Two-Family Optional Method Calculation - Electrician Talk - Professional Electrical Contractors Forum

thegoldenboy · 05-11-2012, 05:19 PM

I've got a customer who's looking to have a back up generator installed at his two family home in which he resides. His service is around 40 years old, both panels are FPE Stab-lok.

I'm pushing him to consider changing out the service first, for ease of future work.

There is no permanent means of A/C installed on either floor but there is 102 linear feet of electric baseboard heat between both floors. For calculation purposes only, I used a figure of 250w/ft for the heat.

102' x 250w = 25,500w or 25.5kw or 25,500va

The house is 2,073 sq. ft. according to the research I've done online.

I'm just looking for a double-check on my calculation.

2,072 sq. ft x 3va 6,219va
(4) Sm. App. @ 1,500va ea. 6,000va
(2) Laundry Cir. @ 1,500va ea. 3,000va
(2) Ranges per Column C T220.55 11,000va
Water Heater 4,500va
(2) Refrigerators 2,800va
Freezer 600va
Dishwasher 1,030va
(2) Disposal 1,380va
Range Hood 400va
(2) Microwaves 3,260va
Pool Pump 2,880va
(2) Clothes Dryer T220.54 10,000va
Hot tub 600va

Total 53,669va

1st 10kva @ 100% 10,000va
Remaining @ 40% 21,468va

Total General Load 31,468va
25.5kva of Heat @ 40% 10,200va

Total 41,668va

41,668va / 240v 174A

Any feedback would be appreciated. I used the calculator over at MH but found it to be a little complicated.

HARRY304E · 05-11-2012, 05:50 PM

I don't see any mistakes..

Do you have to have the whole house tied into the generator?

Remember also they will never be running the house at full load anyway.

thegoldenboy · 05-11-2012, 06:10 PM

Customers looking only to supply his 1st floor with enough of a generator to supply heat, lights and a refrigerator. His elderly mother lives on the first floor, he lives on the second.

I'm working out those calculations right now.

I'm going to tie the generator parallel into each panel so it can run either panel if need be. If either panel falls out, they're both covered this way. If the entire service drops out, only enough to keep them getting by on the first floor.

There's 65 linear feet of baseboard heat on the first floor. 27 linear feet on the second. I'm using 40% of the heat for the calculation, so I'm using the respective figure for the first floor which would be 6.5kw. The second comes in at 3.7kw.

So with the 6.5kw figure, I also need to account for 2 refrigerators and at least 1 circuit for lighting. I used the figure of 2,800va for the calculation so that brings me to 2.8kw for the refrigerators and I'll account for 1,500va for lighting. That brings me to a total of 10.8kw to keep his food fresh and heat on in the first floor.

I've never sized a generator, but those numbers look sound to me.

Bulldog1 · 05-11-2012, 07:39 PM

Quote:

Originally Posted by thegoldenboy

Customers looking only to supply his 1st floor with enough of a generator to supply heat, lights and a refrigerator. His elderly mother lives on the first floor, he lives on the second.

I'm working out those calculations right now.

I'm going to tie the generator parallel into each panel so it can run either panel if need be. If either panel falls out, they're both covered this way. If the entire service drops out, only enough to keep them getting by on the first floor.

There's 65 linear feet of baseboard heat on the first floor. 27 linear feet on the second. I'm using 40% of the heat for the calculation, so I'm using the respective figure for the first floor which would be 6.5kw. The second comes in at 3.7kw.

So with the 6.5kw figure, I also need to account for 2 refrigerators and at least 1 circuit for lighting. I used the figure of 2,800va for the calculation so that brings me to 2.8kw for the refrigerators and I'll account for 1,500va for lighting. That brings me to a total of 10.8kw to keep his food fresh and heat on in the first floor.

I've never sized a generator, but those numbers look sound to me.

What code cycle are you under? You size the generator based on the amps of the circuits you are backing up. You can't feed both panels since the code change in 2008 like you plan. The generator has to be big enough to back up everything it supplies.

thegoldenboy · 05-11-2012, 08:23 PM

Quote:

Originally Posted by Bulldog1

What code cycle are you under? You size the generator based on the amps of the circuits you are backing up. You can't feed both panels since the code change in 2008 like you plan. The generator has to be big enough to back up everything it supplies.

2011. It's one service drop, two meters, two panels. The Inspector of Wires says having the generator in parallel with both panels is fine. What change are you citing?

I have to redo my entire calculation, I just left the guys house and he forgot to mention that the entire 1st floor of the house is baseboard hot water, not electric baseboard heat.

thegoldenboy · 05-11-2012, 08:50 PM

Total General Load 31,468va
9.25kva of heat @ 100% 9,250va

Total 40,718va

40,718va / 240v 170A

I lost 4 amps.

I'm not controlled at 4 locations so I have to take it at 100%. It doesn't change much on the service, but he only wants the furnace to be operational which is on a 120v 15A circuit.

Bulldog1 · 05-12-2012, 10:25 AM

Quote:

Originally Posted by thegoldenboy

2011. It's one service drop, two meters, two panels. The Inspector of Wires says having the generator in parallel with both panels is fine. What change are you citing?

I have to redo my entire calculation, I just left the guys house and he forgot to mention that the entire 1st floor of the house is baseboard hot water, not electric baseboard heat.

The code changed in 2008. I posted what the change says. If you are under 2005 your inspector is correct. If your under 2008 or 2011 he is wrong if your generator is not large enough to supply all loads connected to it. Before 2008 you could use am undersized genny.

thegoldenboy · 05-12-2012, 10:40 AM

I was looking for the Art. # bulldog, so I could read it and better educate myself. I've got 05, 08 and 11 for reference.

Nevermind, I found it.

702.5 Capacity and Rating.
(A) Available Short-Circuit Current. Optional standby
system equipment shall be suitable for the maximum available
short-circuit current at its terminals.
(B) System Capacity. The calculations of load on the
standby source shall be made in accordance with Article
220 or by another approved method.
(1) Manual Transfer Equipment. Where manual transfer
equipment is used, an optional standby system shall have
adequate capacity and rating for the supply of all equipment
intended to be operated at one time. The user of the
optional standby system shall be permitted to select the
load connected to the system.
(2) Automatic Transfer Equipment. Where automatic
transfer equipment is used, an optional standby system
shall comply with (2)(a) or (2)(b).
(a) Full Load. The standby source shall be capable of
supplying the full load that is transferred by the automatic
transfer equipment.
(b) Load Management. Where a system is employed
that will automatically manage the connected load, the
standby source shall have a capacity sufficient to supply the
maximum load that will be connected by the load management
system.

If I'm reading that correctly, it needs to be rated for the equipment it intends to serve at one time. He's not looking to run his entire house at once. A furnace, lights, and two refrigerators. He's going to have to manually shed the load himself. Everything he intends to run will be correctly labeled, and I'm sizing the generator to run what he intends to run, not the size of the service.

I used Kohlers calculator last night and came up with a rating of 8.5kw.

Bulldog1 · 05-12-2012, 01:04 PM

Quote:

Originally Posted by thegoldenboy

I was looking for the Art. # bulldog, so I could read it and better educate myself. I've got 05, 08 and 11 for reference.

Nevermind, I found it.

702.5 Capacity and Rating.
(A) Available Short-Circuit Current. Optional standby
system equipment shall be suitable for the maximum available
short-circuit current at its terminals.
(B) System Capacity. The calculations of load on the
standby source shall be made in accordance with Article
220 or by another approved method.
(1) Manual Transfer Equipment. Where manual transfer
equipment is used, an optional standby system shall have
adequate capacity and rating for the supply of all equipment
intended to be operated at one time. The user of the
optional standby system shall be permitted to select the
load connected to the system.
(2) Automatic Transfer Equipment. Where automatic
transfer equipment is used, an optional standby system
shall comply with (2)(a) or (2)(b).
(a) Full Load. The standby source shall be capable of
supplying the full load that is transferred by the automatic
transfer equipment.
(b) Load Management. Where a system is employed
that will automatically manage the connected load, the
standby source shall have a capacity sufficient to supply the
maximum load that will be connected by the load management
system.

If I'm reading that correctly, it needs to be rated for the equipment it intends to serve at one time. He's not looking to run his entire house at once. A furnace, lights, and two refrigerators. He's going to have to manually shed the load himself. Everything he intends to run will be correctly labeled, and I'm sizing the generator to run what he intends to run, not the size of the service.

I used Kohlers calculator last night and came up with a rating of 8.5kw.

I was assuming you were using an ATS. If you are you can't manually shed the loads. You have to use our load control module or set a panel with the circuits you are backing up and your generator must be big enough to back up all the circuits in the panel.

thegoldenboy · 05-12-2012, 01:29 PM

I was planning on putting in two manual interlock kits, one for either panel with the generator in parallel to both panels.

Out of the first panel, the only thing he wants to run is one or two circuits for lighting, the furnace, a microwave if need-be and the refrigerator.

Out of the second panel, the only thing he wants to run is the upstairs refrigerator and nothing else.

The easiest and most straightforward approach I can come up with is marking the handles of the breakers in red paint, so he knows to leave those on and shut everything else off.

I can't think of a feasible way to bring circuits into one panel from two separately metered locations.

Am I still compliant with 702.5(B)(1) in this approach?