load calculation for 200 amp service feeding two main breaker panels

cubgirl

Member
Location
madison, AL
Hi everyone,

first, let me state that i'm not an electrical professional. i'm a retired engineer but not electrical (industrial & aerospace). i posted these questions at another forum & i think they may be getting too technical for those guys. i hope you all don't mind my asking here.

i have a building, zoned both commercial and residential. i renovated it 2009 & had to meet commercial specs: ADA building, electric, HVAC, etc. it has 2 units but only one 200 amp panel (i upgraded electrical in 2009 and everything was new: service entrance cable, all circuits, etc).

it's currently vacant so i decided use this opportunity to split the utilities so the tenants pay utility bills, not me. i hired an electrical company for the job and requested that they install a new gang meter outside and two 125 amp breaker panels in each unit. they agreed to this but after job was done, i realized they had not met my specs. what they did was leave the old 200 amp, 40 space/circuit panel (siemen's G4040B1200 or G4040B1200CU or G4040L1200 or G4040L1200CU not sure how it's configured) with about 13 empty spaces (uncovered), all circuits are unlabeled and the original 200 amp main breaker was removed (its space uncovered). a 100 amp main breaker was installed on the bus and it does break the entire panel. in the 2nd unit, they installed a new 100 amp breaker panel (siemen's P2020B1100CU) which is also unlabeled, but every space is covered.

before the job began, i had emailed them the specs for a new tankless H2O heater that i bought and planned to install upstairs after the job was done (it requires a 125 amp panel with a 60 amp DP & #6-2 wire). a new H2O heater is necessary this unit for total separation of the utilities. i was specific about the panel size and breaker requirement. however, i was just guessing that 125 amps should be fed to both units.

after realizing they'd installed two 100 amp panels, i became concerned because my requirements weren't met. after researching, i decided to do load calculations on the two units. the results are that unit #1 requires a minimum of 150 amps and the unit #2 requires 125 amps, minimum. so 125 amps for unit #1 was incorrect but it was better than the 100 amps they provided.

i'd like to attach my load calculations to get feedback (do they look correct?) and ask the professionals if load calculations should have been done by the contractor to determine panels and feeder sizes? if the calcs had been done beforehand, this situation could've been avoided. the other forum told me load calcs are NEVER done for residential; and for commercial buildings, an engineer, not the electrician, perform the calculations. any advice on that?

i assume 200 amps can be distributed to 125 amp and 150 amp main breaker panels based on feeder wire size, thermal rating and material. during the job, i asked about the feeders and the contractor told me they were 2/0 and could support 125 amps. however, since but i was misled about panel amps, i assume the feeder wires are not rated for more than 100 amps.

these calculations were pretty easy to do and i'm not sure why the electrician didn't just plug in the numbers and install the correct size panels and feeders.

i've come to the conclusion that the only way to fix this is to hire someone to install a 150 amp feeder to unit #1 and replace the 100 amp main breaker with a 150 amp breaker, if possible. unit #2 will need both a new feeder and new panel, both rated at 125 amp, minimum. i think the gang meters can be left alone since i have 200 amp service.

is there a more elegant solution out there that i'm missing? that'd be great but unlikely.

thank so much for any advice and help you can provide & please forgive that i'm not a professional. :ashamed1:
cubgirl

P.S. i had to convert the spreadsheets to pdf to upload.
 

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Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
Your calcs look good although I did not go over it with a fine tooth comb. Unfortunately , IMO the use of an electric on demand water heater is IMO a big mistake as they draw so much and my experience from others who have used them is the customer is never satisfied with them. If you used a standard residential water 4500 watts then the issues with the service would not be there.

I would also recommend just going with 2- 200 amp panels instead of keeping the service so close. Around here a 150 amp panel would cost me twice as much as a 200 amp panel because they are not stocked.
 

cpinetree

Senior Member
Location
SW Florida
Might also check what model number meter can was installed.
Was it 150 or 200 amps per position? If so, it may not be to bad to make changes.
If it is only 100 or 125 amps per position, you may have a bigger problem on your hands.
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Yes, a load calculation should have been done by the contractor. Yes, it is within the skill set of an electrician to do such calculations. Your calculations appear to be to be reasonable. But I don't know if you have correctly listed everything within each unit, or whether you have inadvertently left something out. So I can't tell you that they are right.

Please be advised that you can't take a panel that has one rating (e.g., 100 amps), replace its main breaker with one rated higher (e.g., 150 amps), and thereby create a new panel that has the higher rating. For starters, you would be voiding the manufacturer's warrantee. You might also not be able to physically fit the higher rated breaker in the space originally occupied by the lower rated breaker. But more importantly, a panel's rating is based on many aspects of its physical construction, not just the rating of its main breaker. Bottom line: you need to replace the panels in their entirety.

A 2/0 wire is good for at least 145 amps. Depending on what type of wire it is, it might be good for 175 amps. You would need to read the label that is printed on the jacket of the wire (it is generally inscribed every foot or so).

Your most significant error in reasoning is that you can take two units that are calculated to have loads of 125 amps and 150 amps, and feed the two of them with a 200 amp source. You have to account for the total load, and your total is 275 amps. You might be able to reduce that calculated value a bit by doing a separate calculation that accounts for everything at once (e.g., use the total square footage of both units at the top of the calculation). But I am confident that that would not bring the total load below 200 amps.

I agree with Dennis that the right path forward would be to install a 200 amp panel in each unit. That would require a feeder of at least size 3/0. It would also require meters that each have a rating of 200 amps, and it would require the serving utility to provide for a load that high from their system to your meters.

I think you were not "done right" by the first electrical contractor. How you deal with them is another problem entirely.
 

donaldelectrician

Senior Member
I feel a electrician Has to be able to do load calc . For just about every bldg. in a city .

To me that is part of the job . Very Important part of the job that I feel strongly about



Don
 

cubgirl

Member
Location
madison, AL
Might also check what model number meter can was installed.
Was it 150 or 200 amps per position? If so, it may not be to bad to make changes.
If it is only 100 or 125 amps per position, you may have a bigger problem on your hands.
i believe it's 125 amps per position. it looks like its this Uni-PAK 2-Gang 125 Amp Tenant Main Breaker Meter Socket.

besides limiting each unit to 125 amps, why is it a bigger problem?

thank you for sharing your knowledge with me
cub
 

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cubgirl

Member
Location
madison, AL
thank you

thank you

Yes, a load calculation should have been done by the contractor. Yes, it is within the skill set of an electrician to do such calculations. Your calculations appear to be to be reasonable. But I don't know if you have correctly listed everything within each unit, or whether you have inadvertently left something out. So I can't tell you that they are right.

Please be advised that you can't take a panel that has one rating (e.g., 100 amps), replace its main breaker with one rated higher (e.g., 150 amps), and thereby create a new panel that has the higher rating. For starters, you would be voiding the manufacturer's warrantee. You might also not be able to physically fit the higher rated breaker in the space originally occupied by the lower rated breaker. But more importantly, a panel's rating is based on many aspects of its physical construction, not just the rating of its main breaker. Bottom line: you need to replace the panels in their entirety.

A 2/0 wire is good for at least 145 amps. Depending on what type of wire it is, it might be good for 175 amps. You would need to read the label that is printed on the jacket of the wire (it is generally inscribed every foot or so).

Your most significant error in reasoning is that you can take two units that are calculated to have loads of 125 amps and 150 amps, and feed the two of them with a 200 amp source. You have to account for the total load, and your total is 275 amps. You might be able to reduce that calculated value a bit by doing a separate calculation that accounts for everything at once (e.g., use the total square footage of both units at the top of the calculation). But I am confident that that would not bring the total load below 200 amps.

I agree with Dennis that the right path forward would be to install a 200 amp panel in each unit. That would require a feeder of at least size 3/0. It would also require meters that each have a rating of 200 amps, and it would require the serving utility to provide for a load that high from their system to your meters.

I think you were not "done right" by the first electrical contractor. How you deal with them is another problem entirely.
wouldn't 200 amp panels in each unit require more than a 200 amp service? i think service is 100, 150, 200 & 400 amps, correct? are you saying 400 amp service would be required to support the 275 amp load calculated? i thought 200A could support two 125A or two 150A panels because all circuits are not on at all times. i thought that was the idea behind sub-panels. but again, i am no electrician.

as far as wires, yes that's a concern... i looked at the NEC tables and the corrections. NEC 310.15B7 says the 100A feed (my current configuration) feed must be minimum of 83A (83% of feed rating). but the upstairs feed goes through the attic (125F ambient). correction for that temp for a 75°C wire is 67%. it must still be 83A, after correction. so the requirement is a 75°C 1 cu (130A * 0.67 = 87) or 2/0 al (135 * 0.67 = 91), minimum. if i don't have that, then it's a violation, correct?

but in order to get 125A i requested, the feed must be 104A (83% of 125A) and the correction is still 67%. @ 75°C, that's a minimum of 2/0 cu (175A * 0.67 = 118) or 3/0 al (155A * 0.67 = 104). so the feed to that panel must be 2/0 cu or 3/0 al @ 75°C to support 125A panel. is that correct? if not, a new feed must be run.

i'll have to purchase a P2020L1125CU ($60) and a MBK125A ($58) to replace the P2020L1100CU breaker panel they installed ($69). so they saved $49 by ignoring requirements. :happysad:

also, since they installed a gang meter with 200A max bus rating and 125A max meter socket rating, i realize i can't convert the downstairs to 150A. ok...but i asked for 125A in both units and still want it.

first, the wire needs to be 104A. i don't think there's any corrections. @ 75°C, that's minimum of 1 cu (130A * 0.83 = 108) or 2/0 al (135A * 0.83 = 112). so the feed to that panel must be 1 cu or 2/0 al @ 75°C to support the 125A panel. if not, a new feed must be run.

second, the OCPD. i found out the old 200A panel (Siemens G4040B1200CU, discontinued) will not accept a MBK125A. the spec for the panel says, "Acceptable main breakers: 200A load centers MBK200A, MBK150A, all Type QP circuit breakers backfed" ah-ha, so now i know why they backfed a 100A main to the panel.

i'll get a 125A Q2125 breaker ($70) and backfeed for 125A. the 100A Q2100 ($28) they installed saved them a whopping $42. woo-hoo!! :dunce:

what i'm realizing is that to save a measly $91 ($49 up and $42 down), the contractor simply ignored my requirements. and if he'd have done a load calc, he might've advised an entirely different solution. i've asked the contractor for a copy of his load calcs. no response.

if you see an error in my logic or calculations, please let me know.

i realize this only solves problems constrained for current gang meters. do you think i should argue that the contractor should have done a load calculation and ask for a whole new system that will support the load? the other forum said this was my fault because i emailed the requirements for the tankless H2O heater and stated i wanted 125A to each unit.

i did so because the quote was vague. all the quote said was this: "This price is to install a 200 Amp two gang meter center, to split up the apartments, to fix wires in the attic, and add a new panel in upstairs apartment. This comes with a 3 year warranty on the panel and breakers. To split up the outside lights is an added cost." That's it. I specified 125A to each unit but i'm not a professional. i contend it's the contractor's responsibility to install a system that will handle the load.

thank you again for sharing your knowledge with me.
i appreciate it very much
cub
 

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cubgirl

Member
Location
madison, AL
Your calcs look good although I did not go over it with a fine tooth comb. Unfortunately , IMO the use of an electric on demand water heater is IMO a big mistake as they draw so much and my experience from others who have used them is the customer is never satisfied with them. If you used a standard residential water 4500 watts then the issues with the service would not be there.

I would also recommend just going with 2- 200 amp panels instead of keeping the service so close. Around here a 150 amp panel would cost me twice as much as a 200 amp panel because they are not stocked.
hi & thank you...

yes that's what i'm hearing, two 200A panels. wouldn't that require more service amps? does utility company charge the same the rate regardless of the size of service? the only cost is the equipment? if that's so, i see now why a load calculation should've been done. i could've gotten different EQ installed to handle the load.

thanks
cub
 

GoldDigger

Moderator
Staff member
There is no explicit violation in driving two 200A mains and panels from a single 200A service. It all depends on the calculated load.
But, on the other hand, POCO may not want to connect a 100A service to a panel with a 200A main. :)
 

JFletcher

Senior Member
Location
Williamsburg, VA
This is an older article, and I do not have a 2014 NEC, so I dont know if the information is still good, but:

http://ecmweb.com/code-basics/dwelling-unit-feederservice-conductor-calculations

there is mention of an optional calculation:

"Optional method. You can use the easier optional method found in 220.30 only when the total connected load is served by a single 3-wire, 120/240V or 208Y/120V set of service or feeder conductors with an ampacity of 100A or greater. Because this condition describes the typical residential service, the optional method is likely to apply. Using it can simplify the design process and save you time because you have so many fewer sets of calculations.

General loads. The calculated load shall not be less than 100% for the first 10kW, plus 40% of the remainder of the following loads:

Small-appliance and laundry branch circuits: 1,500VA for each 20A circuit.

General lighting and receptacles: 3VA per sq ft

Appliances: The nameplate VA rating of all appliances and motors fastened in place (permanently connected) or on a specific circuit. Be sure to use the range and dryer at nameplate rating."

Which I believe winds up, in the OP's case (weclome cubgirl!), to be less load than what she calculated.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Wouldnt the refrigerator go on the SABC? and the oven demand factor would be 8kw? (220.55) Also, would 220.60 (2008 NEC) allow to take the higher of two loads that are unlikely to be used simultaneously? i.e, the dishwasher and garbage disposal arent likely to be on at the same time imo.
 

cubgirl

Member
Location
madison, AL
yes thanks I heard about the optional method... however, there are grouped (2) disconnects near the gang meters, one for each panel. This would be each unit's main disconnect. i heard in that situation, there needs to be a 4 wire to the panels. if that's so, does using a 4 wire preclude using the optional method because it requires that the total load is served by a single 3 wire??

"You can use the easier optional method found in 220.30 only when the total connected load is served by a single 3-wire, 120/240V or 208Y/120V set of service or feeder conductors with an ampacity of 100A or greater. Because this condition describes the typical residential service, the optional method is likely to apply."

I also thought I couldn't use it because this is a two-family, zoned residential/business. But I'm stumbling around in the dark here.

thanks
cub
 

Glock23gp

Member
Location
United States
Standard calc I have a total feeder load of 203.25 amps if my math is correct per 2014 NEC requiring you to have a 400a or 320 continuous service.

You have several things listed in the wrong categories making your numbers slightly off.

Ref and ice machines are not fastened in place therefore are not included in the appliances (220.53).
Disposal isn't in the motor category it's a fastened in place appliance. (220.53)

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cubgirl

Member
Location
madison, AL

cubgirl

Member
Location
madison, AL
Standard calc I have a total feeder load of 203.25 amps if my math is correct per 2014 NEC requiring you to have a 400a or 320 continuous service.

You have several things listed in the wrong categories making your numbers slightly off.

Ref and ice machines are not fastened in place therefore are not included in the appliances (220.53).
Disposal isn't in the motor category it's a fastened in place appliance. (220.53)

Sent from my SAMSUNG-SM-T337A using Tapatalk
you must be talking about unit 1. that's quite a big difference -- 60 amps. what caused it?

the ice machine is fixed in place. it's a Kitchen Aid 18" under cabinet model with an electric drain pump with an attached drain (it's upstairs so it has to have a drain pump system attached) and a copper waterline attached. as far as refrigerators, if they have a copper waterline, i consider it fixed. :happyyes:

this load calc was a spreadsheet i got off the net from the city of Naperville. the items are listed in a drop down on the sheet so i followed it. i can edit it and put the disposer under fixed appliances. i see Holt's ECM page says disposers are a fastened appliance but it doesn't mention refrigerators. also, what about counter top microwaves and stand alone freezers? where do they go? are they considered a part of 220.52A, kitchen circuits? if so, i should remove the refrigerator and microwave from the appliances section 220.53.

thanks for sharing your knowledge
cub
 

Glock23gp

Member
Location
United States
Fastened in place is just as it sounds. Physically secured with bolts or screws. So counter top micros, fridge, freezers all do not count.

They all plug into your 1500va small appliance circuits.

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cubgirl

Member
Location
madison, AL
Fastened in place is just as it sounds. Physically secured with bolts or screws. So counter top micros, fridge, freezers all do not count.

Sent from my SAMSUNG-SM-N910A using Tapatalk
right but my question was are they left off completely because they're embedded in 220.52A with the kitchen circuits??

they have to be part of the load somewhere... i'm thinking they're hidden in 220.52A

anyway, i took them off the fixed appliance section. made the amp go down a little.
 

Glock23gp

Member
Location
United States
I would add an extra small appliance circuit for them just like how I would wire it if it were new with a dedicated circuit feeding them

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