Three electrcians all give different answers on service conductor ampacity

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Hi. We are working on a kitchen remodel in a brand new building in San Francisco. They want to upgrade all appliances and believe they need 175A service. The unit is only 550 sq.ft., but they are installing all high-end appliances, including induction cooktop, Miele ovens, 2 undercounter SubZero fridge/freezer drawers, etc. The unit was originally set up with 100A service on three wires (supposedly single phase).

The building is 208Y 120V 3 phase 4 wire supplied by PG&E to the each of 5 main panels rated at with 800A 120/240V rating. Each main panel services 5 separate units.

The conduit between the main panel and the unit subpanel is 1?" EMT conduit with about five 90? elbows and two 135? elbows for about a total run of 75'-100'.

The conductor is #2 AWG copper stranded wire with THHN sheath.

The appliances the client has includes the following:

Fridge/Freezer combo (Sub-Zero ID30C): 115V/3.5A/60Hz/420W (15A circuit)
Fridge (U-Line U-3018RF): 115V/2.4A/60Hz/290W (15A circuit)
Dishwasher (Miele G 4580 SCVi): 120V/12.5A/60Hz/1,500W (15A circuit)
Hood (Zephyr arc duo): 120V/7A/60Hz/840W ... draws 7A with blower & light (15A circuit)
Oven (Miele H6800BM): 208-240V/22A/60Hz/5,300W (25A circuit)
Cooktop (Bosch NITP066UC): 208-240V/30A/50-60Hz/7,200W (30A circuit)
Drier (GE DCVH480EKWW): 240V/24A/60Hz/2,200W (25A circuit)
Washer (GE WCVH4800KWW): 120V/10A/60Hz/1,200W (15A circuit)
Towel Warmer (Virtu VTW-128A-BN): 120V/0.67A/60Hz/80W (part of bathroom 15A GFCI circuit)
Kitchen GFCI: (20A circuit)
Bathroom GFCI: (15A circuit)
Bedroom plugs: (15A circuit)
Living room plugs: (15A circuit)
Lighting: (15A circuit)
Consumer Electronics (Apple Magsafe): 120V/1.5A/85W
(Apple iPad charger): 120V/0.45A/10W
(Grandstream CCTV): 0.4A/50W
(Apple Time Capsule): 120V/0.25A/30W
(Apple Cinema Display): 120V/0.75A/90W
(Medella Pump In Style breast pump): 120V/0.15A/18W
(Revlon hair dryer): 120V/16A/1,875W
Ceiling LED Lights (average guestimate at 12 total): 24W*12 ? 288W = 2.4A
======================================================================
Max theoretical demand ? 43A @ 120V (plus 16A hairdrier) + 76A @ 240V
or, max of 21,475W

Am I on the right track here? What is the total ampacity of the #2 AWG conductor? How much will the client need to cut back (if any) on the appliances, if we don't upgrade the service wire? Unfortunately, the conduit runs through a medical office, and it will cost a lot in access fees and be a huge hassle for us to get to the conduit pull boxes. So, we hope to stick with the existing wire, if possible. Thanks!
 

Dennis Alwon

Moderator
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Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
You are not calculating your loads correctly. You need to trust on the guys you hired. My bet is that a 100 amp service may work but there is not enough info. FWIW, I have never seen a 175 amp service
 
continuous/non-continuous loads

continuous/non-continuous loads

Thanks for your response. Continuous loads in this small condo will be minuscule, perhaps 15-20A or less if one assumes all lights are left on and fridge doors left ajar. So multiply by 125% is still insignificant. Question is what happens when friends/family comes to visit for Thanksgiving or the Superbowl and everything is being used at the same time? Of course our electricians will perform and be responsible for all work, but their de facto insistence that all small condos need less than 100A seems unproductive/imprecise.

BTW, here is a shot of the service wire at the sub-panel...
1600Mkt-202-electrical_subpanel.jpg
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
#2 cu is good for 115 amps at 75 degree Celsius. Assuming this is conduit which it looks to be so. Is the conduit EMT or at least metallic?
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
I'm not gonna do the math but there is no way a 550 sq ft condo calcs out at 175A, and there is no way that it draws over a 100A unless someone were to make it their sole purpose in life to do so. And they would need to bring in some outside equipment.

I fired up every thing I could in my house (2800 sq ft) and that includes a double electric oven, two electric dryers, microwave, toaster, toaster oven, hair dryers and curling irons in both bathrooms, coffee pot, all the lights in every room, three tv's, two computers, and my awesome stereo system..... 76A is what my clamp on meters read at the service.

I was rummaging around in my shed for some power tools to plug in to see if I could break 80A when my wife came out and asked me what I was doing.

"Important research", I said. She went inside and started turning things off.
 

PetrosA

Senior Member
The only people I've ever met who would buy those appliances to put into a 500 sq. ft apartment don't know how to use them in the first place, so my guess is you could probably just hook up the fridge and the microwave and just set the other appliances in place and call it a day. No upgrade needed.
 

Ponchik

Senior Member
Location
CA
Occupation
Electronologist
Remember a 100Amp breaker will hold 80Amps continuously. That is whole lot of amps for a 550 SqFt condo.

Based on NEC you do not need an upgrade unless your calculation requires it.

On the other hand if the owner wants an upgrade then by all means put one in.
 

Ponchik

Senior Member
Location
CA
Occupation
Electronologist
If they need a panel upgrade because of breaker space capacity, then replace the existing with a 40 circuit breaker space.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
... Question is what happens when friends/family comes to visit for Thanksgiving or the Superbowl and everything is being used at the same time? ...
That's one HECK of a Thanksgiving party!

Fridge/Freezer doors are both ajar, continuously, because 6 teenagers are staring at the contents as they contemplate what to eat before dinner
Dishwasher has JUST been turned on to clean the breakfast dishes and is heating the cold water WHILE the pump motor is running
Hood and hood lights are on full blast
Oven is browning the turkey, so all elements are on full blast
Cooktop is boiling the potatoes and steaming the beans
Drier is drying the 1st load of dirty clothes from the men and boys having just returned from the local park after playing football (no yard, it's a condo)
Washer is in the spin cycle of the second load of dirty clothes.
Towel Warmer is going full blast after they have all taken showers
Kitchen GFCI has an espresso maker and blender plugged in, both running full blast as they make iced espresso frappes for the adults and teenagers
Bathroom GFCI is running a hair dryer
Bedroom plugs are supporting the chargers for a dozen extra iPads and smart phones brought by guests as the kids are texting, reading their Facebook pages and playing candy crush
Living room plugs are all loaded because the lady of the house is having a nervous breakdown from all the commotion and calms herself by vacuuming while the music system is blaring
Lighting is turned on in every room, every lamp, all dimmers at full brightness
Consumer Electronics (various listed) are ALL in use by various people trying to pretend they don't notice the nervous breakdown of the lady of the house

I'd love to see all of that. Or, maybe not...

The point I am making is that the concept of "load factor" is what comes into play here. The scenario in which that total theoretical load can be at that level is virtually impossible to attain.
 

suemarkp

Senior Member
Location
Kent, WA
Occupation
Retired Engineer
Your laundry circuit also needs to be 20A. I would put the fridge on one of the two 20A kitchen circuits, otherwise you'll have to count it separately. Watt value for dryer is inconsistent with the volt and amp values listed. Hopefully it will work OK at 208V... Note that the cooktop and oven most likely draw less power at 208V than 240V. Is their nameplate watt value slashed? If so, use the lower number next to the slash.

Why did they put in 800A panels when each only feeds 5 units and these units aren't very large load wise (and the 1.25" conduit will prevent it from getting very large)? Seems like a 200A 3 phase panel would cover 6 units.

You don't do a load calc by adding up the circuit breaker handle ratings. Most of your loads will be based on the 500 sq ft (x 3 VA per square foot). Then the two kitchen 1500VA circuits, the laundry 1500 VA circuit, and then the large items (most of which will get a 40% demand factor applied). Calculate in VA and ignore 120V or 208V. No water heater or space heating? If not, I would think this would all easily fit on a 100A feeder.

You have no continuous loads in this dwelling feeder calculation. If you had a water heater, its circuit would be treated as a continuous load but not at the feeder level.

Houses are done all the time with NEC calculation values and they don't trip the main at thanksgiving. If the calc comes out to 80A, then 100A would provide plenty of margin for thanksgiving...
 

mwm1752

Senior Member
Location
Aspen, Colo
Hi. We are working on a kitchen remodel in a brand new building in San Francisco. They want to upgrade all appliances and believe they need 175A service. The unit is only 550 sq.ft., but they are installing all high-end appliances, including induction cooktop, Miele ovens, 2 undercounter SubZero fridge/freezer drawers, etc. The unit was originally set up with 100A service on three wires (supposedly single phase).

The building is 208Y 120V 3 phase 4 wire supplied by PG&E to the each of 5 main panels rated at with 800A 120/240V rating. Each main panel services 5 separate units.



no heating or cooling loads mentioned -- used your figures( which are questionable), added disposal to my handy dandy 220.82(B)(2) spreadsheet I made up @ 208 y/120 = 37.75 amps
Am I allowed to use this option as the load is not over 100 Amps? or is the use based upon the service/feeder size?
Feeder/service load calculations do not need continuous adjustment of 125%.
 
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suemarkp

Senior Member
Location
Kent, WA
Occupation
Retired Engineer
I think you have to install a 100A service/feeder if the optional calc comes in under that.

Using the standard calc, and the equipment given (no central or water heating), I get 96 amps. With the optional calc, I get 80A. I don't see how you could ever get as low as 37.75A -- there's a 10KVA floor to that calc where everything is 100%. That's 48A right there and the oven/cooktop are 12KW on their own.

I think #2 copper is plenty on either a 100A, 110A, or 125A breaker.
 
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jxofaltrds

Inspector Mike®
Location
Mike P. Columbus Ohio
Occupation
ESI, PI, RBO
Hi. We are working on a kitchen remodel in a brand new building in San Francisco.
Thanks!


You are not calculating your loads correctly. You need to trust on the guys you hired. My bet is that a 100 amp service may work but there is not enough info. FWIW, I have never seen a 175 amp service

Think the old 'man" said it best.
 

junkhound

Senior Member
Location
Renton, WA
Occupation
EE, power electronics specialty
Loads do tend to even out.

Last habitat homes I helped wire had 5 units, each with 200 A panel, gas heat and WH, otherwise all electric kitchens.

The habitat EE was concerned that the POCO only set a 25 kVA transformer for the 5 units total! Poco said the xfmr they used was rated 150% for long term overcurrent, so really 37.5 kVA but more voltage droop when fully (over)loaded. POCO likes to keep core losses of dist xfmrs low, so goes with the smallest their experience dictates is satisfactory.

That was 2 years ago, have not heard of poco upgrading, maybe should drive by and look to see.

OTOH, have seen my own house hit 144A. Tried using 300A welder at same time as DW was pressure washing once and tripped 100A subpanel breaker that feeds both one time after a few minutes of continuous welding.

5 ea 3000 sqft. houses here are on the same 37.5 kVA xfmr, only self and one neighbor on heat pumps, other 3 gas, no big problems, have never burnt up that poco xfmr. Only one other neighbor has a (much smaller) pressure washer and no other big welders. Other 4 houses have gas water heaters.

No <600 sq ft condo I have ever heard of has a 4000psi 4gpm pressure washer and a 300A dc welder sitting in the hall closet <G>
 
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mwm1752

Senior Member
Location
Aspen, Colo
I think you have to install a 100A service/feeder if the optional calc comes in under that.

Using the standard calc, and the equipment given (no central or water heating), I get 96 amps. With the optional calc, I get 80A. I don't see how you could ever get as low as 37.75A -- there's a 10KVA floor to that calc where everything is 100%. That's 48A right there and the oven/cooktop are 12KW on their own.

I think #2 copper is plenty on either a 100A, 110A, or 125A breaker.

3 watts per sq ft x 500 = 1500w -- where do you get 10,000 watts --- oven, cooktop are per his load calc numbers -- more than 4 appliances @ 75% -- 3 phase I"ll recheck anyway but I thought he was just figuring for the unit feeder not the service my bad.
 

david luchini

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Location
Connecticut
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Engineer
3 watts per sq ft x 500 = 1500w -- where do you get 10,000 watts --- oven, cooktop are per his load calc numbers -- more than 4 appliances @ 75% -- 3 phase I"ll recheck anyway but I thought he was just figuring for the unit feeder not the service my bad.

I come up with 80A as well (for the unit feeder) at single phase, not three phase, using the optional method.
 
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