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Discussion Starter · #1 ·
Hello.
Info: Im in New York state. Main panel is 100 amps with no 240 circuits currently. Circuit panel is in the front of the house. Everything I would like to hook up is 240 volts and in the back of the house.

Need to hook up

Hot water tank – 20 amps
Electric cook top - 30 amps
Electric oven – 15 amps

My question is

Should we
A)Run wiring to a 70 amp Subpanel in the back of house with (3) 240 circuits and hook everything to that?
Or
B) Run 3 separate lines off the main panel to the appliances?
C) Or something else entirely?
Any guidance, suggestions and help is much appreciated. Thanks.
 

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Discussion Starter · #3 ·
First, thanks for your response. To answer your questions, yes there is room in the main panel. The runs would be approximately 75 ft each from front to back of house. Difficulty would be quite easy I'd assume. Just a straight shot through the attic and down to each appliance.
 

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First, thanks for your response. To answer your questions, yes there is room in the main panel. The runs would be approximately 75 ft each from front to back of house. Difficulty would be quite easy I'd assume. Just a straight shot through the attic and down to each appliance.

if it was my house, i would wire them all back to the main panel, and forget about the sub panel.....but thats just my opinion
 

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I'm a huge fan of subpanels, and in particular, I'm a huge fan of BIG subpanels. Now I see where you totaled up your loads and got 65A, and you're like "70A is a panel size". Now I'm not sure where you think you have to go exactly 70A, or just are cheap. First, you're allowed to go bigger, and second, there are lots of great places to be cheap in electrical (in fact I'm about to introduce you to one, but panels are not the place.

You want a nice panel, and what defines "nice" is lots and lots of spaces. Your math may say you need a 6-space (or even a 4-space using double-stuff breakers) - I'm telling you to get a 24-*space*.


Go insane. Spaces are dirt cheap (when speccing a panel) - but boy, when you're trying to add something later, and just don't have the spaces, regrets are expensive. Set it up so you never have that problem. Ever.

You don't need a main breaker in a subpanel - they're often used as the cheap way to get mandatory disconnect switches, but you don't need that here. So a main-lug panel will be fine.

The bus current rating (e.g. 70A) is like the speed rating on tires - you'd rather be driving 112 mph rated tires than 85 mph rated... right? More margin of safety. So don't worry about busing that is much larger than 70A. Can't hurt, gives you safety margin. Like I say I'd use a 24-space 200A bussed panel myself. The advantage is huge, and the coin isn't enough to care about.

As far as current factoring, since cooktop and oven are involved, the math gets really weird. I'm not an expert in that but someone will speak up. Giving them a full 45 amps of capacity might be overkill, but it's a solid choice. On the water heater, whatever the instructions say, you have to do that. Otherwise read the amps off the nameplate, multiply by 125% (i.e. add 25% more) and round up to the next available breaker. I usually expect 30A for a water heater, so I would run #10 wire in case the *next* water heater is 30A.

Now, on the feeder... you say none of your loads need neutral. It'd technically be possible to run this subpanel with no neutral whatsoever. Whenever you wire a 240V-only panel with no neutral, it's inevitable that someone will come along, go "ooh, subpanel!" and slap a 120V circuit in there. They will just assume it's a pre-1989 subpanel with common neutral-ground, and slap the neutral on the ground bar. This is inevitable. That creates a dangerous situation (which is why it was outlawed), so I strongly advise bringing neutral to this panel even though you don't need it right now. If you don't want to do that, I understand, but then I really do advise making it a 4-6-space panel and totally filling it, using a separate ground bar (so the neutral bar is EMPTY), and stick warning labels inside and out. Since it is 240V-only you do not need common-trip breakers.

Ah, but look at me, spending your money. I promised savings!

Aluminum wire. Seriously. It's true that on small branch circuits, it played badly with copper terminations. It has always worked well for inter-panel feeder. (not least, because the lugs are aluminum lol. Aluminum lugs play well with both Cu and Al wire). I laugh when people insist on spending 3x as much on copper feeder "to avoid dissimilar metal problems" then land it on aluminum lugs and don't even realize it.

If it were me, I'd run #1 Al wire so I could run full 100A. Otherwise for 65A actual load, you can run #4 Al or #6 Cu wire (other than NM-B or UF-B; you need wire that is allowed 75C thermal rating). This can be breakered at 70A in the main panel. Price it all 3 ways (#1Al #4Al #6Cu but remember the #6 cannot be NM-B).
 

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Full-size subpanel with 100A aluminum 4-wire feeder.
 

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I would run all 3 back to the service panel.

Have you done a load calculation to see if you have enough capacity?
 

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Discussion Starter · #9 ·
I really appreciate the extensive input guys!!! So I will try to respond to everyone. But before I do I see the answers are split down the middle which leads me to another question. Now that I know these two options are both acceptable, what are the pros/cons for these two options? What would make you choose one over the other?

seharper
haaa yes I like to cut costs when possible but I just assumed 70 amps was good according to the math I did. After reading your response I now realize, especially with the tire analogy, that I should go higher amps. So you're saying I should run 100 amp subpanel from a 100 amp main panel correct?

rjniles
I am told this but my model says 18.75 so I just rounded up to 20. Should I still do 30amps? The model is @ this link if you're interested. https://www.homedepot.com/p/Rheem-P...ric-Tank-Water-Heater-XE40M06ST45U1/205810725

Jim Port
Yes I've done a load calculation and it seems doable.
 

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A "Handy Husband"
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A water heater is a continuous load. Rated current is increased to 125% of normal amps. 18.75 times 1.25. Requires 30 amp circuit.

Sent from my RCT6A03W13E using Tapatalk
 
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seharper
haaa yes I like to cut costs when possible but I just assumed 70 amps was good according to the math I did. After reading your response I now realize, especially with the tire analogy, that I should go higher amps. So you're saying I should run 100 amp subpanel from a 100 amp main panel correct?
Yeah, I'm generally saying don't sweat subpanel amp rating. Get a panel you like that'll provide for future expansion, and if it's 100A that's cool, if it's 200A that's cool too. You can feed a 100A-bussed sub from a 100A panel. You can feed a 200A-bussed sub from a 100A panel. The safety is provided by the circuit breakers, just like the vehicle safety is provided by the speed limits (kinda, I admit that one is arguable but you get my point. The 200A panel doesn't cause 200A to flow, just as the 130mph tires don't make your car go 130 mph).

rjniles
I am told this but my model says 18.75 so I just rounded up to 20.
Yup, that's what gotcha. You needed to read the instructions.

They plainly say for a 4000W or a 4500W water heater (which yours is), same answer for both, you MUST use 10 AWG wire, and they RECOMMEND a 25A Breaker (though a 30A will be alright).

Jim Port
Yes I've done a load calculation and it seems doable.
Because load calcs on ranges and ovens are *super weird*. They actually let you oversubscribe by quite a lot.
 

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Discussion Starter · #13 ·
Thanks everyone! 30 amp for the water tank it is! So, for all electrical devices, as a general rule you should increase to 125% normal amps as rjniles suggested for the water tank? I will have to do some recalculations for these appliances if so. So far I am leaning towards running straight from the main panel but would like a couple more opinions before that decision is made. Thanks everyone for your time.
 

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Not all appliances are a continuous load. In residential the WH is about the only one; electric cook top and oven are not.

I would not install a sub unless you are short on breaker spaces. Run 10/2 for the WH, run 10/3 for the cook top, Run 10/3 for the oven. You could use 12/3 for the oven but 10/3 will future proof the install.
 
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Your amperage requirements look a little low for those appliances.
Where are you getting those numbers?
As for wiring runs....if you only need 3 circuits just run 3 circuits.
A sub-panel is great if you are putting on an addition or wiring an outbuilding.
But within your home it looks like you have specific needs.
Why add a large sub panel for 3 circuits (6 brewers)?

But your main panel only being 100 amps is the issue I would like to address.
With electric appliances you should have a 200 amp service.

I would forget the sub-panel idea and upgrade your service to 200 amp.
That would give you more than enough room and capacity to add your appliances and have room for any future plans.
Did you mention how large this home is?
Unless it's a small cabin, a 100 amp service just doesn't cut it anymore.
 

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Discussion Starter · #16 ·
Headed to the hardware store now to pick up the 10/3. Thanks!
Not all appliances are a continuous load. In residential the WH is about the only one; electric cook top and oven are not.

I would not install a sub unless you are short on breaker spaces. Run 10/2 for the WH, run 10/3 for the cook top, Run 10/3 for the oven. You could use 12/3 for the oven but 10/3 will future proof the install.
 
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