Two Family Residence Rewire

[Ryan Currin]

A 2400 Square foot two family residence I'm rewiring has an existing service of 100 amps feeding a two bank meter socket and two tenant panels with 60 amp mains. I've installed a new 3 bank meter enclosure w/public bypass. Each unit has a 100 amp panel board consisting of 2 small appliance circuits, 4 lighting/outlet branch circuits, microwave, dishwasher, disposal, 30 amp 240 volt condenser, and 1 energy efficient 120 volt air handler (5 amp draw). There are no electric ranges, no electric baseboard heat, and no washer/dryers in each unit. The public panel feeds 1 dryer, 1 washer, 1 water heater (30 amp 120v), and 1 lighting circuit in the basement. I would like to use the existing 100 amp service to feed the new 3 bank meter enclosure. The loads in this residence are very minimal so I think this would be suffice. My local inspector said he would pass the installation but did recommend a 200 amp service but I think that would be overkill. All the appliances/bulbs are energy efficient and the owner does not want to pay for a new 200 amp service upgrade which I agree. Also, would I need to protect each unit panel feeder with a 60 amp breaker max or could I use a 100 amp breaker instead. I plan on using a 40 amp breaker for the public panel.

Thanks

 

[K_Buz]

wow...I was WAY off base...sorry...I'll go back to my corner now

 

[Dennis Alwon]

Ryan did you do a service calculation. If the entire load is 100 amps or less then I don't see an issue but you need to make sure.

 

[Ryan Currin]

Ryan did you do a service calculation. If the entire load is 100 amps or less then I don't see an issue but you need to make sure.

Yes, I did try and calculate the entire load but I'm not sure how to combine the two units. Obviously the small appliance and ac loads, etc will not be operating at the same time so I can omit certain loads, but again I'm not sure if my calculation is correct. This residence by code is not considered a "multi family" because it is below 3 floors so I'm assuming to treat it as a one family dwelling calculation. Do I multiply the small appliance demand factors by 2 as well as the other loads I stated in my question? The service load calculations in the code specify including range loads and electric space heating loads which I have none because everything is gas.

Thanks

 

[Dennis Alwon]

By the electrical code, NEC, a multifamily dwelling is a building that contains three or more dwelling units. So I think it is a MF dwelling. You also must have a washer dryer on the premise or provisions for one and you may possibly need a house panel.

 

[Ryan Currin]

By the electrical code, NEC, a multifamily dwelling is a building that contains three or more dwelling units. So I think it is a MF dwelling. You also must have a washer dryer on the premise or provisions for one and you may possibly need a house panel.

Dennis, please read my first post, this is only a two family (only two dwelling units) residence and there is only one dryer/washer machine in the basement for all residents and is fed from a house panel.

 

[kwired]

Yes, I did try and calculate the entire load but I'm not sure how to combine the two units. Obviously the small appliance and ac loads, etc will not be operating at the same time so I can omit certain loads, but again I'm not sure if my calculation is correct. This residence by code is not considered a "multi family" because it is below 3 floors so I'm assuming to treat it as a one family dwelling calculation. Do I multiply the small appliance demand factors by 2 as well as the other loads I stated in my question? The service load calculations in the code specify including range loads and electric space heating loads which I have none because everything is gas.

Thanks

I come up with load calc of 22920 VA = 95.5 A @ 240 volts. This assumes a few things - 1/2 of the area you mentioned - per dwelling unit - may be less if your area covers common area also, but I did not figure any general lighting for common area so it is probably about same anyway, also assumed 5000 watt dryer, 2500 watt water heater ( assumed 30 amp was your breaker size and not load of the unit, same with air conditioners 30 amp breaker size is probably about a 4500 VA AC unit at largest. Better details could make the 95.5 go up or down - we are close to 100 amps with what we do know so far.

Why you think it needs to be multiple levels to be a multi family dwelling? There are many that are all on one level.

By the electrical code, NEC, a multifamily dwelling is a building that contains three or more dwelling units. So I think it is a MF dwelling. You also must have a washer dryer on the premise or provisions for one and you may possibly need a house panel.

His "public panel" is the house panel. He has a two family dwelling.

 

[Ryan Currin]

I come up with load calc of 22920 VA = 95.5 A @ 240 volts. This assumes a few things - 1/2 of the area you mentioned - per dwelling unit - may be less if your area covers common area also, but I did not figure any general lighting for common area so it is probably about same anyway, also assumed 5000 watt dryer, 2500 watt water heater ( assumed 30 amp was your breaker size and not load of the unit, same with air conditioners 30 amp breaker size is probably about a 4500 VA AC unit at largest. Better details could make the 95.5 go up or down - we are close to 100 amps with what we do know so far.

Why you think it needs to be multiple levels to be a multi family dwelling? There are many that are all on one level.



His "public panel" is the house panel. He has a two family dwelling.

Thanks for your response and calculation. That's funny, I came up with 98 amps for my service load. Also, I think we should remember that residential loads are really calculated at 208 volts rather that 240 volts, because most utility pole/underground transformers most often only provide 240 volts to commercial businesses, and residences are kept to a lower voltage of 208. Dennis, do you think there's and validity behind my theory on the service for this residence? As electricians, we often do sometimes install new services based on a comfort zone and an over-simplified approach rather than a true theory on what is exactly the requirement. In definition, this does have an impact on our overall cost when it's all said and done.

Thanks

 

[kwired]

Also, I think we should remember that residential loads are really calculated at 208 volts rather that 240 volts, because most utility pole/underground transformers most often only provide 240 volts to commercial businesses, and residences are kept to a lower voltage of 208.

Where did you get this idea??

Any load is calculated at the nominal applied voltage as far as art 220 is concerned, read 220.5(A). A dwelling could be supplied by about any voltage imaginable - there are requirements for certain voltages at specific places particularly 120 volts nominal, which makes 120/240 or 120/208 the preferred supply voltages.

208 volts is almost always derived from three phase wye system and is more common to see in commercial or multi family dwellings. 120/240 single phase is more typical in single family and smaller multi family dwellings, but is also seen a lot in small commercial buildings.

 

[Ryan Currin]

Thanks for your response and calculation. That's funny, I came up with 98 amps for my service load. Also, I think we should remember that residential loads are really calculated at 208 volts rather that 240 volts, because most utility pole/underground transformers most often only provide 240 volts to commercial businesses, and residences are kept to a lower voltage of 208. Dennis, do you think there's and validity behind my theory on the service for this residence? As electricians, we often do sometimes install new services based on a comfort zone and an over-simplified approach rather than a true theory on what is exactly the requirement. In definition, this does have an impact on our overall cost when it's all said and done.

Thanks

Your correct, a multifamily dwelling is considered 3 or more dwellings, and is not based on the amount of floors, but the dwelling I'm referring to is only a two family.

 

[Ryan Currin]

Where did you get this idea??

Any load is calculated at the nominal applied voltage as far as art 220 is concerned, read 220.5(A). A dwelling could be supplied by about any voltage imaginable - there are requirements for certain voltages at specific places particularly 120 volts nominal, which makes 120/240 or 120/208 the preferred supply voltages.

208 volts is almost always derived from three phase wye system and is more common to see in commercial or multi family dwellings. 120/240 single phase is more typical in single family and smaller multi family dwellings, but is also seen a lot in small commercial buildings.

True, but I've worked on many different sources of voltage from the utility companies and the voltages do vary, with mostly commercial voltages being higher due to motor concerns and equipment.

 

[kwired]

True, but I've worked on many different sources of voltage from the utility companies and the voltages do vary, with mostly commercial voltages being higher due to motor concerns and equipment.

Voltages will vary some, there is not really that many 'nominal' voltage systems that are common. Again read 220.5.

For residential and commercial about all you will ever run into is 120/240, 120/208, 277/480 - especially at the service. If something else is needed it is usually separately derived with owners equipment and usually supplies a specific load.

You run into corner grounded systems, ungrounded systems, and systems with a high leg - but they typically all still have nominal voltages covered by 220.5.

 

[JDB3]

Is the dryer electric heat (120 for the motor & 240 for the heating element) if so, then most pull between 21 to 23 amps. Washing machine between 8 to 12 amps. You said water heater 120 volt / 30 amp. While doing the washing & drying, I can easily see close to 40 amps just on water heater & washer. I would say that I would be concerned with a 100 amp service. I have told customers that I will not do a structure because of them wanting to save $$$.00, and scrimp on not having an adequate service size.

 

[kwired]

Is the dryer electric heat (120 for the motor & 240 for the heating element) if so, then most pull between 21 to 23 amps. Washing machine between 8 to 12 amps. You said water heater 120 volt / 30 amp. While doing the washing & drying, I can easily see close to 40 amps just on water heater & washer. I would say that I would be concerned with a 100 amp service. I have told customers that I will not do a structure because of them wanting to save $$$.00, and scrimp on not having an adequate service size.

You are correct with all of that. Outside of a few details, OP is still very close to having 100 amp supply being acceptable. I probably would suggest going with 200 amps myself, but if the load calc says 100 is acceptable then there is no code violation.