Subpanel Sizing

[jeff43222]

I recently rewired a kitchen in a single-family dwelling, and today I went in to install a subpanel to accommodate all the new circuits. Unfortunately, I discovered that the existing main panel (100A, QO, 20 space, almost full) can only handle #8 wire on the neutral bar, but I was planning on putting in something bigger than that.

My concern is that a 50A breaker protecting the feeder to the subpanel might not be enough for the potential loads that might be encountered. Here is what I installed in the kitchen:

15A ckt for undercab Xenon lights (80 watts total)
15A/15A circuit for split-wired disposal/dishwasher recp
2 20A ckts for small appliances
1 20A ckt for the built-in microwave
1 30A ckt for the dual-fuel range

If it weren't for the range, I'd say 50 amps is plenty, but I was thinking that if all the circuits were pushing the envelope, it wouldn't be too surprising if the 50A breaker tripped. Homeowners said they do a lot of cooking/baking, and they use the usual countertop appliances (coffee machines, toasters, etc.).

I'd really prefer to give them an installation where they wouldn't have to think twice about running the coffee maker and the microwave at the same time the dishwasher and range are on, but the only way I can see that is if I swap out the main panel.

Anyone care to offer up an opinion?

[ August 24, 2005, 03:51 PM: Message edited by: jeff43222 ]

 

[stud696981]

Re: Subpanel Sizing

I would probably just go with a 100 amp sub panel and feed. Use #3 Thhn copper. You don't have to go that big, but there would not be much difference in price so you could easily sell it to the customer. As far as the neutral bar, you can buy an adapter that goes on their to accept the bigger wire. If you go to the supply house and they don't know what it is just look through the Square D catalog.

 

[peter d]

Re: Subpanel Sizing

I would use 2/3 SER alumnium and protect with a 90 amp breaker. That would be more than adequate, and the aluminum cable is quite cheap. (About $1 - $1.50 per foot)

As studs suggested, Square D makes lugs to adapt larger wire onto the neutral/ground bars for both QO and Homeline panels. I have even seen them at Lowes and Home Depot, so they are readily available, and cheap too.

 

[charlie b]

Re: Subpanel Sizing

Do a formal calculation of loads. But when you calculate the neutral current, you can take a 70% demand factor for the range load. Here's an example, using fictitious loads (I don't know the ratings of the equipment):

Ungrounded Conductor Sizing
Xenon lights ? 80 VA
Disposal - 500 VA
Dishwasher ? 1500 VA
SA Circuits ? 3000 VA
Microwave ? 1200 VA
Range ? 8000 VA
Total = 14,280 VA (59.5 Amps)
Use a 60 amp or 70 amp main breaker on the subpanel (or on the main panel, feeding an MLO subpanel)

Grounded (Neutral) Conductor Sizing
Xenon lights ? 80 VA
Disposal - 500 VA
Dishwasher ? 1500 VA
SA Circuits ? 3000 VA
Microwave ? 1200 VA
Range ? 5600 VA (70% of 8000)
Total = 11,880 VA (49.5 Amps)
Use a 50 amp neutral wire (i.e., #8)

You will have to do your own math. This is only an example.

 

[jeff43222]

Re: Subpanel Sizing

I was not aware of these adapters. I think a trip to the local supply house is in order. They are a Square D distributor, and the counter guys do a great job hooking me up with what I need. I avoid Big Orange as much as possible. About the only thing I buy there is wood for mounting panels on foundations.

If this adapter thing works out, I think I'll use #4 Cu THHN, since I always have plenty of that lying around.

 

[dillon3c]

Re: Subpanel Sizing

the adapter thingy--add-a-lug kit.

 

[peter d]

Re: Subpanel Sizing

Originally posted by jeff43222:
I was not aware of these adapters. I think a trip to the local supply house is in order. They are a Square D distributor, and the counter guys do a great job hooking me up with what I need. I avoid Big Orange as much as possible. About the only thing I buy there is wood for mounting panels on foundations.

If this adapter thing works out, I think I'll use #4 Cu THHN, since I always have plenty of that lying around.

I don't have a convenient Sq D distributor near me, so I am forced to rely on HD for that stuff. And a lot of the local suppliers leave a lot to be desired. (RE: They're awful)

Is this "subpanel" mounted next to the main, or is it remote? I suggest the 2/3 ser cable, but I was assuming it was remote.

 

[jeff43222]

Re: Subpanel Sizing

I like the idea of doing the load calc, but as yet I haven't seen the specs on the appliances, as they are supposed to be be delivered today (I'm sitting in my office waiting for the homeowner to call). Lucky for me he lives in my neighborhood.

I might be able to squeak by with a #8 ground and neutral. I couldn't find a code reference that specifically allows this for subpanel feeders, though. I usually use the same size ungrounded and grounded conductors on the same circuit/feeder.

One complication: There is no disposal, just a switched receptacle in case one is installed someday (homeowners like to compost). Most disposals I've wired have been in the 7-8 amp range. Should I include this hypothetical value in the calc since it might become a load at some point?

And is the range calc affected on Thanksgiving, when it could conceivably be on for more than three hours?

[ August 24, 2005, 04:38 PM: Message edited by: jeff43222 ]

 

[dillon3c]

Re: Subpanel Sizing

Jeff
I noticed in your first post,exsisting 100 ampere rated QO pnl. (*almost full)

In terms of almost. Can you add the range circuit into the 100 ampere panel,and in removing it from, the 50 amp sub pnl.?

 

[jeff43222]

Re: Subpanel Sizing

Originally posted by peter d:
I don't have a convenient Sq D distributor near me, so I am forced to rely on HD for that stuff. And a lot of the local suppliers leave a lot to be desired. (RE: They're awful)

Is this "subpanel" mounted next to the main, or is it remote? I suggest the 2/3 ser cable, but I was assuming it was remote.

Too bad about your supply houses. The two I use do such a great job that it really makes it obvious how lacking Big Orange is.

The subpanel in question is going to be mounted immediately adjacent to the main panel, with a 1-1/4" PVC nipple connecting them, which is why I'm going to use the #4 THHN.

 

[jeff43222]

Re: Subpanel Sizing

Originally posted by dillon3c:
Jeff
I noticed in your first post,exsisting 100 ampere rated QO pnl. (*almost full)

In terms of almost. Can you add the range circuit into the 100 ampere panel,and in removing it from, the 50 amp sub pnl.?

Looks like someone is wearing his smart hat today. That's a brilliant idea!

The existing main panel has two open slots, which I'll need for the breaker feeding the subpanel (no extra lugs on the main panel busses), but when I looked closely at the existing circuits, I found that there's a 120V circuit being protected with a two-pole breaker (the other pole isn't used), plus there's another breaker protecting a piece of NM that exits the panel and is immediately terminated with wire nuts. I'm assuming I can get rid of that one without anyone being upset.

That would leave the old main panel completely full once I put in the range breaker and the subpanel feeder breaker, but there will still be space in the new panel in case someone wants to add a few circuits down the road.

Homeowner just called to let me know the appliances aren't there yet (it's after 4:00, and they claimed they'd be there between noon and 3:00). I shared the news about not having to go for a panel upgrade. She sure was happy.

Thanks for the great idea! I'm going to go look for my smart hat now ...

[ August 24, 2005, 05:09 PM: Message edited by: jeff43222 ]

 

[charlie b]

Re: Subpanel Sizing

Originally posted by jeff43222:I might be able to squeak by with a #8 ground and neutral. I couldn't find a code reference that specifically allows this for subpanel feeders, though. I usually use the same size ungrounded and grounded conductors on the same circuit/feeder.

So do I. I was merely offering an alternative, for your consideration. But for this thread, the "Good Thinking, Why Didn't I Think of That Prize" goes to dillion3c.

220.22 (2002 version) allows the 70% demand factor for feeders. By definition, the conductors going to a subpanel are "Feeders." It applies to your situation.

One complication: There is no disposal . . . Should I include this hypothetical value in the calc since it might become a load at some point?

Yes. The outlet is in place, so you need to count its load in some manner.

And is the range calc affected on Thanksgiving, when it could conceivably be on for more than three hours?

It is not. If the code authors had in mind to worry about a range being a "continuous load," then they would have built that into the table. In other words, instead of calling a single range "8000 VA," they would have said, "a single range is 6400 VA, but add 25% for it being a continuous load."

 

[jeff43222]

Re: Subpanel Sizing

Originally posted by charlie b:

Originally posted by jeff43222:I might be able to squeak by with a #8 ground and neutral. I couldn't find a code reference that specifically allows this for subpanel feeders, though. I usually use the same size ungrounded and grounded conductors on the same circuit/feeder.

... But for this thread, the "Good Thinking, Why Didn't I Think of That Prize" goes to dillion3c.

Hear, hear!

220.22 (2002 version) allows the 70% demand factor for feeders. By definition, the conductors going to a subpanel are "Feeders." It applies to your situation.

I just read through 220.22. I'm not entirely convinced it's OK to use the 70% demand factor in this case. The section talked about it being permissible for feeders that supply cooking equipment and dryers, as well as for the unbalanced load in excess of 200A on single-phase AC systems. Since the house only has 100A service, doesn't that mean I have to keep the neutral as big as the hots, except when taking cooking equipment and dryers into consideration? According to the example calc above, only the range neutral was calculated at 70%, and the examples in Appendix D do the same thing. Are you saying I can size a subpanel feeder neutral at 70% in all cases?

One complication: There is no disposal . . . Should I include this hypothetical value in the calc since it might become a load at some point?

Yes. The outlet is in place, so you need to count its load in some manner.

I figured as much, but I thought I'd run it past the experts, seeing as how my smart hat is nowhere to be found today.

And is the range calc affected on Thanksgiving, when it could conceivably be on for more than three hours?

It is not. If the code authors had in mind to worry about a range being a "continuous load," then they would have built that into the table. In other words, instead of calling a single range "8000 VA," they would have said, "a single range is 6400 VA, but add 25% for it being a continuous load."

That makes sense. I'm a little rusty on the range calc rules since I usually only deal with one range per job.

 

[jimwalker]

Re: Subpanel Sizing

I think you worry to much.It's not the end of the world if a breaker trips.That range has a 30 amp
2 pole but what would it actually use ? and them 2 SA won't likely be loaded to max.Perhaps try to remove that dish washer and pig and run them back to the main panel.

 

[torcho]

Re: Subpanel Sizing

Jeff, when sizing the neutral load the code states it is sized to carry the unbalanced load of the phase conductors. Most dryers and ranges have some 120v devices inside, but the majority of the cooking elements are still 240v, that is why we are allowed a 70% factor for the neutral (i.e a 8000va range is rated at 33.33amps, and the calculated neutral load is allowed to be 23.33amps) all of your other 120v loads need to be included in the load calculation. Without knowing all the specs, I'm not sure if you will be able to undersize the neutral by much. Most of the SER cable we use has both phase conductors and the neutral sized the same, the equipment grounding conductor is typically 1 or 2 wire sizes smaller. I agree with the other posts, see if you can locate the range in the main panel for hopes of having a smaller sub. Just my opinion.
Looking back on your original post, this is a 240v range right?

[ September 07, 2005, 02:08 AM: Message edited by: torcho ]

 

[jeff43222]

Re: Subpanel Sizing

The range is one of those new dual-fuel models (gas cooktop, electric range). The specs called for a 30A 240V circuit, which is what I installed. Based on conversations with the homeowners, I determined that there would be a lot of current used in the kitchen, so I wanted to put together an installation where tripped breakers were less likely.

I ultimately found a way to put the range circuit in the main panel and the rest of the new kitchen circuits in the subpanel.