New service and possible subpanel

Reply

  #1  
Old 01-24-05, 02:35 PM
Tim Martin
Visiting Guest
Posts: n/a
New service and possible subpanel

So during the course of a complete kitchen remodel, we realized we'd need more circuits in the kitchen than we had before. We don't really need that much more current, it's just that code now requires more dedicated circuits than it did when our house was built ('69).

We seem to have a couple choices to make:

1) we use the existing conduit which currently has a 8-gauge and 10-gauge conductors in it for a two 240 volt circuits (30 and 40 amp). This means pulling out those wires and pulling a bunch more in because they would be de-rated to 70% since we'd need at least 7 conductors for the resulting four circuits:
40 amp (240 volt) = 3 6-gauge conductors
30 amp = 1 8-gauge conductor + 1 shared 8-gauge neutral
15 amp = 1 10-gauge conductor + 1 shared 8-gauge neutral
15 amp = 1 10-gauge conductor + 1 shared 8-gauge neutral

As you can see, the 30 and 15 amp circuits would share an 8-gauge neutral where the 30 is one phase and the two 15s are the other phase. I think this is allowed, right?

This would require us to add a few breakers to our current panel. The panel is rated for 125-amps, but interestingly enough is fed by a 90 amp service entrance panel at the meter. By my calculations we're getting pretty close to that 90 amp limit. We'd end up with 265 amps of breakers in the 125 amp box, but using some kind of black magic electrician's math it works out to about 80 amps at any one time.

2) Our second option is to add a subpanel in (or outside) the kitchen. This would involve either chaining it off the existing 125 amp panel or running entirely new conduit all the way from the service entrance all the way to the kitchen. This makes the circuit wiring for the kitchen pretty straight forward and gives us room to grow, but is the most expensive option.

Either way it looks like we might have to upgrade our service to at least 125 amps (might as well go to 200?). Which looks like it won't be cheap.

Can anyone give us advice as to which way to go?
 
Sponsored Links
  #2  
Old 01-24-05, 02:48 PM
Member
Join Date: Sep 2000
Location: United States
Posts: 18,497
A 70% derating usually doesn't require larger wire. But it might require newer (90 degree) wire of the same size. Conduit size is also a factor. How big is it?

You seem to say both that 90 amps is enough and that it is not enough. Which is it?

What do you have in that kitchen that needs a 30-amp 120-volt circuit?
 
  #3  
Old 01-24-05, 03:05 PM
Tim Martin
Visiting Guest
Posts: n/a
Originally Posted by John Nelson
A 70% derating usually doesn't require larger wire. But it might require newer (90 degree) wire of the same size. Conduit size is also a factor. How big is it?

You seem to say both that 90 amps is enough and that it is not enough. Which is it?

What do you have in that kitchen that needs a 30-amp 120-volt circuit?
It's a 1" conduit. I've found it hard to find some of this info, but I understand that I didn't do the math, I just figured i'd use wires that could handle 30% more power. Should I have used the more accurate cross-section area math?

I do not *think* 90 amps is enough. Using the load calculations from the NEC (shown here http://www.usinspect.com/Electrical/...ctionItems.asp) I calculate my total load to be around 100 amps.

As for the 30-amp circuit -- Dishwasher and Disposal. I could run two 15-amp circuits but I'm not required by local code to do so. I thought it might be easier to just run one circuit to handle both (8.1 amp disposal and 9.6 amp dishwasher = 17.7 amps)
 
  #4  
Old 01-24-05, 04:04 PM
Member
Join Date: Sep 2000
Location: United States
Posts: 18,497
Neither the dishwasher nor the disposal may be placed on a 30-amp circuit. The manufacturers require either 15 or 20 amp circuits. Also, you show two 15-amp circuits. I don't know what these are for, but in the U.S., you need two 20-amp circuits for your countertop receptacles.

Read up more on ampacity derating. I don't think you understand how it works.
 
  #5  
Old 01-24-05, 04:39 PM
Tim Martin
Visiting Guest
Posts: n/a
Originally Posted by John Nelson
Neither the dishwasher nor the disposal may be placed on a 30-amp circuit. The manufacturers require either 15 or 20 amp circuits. Also, you show two 15-amp circuits. I don't know what these are for, but in the U.S., you need two 20-amp circuits for your countertop receptacles.

Read up more on ampacity derating. I don't think you understand how it works.
If i have two devices that manufacturers recommend 15-amp circuits, why wouldn't I be able to put both of them on a 30-amp circuit (providing that it doesn't violate code)?

The circuits I've mentioned here are only the additional ones I'll need. I already have two 20-amp circuits dedicated to countertop GFCI receptacles. There's also a 40-amp 240v for the range.

I've done the planning and calculations for a subpanel. The result would be:
50-amp 240v (range)
2x 20-amp 120v (counter tops)
4x 15-amp 120v (microwave, disposal, dishwasher, frig/lighting)

Using 75% of the countertop ratings and the individual ratings of all those appliances we come up with 71 [email protected] Load is considered 80% (right?), so that's about 57 amps. So I figured I could get a 100 amp panel and put the 7 breakers in it.

If i go the non-subpanel route, I'd need to run the 50-amp 240v and all four 15-amp 120v circuits through that 1" conduit. Is that possible? I think you're right, I don't understand the derating well enough.
 
  #6  
Old 01-24-05, 04:55 PM
Member
Join Date: Jan 2002
Location: Chicago
Posts: 1,915
You can't use a 30A feeder to supply power to 2 15A devices. It doesn't work that way. The instructions tell you to use a 15A or 20A circuit, which implies a 15 or 20A breaker. The wire size is irrelevant as long as it's at least enough to handle the breaker (at least 14 gauge for a 15A breaker, 12 gauge for a 20A breaker). If a partial short were to happen in one of your devices, and you have them on a 30A breaker the appliance will probably catch fire way before the breaker trips.

You can solve your problems by running 1 20A multiwire and 1 15A (shared neutral circuits). You have EMT, so a separate ground is not required. That will be 9 12 gauge conductors. If you run a 3/4" conduit I don't believe you'll need to derate anything.

Personally I only run 20A circuits for the kitchen. The kitchen in my house has the setup I recommended.
 
  #7  
Old 01-24-05, 05:06 PM
Member
Join Date: Sep 2000
Location: United States
Posts: 18,497
providing that it doesn't violate code
That's precisely the problem. It does violate code. If the dishwasher malfunctions and starts drawing 29 amps, it will catch on fire and the breaker won't trip.

I agree with trinitro. I suggest you use no 15-amp circuits at all. Put all those things you plan for a 15-amp circuit on a 20-amp circuit instead.

Load calculations are not a percentage of breaker size. In fact, the calculation is completely independent of breaker sizes.
 
  #8  
Old 01-24-05, 05:15 PM
Member
Join Date: Jan 2002
Location: Chicago
Posts: 1,915
I meant 1 20A and 2 15A multiwire. But I would run 3 20A multiwire circuits, even though it's more expensive. All 9 conductors should fit in a 3/4" EMT if you must.
 
  #9  
Old 01-25-05, 08:53 AM
Tim Martin
Visiting Guest
Posts: n/a
Originally Posted by John Nelson
That's precisely the problem. It does violate code. If the dishwasher malfunctions and starts drawing 29 amps, it will catch on fire and the breaker won't trip.

I agree with trinitro. I suggest you use no 15-amp circuits at all. Put all those things you plan for a 15-amp circuit on a 20-amp circuit instead.
I'm confused, you just told me not to put two 15-devices on a 30-amp breaker because I should stick to the mfg recommendation; then you suggest I put them on 20-amp circuits?


Load calculations are not a percentage of breaker size. In fact, the calculation is completely independent of breaker sizes.
I didn't do percentages of breakers. I calculated based on the nameplate requirements of each appliance. I provided a link to the exact process I used to do the load calculation -- is there something wrong with doing it that way -- it's taken from the NEC itself.
 

Last edited by Tim Martin; 01-25-05 at 09:09 AM.
  #10  
Old 01-25-05, 09:06 AM
Tim Martin
Visiting Guest
Posts: n/a
Originally Posted by trinitro
I meant 1 20A and 2 15A multiwire. But I would run 3 20A multiwire circuits, even though it's more expensive. All 9 conductors should fit in a 3/4" EMT if you must.
It sounds like your making a suggestion based on the "no subpanel" option. But three new circuits isn't enough. My countertops are already handled by existing circuits. So here's what ISN'T currently powered by existing circuits:

Frig (8.5 amps)
Microwave (14 amps)
Disposal (8.1 amps)
Dishwasher (9.6 amps)
Range (currently a 40 amp circuit is installed, I'd like to upgrade it to 50 amps)

So that's five circuits. How about one 50-amp circuit two 20-amp (multiwire) and two 15-amp (multiwire)? That's still 9 conductors, can I run them all through a 1" conduit? I will stick with 15-amps for the disposal and dishwasher as that is exactly what the manufacturer recommends.
 
  #11  
Old 01-25-05, 09:25 AM
Member
Join Date: Sep 2000
Location: United States
Posts: 18,497
I'm confused, you just told me not to put two 15-devices on a 30-amp breaker because I should stick to the mfg recommendation; then you suggest I put them on 20-amp circuits?
Yep, 30 and 20 are different numbers. Different rules. We don't make this stuff up. It's all spelled out in the code.

I will stick with 15-amps for the disposal and dishwasher as that is exactly what the manufacturer recommends
Check the installation instructions carefully. Most dishwashers I've installed ask for a 20-amp circuit.
 
  #12  
Old 01-25-05, 09:34 AM
Tim Martin
Visiting Guest
Posts: n/a
Originally Posted by John Nelson
Yep, 30 and 20 are different numbers. Different rules. We don't make this stuff up. It's all spelled out in the code.
No need to be rude John. I don't know the rules, I don't have the code, I'm just a homeowner doing some planing and evaluations. I posted here to get information, insight, and help. I don't expect nor do I appreciate to have the moderator of the forum telling me that "30 and 20 are different numbers".

So if I read between the lines, what're you're telling me is that it's okay to put 15-amp devices on 20-amp circuits, but it's not okay to put them on anything greater than that? The reason I'm double checking is that the plan checker at my permit office said it was okay to put the dishwasher and garbage disposal on the same circuit as long as the circuit was rated high enough for both devices to be on at the same time. This is what led me to a 30-amp circuit which she said would be just fine.

Thanks.
 
  #13  
Old 01-25-05, 09:38 AM
Member
Join Date: Sep 2000
Location: United States
Posts: 18,497
Sorry. I did not intend any offense. I am fully aware that you know that 30 and 20 are different numbers. I was only emphasizing the fact that the difference is significant in this case.

I suggest you pick up a copy of the $6 green paperback Wiring Simplified, available in the electrical aisle at Home Depot. It covers most of the codes you need to know to do this project. The kitchen and bathrooms are the two most electrically regulated rooms in a house, and rightly so.

Either the plan checker made a mistake, or you misunderstood. Virtually all 15-amp devices are allowed to be put on a 20-amp circuit, but no higher.
 
  #14  
Old 01-25-05, 10:01 AM
Tim Martin
Visiting Guest
Posts: n/a
Thanks for the info. I've been looking for a good book, most of them seem *too* simplified.

I'll check out the one you recommend.

All in all, can you say if you'd suggest pulling more circuits through the conduit or installing an entirely new subpanel (which would be connected to the service entrance)?
 
  #15  
Old 01-25-05, 11:24 AM
Member
Join Date: Sep 2000
Location: United States
Posts: 18,497
Adding a subpanel vs. pulling more branch circuits from the main panel is one of those "six of one, half dozen of the other" issues. Once you finalize exactly how many circuits and what their sizes are, the balance may tip one way or the other. Other factors include how much extra space you have in the main panel, and if you have any good place to put a subpanel in a location that meets the numerous code requirements for panel placement.
 
  #16  
Old 01-25-05, 02:23 PM
Member
Join Date: Jan 2002
Location: Chicago
Posts: 1,915
I would not go the subpanel route.. it is not a nice thing to see in most cases...and it's hard to find a suitable location for it usually.

On the 20A vs 15A... it's just a matter of preference. I almost always run 20A circuits for several reasons. Most electricians however will run the minimum required, which is 15A in most cases. If cost is an issue (which doesn't seem to be in your case) then go with 15A. If not, go with 20A.

This is how I would feed the kitchen.

1 12-3 multiwire (2X20A circuits) to feed the coutertop receptacles (probably already run).

Frig (8.5 amps) + Microwave (14 amps) on a 12-3 (2X20A) circuits). You have to feed the microwave on a 20A circuits due to the size (how big is the thing?)

Disposal (8.1 amps) and Dishwasher (9.6 amps) on a 12-3 (2X20A) circuits).


Range (currently a 40 amp circuit is installed, I'd like to upgrade it to 50 amps) ---> 50A 220V circuit.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes
'