I'm finishing my basement and need to add a sub panel. My main panel is 150A and the only size I could find for a sub was 125A. What do I need to buy to connect the two together? From what I understand, I can't take a 125A breaker to power the sub because it's more than 80% of the load but is it okay to to just use a smaller breaker? What else do I need to buy to connect them? I have an electrician coming to do the actual work but I'm supposed to have the supplies ready before he comes.

 

With out knowing the loads on the subpanel there is no way to accurately guess the answers to your questions but reading between the lines of your post I sense some confusion on the subject. Why do you say you couldn't find anything but a 125 amp subpanel. Subpanels are readily available at the home centers in sizes from 60 amp and up.

You do not need to supply a 125 amp subpanel with a 125 amp circuit. More commonly for what you seem to be doing you would use a 100 or 125 amp panel to give plenty of breaker space but supply it from a 60 amp breaker using #6 wire.

Also if both main and sub are in the same building you could use a main lug only panel. Often panels with main breakers are used in that situation only because they can be bought for less then a main lug panel and come with breakers.

 

Lost me here!? Are you planning the project, or the electrician. I believe you need to get your electrician involved in what you need!!! If he or she doesn’t know, then you need to start looking for a new electrician. Just being honest!!! I find it strange you are here asking us questions that your electrician should be able to “better” answer!!! It would be different if you was personally doing the project, or just wanted to check on the validity of your contractors (Electrical) knowledge of code. Note: Communication with trust is number one. Make sure you have it, or your project want go smoothly!!!

 

The 125 amp is the max feed to the panel. You can install a lesser size feeder.

 

Clarification: I have 12 new circuits. Due to the lack of spaces available in my main panel, I had to have a sub with at least 9 slots and the only one available with that many was the 125A.

 

Doesn't change anything. You could install a 200 ampere panel with 42 spaces and still feed it with a 60 ampere supply.

What you DO need to do is a load calculation on the new panel (not just add up the circuit breaker ratings) and then select the feeder circuit breaker sufficient to make the maximum expected load. As an example, I have an auxiliary panel that serves all the loads that can be supplied by my generator. It has in excess of 150 amperes of combined circuit breaker ratings but is fed from a 60 ampere circuit breaker in the service panel.

 

Please note we are all using "60 amp" in our reply as an example. As Furd wrote you have not given us the information needed to do more then guess. We need to know at least if it is next to the main panel and if any high amp loads will be added.

To reiterate what SeaOn wrote I also find the fact that the electrician is having you buying the supplies suspect. Is he licensed or "jack-leg".

 

Obviously the OP is doing the work.
I agree with the others.

 

How do I calculate the load on the new panel? What info would you need? The person installing is an uncle that is a retired electrical engineer. My husband promised him we would have everything ready for him if he would just come hook it up for us.

 

Sorry to be blunt but that does not make him an electrician. May make him more dangerous then someone who knows they don't know what they are doing.

Here is an on line load calculator to help you determine the load. Single Family Dwelling Electrical Load Calculator

I'd suggest you come back when you finish with the load calc and let the pros walk you through what needs to be done. It would probably be best if you did it yourself, not that hard.

 

Okay, not sure what numbers you needed but here is what I calculated:
Total Computed Load
8825 Volt-Amps
Computed Amperage
8825 divided by 120 = 74Amps
Total Neutral Load = 7385VA
Total Neutral Amperage = 62Amps
Basically, we have about 1700 square feet with a kitchen including fridge, dishwasher, disposal, and convection oven (no stovetop). There is no laundry. Let me know if there is any other info you would need. Thank you so much for your help with this. You are wonderful!

 

Is it 240v? What is the amps on the name plate? Is the new subpanel going to be next to the main panel or at a distance? Is there space for only one 240v breaker in the main breaker panel? Does it take tandem breakers? If you don't know please post the make and model number of the panel.

Assuming the convection oven is 240v (or 120/240v) and the subpanel is next to the main panel I would suggest connecting it to the main panel and if you don't have room for the oven breaker in the main panel shifting low amp loads such as lights to the subpanel to make room. That would probably mean you only need a 60 amp feed to the subpanel.

 

The oven is 120V (here is a link for the info: PSB1201NSS | GE Profile Advantium® 120V - 30 in. Wall Oven | GE Appliances). The new subpanel is next to the main panel. The main panel is Siemens G2030B1150. Yes, it takes tandem breakers. I don't know if there is room for another 240V breaker but I am assuming that doesn't matter since my oven is 120.

So you are thinking I only need a 60 amp feed? And then I also need a wire as well, don't I? What size would I need?

 

It sounds like a 60 amp would be fine but over kill and maybe no subpanel needed.

Fridge=15amp dedicated circuit (optional. can share)
Diswasher/Diposal=20 amp shared circuit unless one is a larger amp draw then normal
120v Microwave convection combo=20 amp dedicated circuit.

By code you also need two 20amp GFCI protected counter top circuits. Also, best practice, you will need a 15 amp lighting circuit.

When you lay it out a 60 amp panel will be more then enough. However you may not even need a sub panel. Depending on how many tandem breakers already in your main panel and how many allowed you may be able to just run from your main panel. Based on above breaker count you could use six tandems in three spaces in your panel assuming permitted by panel guidelines.

The highlighted numbers indicate 20 full size breakers or 30 circuits max using tandems so you can use up to 10 tandems. How many tandems and open spaces do you have?

 

Our main panel is almost completely full already. The stupid ARC breakers that take up a full space is what really kills us. We have a total of 3 ARC circuits, 5 circuits in the kitchen (as you mentioned), and 3 additional circuits for family/game room. So a total of 11 new circuits, 7 spaces needed if we use tandems.

 

You posted as I was editing my post above. Please see edit. You can use up to 10 tandems. How many do you now have? Does local code require ARC in rooms other then the bedroom?

I'm leaning more toward a subpanel for now and to allow for future expansion if needed. A 100 amp subpanel with a 60 amp feed should give you more then enough spaces and enough power.