that it will pass the inspection if I do all the work first.

[transferguy]

posted by: navyguy

Hi Jack

Do I live down the street from you? :-) ... Where in Canada are you located?

Regardless of the style or type of installation, you need to do the load calculation. Really it needs to be done twice, one for the entire property so that you know the main service can handle the intended loads and then a recalculation for the area to be serviced by the sub panel.

To answer your question about the sub-panel; it is just an extension of the main panel. The breaker(s) in the main panel are there to protect the wire from being overloaded. The breaker in the main panel has to be sized for the cable feeding the sub-panel. It does not matter what amperage rating the sub-panel has as long it is equal to or greater then the wire and breaker feeding it from the main panel.

As an example, you use a 60 amp breaker and #6 wire – you can install a 60, 100, 125 or 200 amp panel. You cannot install a 30 amp panel. If you install a 100 amp breaker and #3 wire, you could install a 100, 125 or 200 amp panel but not a 30 or 60 amp panel. The breaker in the sub-panel is only acting as a “disconnect switch” because the wire and the sub-panel are protected by the breaker in the main panel.

Hope that makes sense.

Cheers

John

[transferguy]

posted by: suemarkp

In the US, from what I've seen main lug only panels are cheaper than main breaker panels. If you need or want a main breaker panel, that is the cheapest way to buy them rather than adding a main after the fact to a main lug panel (I may even buy a main breaker panel and remove a backfed main to add to my breaker supply -- much cheaper than buying it individually). Generally, I don't care about a main in a sub panel unless it is far away from the main panel (e.g. large commercial building). If you have a detached building, it needs a main breaker. But again, you could have a 100A breaker feeding it with 100A wire and a 200A main breaker remote panel. It is just acting as a disconnect, and its overcurrent capability will not be used.

Mark
Kent, WA

[transferguy]

posed by: jackp

Yes I understand now, having a higher rated main breaker in the secondary panel will just be a disconnect and the breaker feeding the secondary from the main panel is actually the overcurrent protection to the secondary panel. Then the individual circuits added to the secondary panel protects each individual circuit. It all makes sense now. The secondary panel will be located close by the main service panel. Checked the breakers on the existing panel and even with changing some breakers to twins to get more in less slot space and it would work for some but it seems to be 1 to 2 short of what I wanted, I will recheck it again.

I could never see a situation that the home would ever be using more than 130 amps or so with most everything on at any given time, I have not yet actually sat down and figured the max it could be but it is a quick number in my head. The secondary panel would likely not have circuits adding any more than 60 to 65 amps, but when would you have every single heating element in use at the same time on a cooking range, so a 100 amp feeding the seconding panel should be fine. Usually the worse case scenario for a home as a whole would be having main floor and basement cooking ranges with most elements in use, a couple of cookers / fryers on a counter top, a couple of fridges going and the central air at full capacity running.

Thanks everyone for the information.

[transferguy]

posted by: navyguy

Here is a little DIY calculator to see what the demand is for yourself. It can't be used for ECs, but it will be good enough for you.

WEB LINK

Cheers

John

[transferguy]

posted by: dkerr

Navguy I may put a link on that load demand site or make a sticky with that link, it could be quite useful for a lot. If there are any others let me know.

Don