Maximize 40A Subpanel

[2Htm]

How many more 20A circuit breakers can safely be added to an existing 40A subpanel, used solely for a residential kitchen’s AFCI/GFCI dedicated circuits: DW, GB, Hood, Range, and 8 Countertop receptacles, No Island? The 40A subpanel is fed via #8 AWG, copper, from 200A Service Equipment, all in one structure.

Thank you!

 

[retirede]

The number of breakers is irrelevant. It’s all about the calculated load.

 

[LarryFine]

My best guesses:

If an electric range, zero; it's already overloaded.

If a gas range, how many open spaces are there?

 

[2Htm]

Thank you for responding - Gas Range, I believe 4 open spaces. Can you share a simple load calc?

 

[ramsy]

40A subpanel, used solely for a residential kitchen’s AFCI/GFCI

Here is the perfect feeder breaker for that sub panel. QO 40A AFCI with no reset button.

If inspector wont believe its AFCI protected, show him how this web page description eliminates nuisance trips.

 

[petersonra]

The panel over current protection will prevent any hazard due to excess current so you can put as many breakers in it as you want. If you end up with too much current a CB will trip to protect the wiring or the panel board.

 

[ramsy]

The number of breakers is irrelevant. It’s all about the calculated load.

The panel over current protection will prevent any hazard

Load calcs wont apply to branch circuits, since 20A SABC appliances exceed NEC load calcs of 1500VA each.

18A is anticipated for DW + Disposer from NEC Appendix D1(b), and are calculated to run together.
Appliance loads are also cumulative, since calculated to run together:
1500W Toasters + (1500W Coffee pot /or/ insta-hot water for tea) + 1200W microwaves typically run together.
1800W Air fryer / Electric Waffle iron + 1500 rice cooker / Toaster oven, may also run together.

How many more 20A circuit breakers can safely be added to an existing 40A subpanel, used solely for a residential kitchen’s

Construction-defect callbacks would be likely with kitchen appliances, especially during holiday cooking.

 

[Birken Vogt]

The panel over current protection will prevent any hazard due to excess current so you can put as many breakers in it as you want. If you end up with too much current a CB will trip to protect the wiring or the panel board.

This is bad advice. The feeder needs to be sized to the load that is calculated. Circuit breakers are a last resort.

 

[petersonra]

This is bad advice. The feeder needs to be sized to the load that is calculated. Circuit breakers are a last resort.

I agree that is the way you are supposed to do it, but no one does.

If a hazardous over current condition occurs, a CB will trip. It's not a last resort. It's what they are there for

 

[ptonsparky]

How many 60 Amp Main/Range panels are still out there that include sub feeds to Dryer and WH? A 40 will certainly work well enough for what has been described.

 

[infinity]

The feeder needs to be sized to the load that is calculated.

I agree. The question is about the adequacy of the 40 amp feeder when compared to the load. The only way to know if it's large enough is to calculate the feeder load. Since it's #8 AWG if this were fed with something other than NM or UF you could bump up the feeder to 50 amps.

 

[readydave8]

Can you share a simple load calc?

Aren't there still examples near back of NEC?

 

[Birken Vogt]

I agree that is the way you are supposed to do it, but no one does.

If a hazardous over current condition occurs, a CB will trip. It's not a last resort. It's what they are there for

No professional would build something that can cause a breaker to trip in normal operation. I don't know where you get the idea that nobody does load calcs, it is in the code just like wire size and breaker height and any other number of things, and it is easy to do for a panel this small, easy for anything really.

 

[ptonsparky]

That breaker will most likely not break under 'normal' conditions. Is it normal for it to trip when overloaded, yes, but overloaded is not normal.

 

[petersonra]

No professional would build something that can cause a breaker to trip in normal operation. I don't know where you get the idea that nobody does load calcs, it is in the code just like wire size and breaker height and any other number of things, and it is easy to do for a panel this small, easy for anything really.

I would be willing to bet that the majority of circuits added to single family homes do not involve more than a cursory glance at existing loads. The load calculations are so conservative that in most cases you could double or triple the calculated load on feeders and never go into a load that is dangerous.

You could also have someone decide to plug in a bunch of high draw appliances on the same circuit leading to the OCPD tripping. That is not a dangerous condition. It is just a nuisance.

 

[Birken Vogt]

Granted, you can add a 20 amp receptacle circuit to a 100A or 200A panel without much concern. But we were talking about a 40 amp sub which would, in my mind, require scrutiny if anything substantial was to be added to it. There is much less overhead than something larger.

Remember the OP is an inspector, there is already a range and a bunch of other stuff on this panel, it is probably already overloaded. He should probably not allow it or require an upgrade to remain within code.

I do generators for a living, careful scrutiny of loads to avoid overpaying/underloading is the norm here.

 

[curt swartz]

Remember the OP is an inspector, there is already a range and a bunch of other stuff on this panel, it is probably already overloaded. He should probably not allow it or require an upgrade to remain within code.

The OP stated the range is gas. If this panel is just feeding the kitchen general lighting, (2) small appliance circuits, dishwasher, disposal, hood and microwave 40 amp is definitely OK.

As an inspector if they feel the load on the panel is questionable they should request the EC provide a load calculation prior to passing inspection.

 

[Birken Vogt]

As usual I should have read back closer. I just take exception to the idea that the breaker tripping is any consideration in the loading of a panel.

 

[suemarkp]

You have 9600VA to work with (40A at 240V). Two kitchen small appliance circuits, gas range outlet, and the kitchen lighting would be a bit over 3 KVA. In that calculation section, you take the first 3KVA at 100% and the rest at 35%. So the kitchen lighting (3VA per sq ft), which can include the gas range, is almost inconsequential.

Add to this the fixed appliances. You need the nameplates for these:
Dishwasher (assume 1200VA)
Garbage Disposal (assume 1000VA). If really picky, may need to add 25% to it bacause its is a large motor.
Range hood (these vary a lot, assume 400VA, but a rangehood microwave could be 1500VA)

These last ones go in at 100% nameplate also. If you can get over 4 fastened in place appliances, then a 75% factor can be applied to those loads.

So you have 3000 + 1200 + 1000 + 400 = 5600 VA. That leaves 4000VA remaining for new circuits and even more if you can use a 75% factor. So adding 2 to 4 circuits is probably not a problem.

Counter top appliances do not matter like coffee pots and air fryers. These all connect to the two kitchen small appliance circuits which go in load calcs at 1500VA each. The refrigerator is also covered as part of those kitchen appliance circuits.

I wish the code would let you eliminate short duration appliances from the calculation like a garbage disposal. It runs for such a short time a feeder is not even going to notice it.

 

[ramsy]

40 amp is definitely OK

Service electricians are more familiar with appliance overloads. Kitchens & garages with laundry may be the most common.

While 210.52(B) only says 20A SABC, not individual branch circuits, nor restricted by feeder size, IMHO 220.18 would settle this debate if appliance loads were considered after occupancy, which is not required by the minimum NEC standard.