Question for electricians about wire gage.

[SketchMaster]

We are replacing our water heater, the old heater was a single element 3500W/240V. The new heater is a dual element 3500W/240V and my father and I are trying to figure out if the 12G wire that was being used with the last heater will be safe. The manual instructs to only use 10G wire but the odd thing is that the wire coming out of the heater IS 12G wire, so is it O.K to use the existing wire with this unit?

 

[mobobuff]

I would say yes, but I'm only drawing from the same basic electrical knowledge shared by most ATOTers.

 

[QED]

Originally posted by: SketchMaster
We are replacing our water heater, the old heater was a single element 3500W/240V. The new heater is a dual element 3500W/240V and my father and I are trying to figure out if the 12G wire that was being used with the last heater will be safe. The manual instructs to only use 10G wire but the odd thing is that the wire coming out of the heater IS 12G wire, so is it O.K to use the existing wire with this unit?

According to the Handbook of Elecronic Tables, 12-guage wiring is suitable for up to 41 amps for chassis wiring, but only 9.3 amps for power transmission wiring. 10-guage wiring is good for 15 amps of power transmission-- and your heater uses 14.5 amps.

 

[dmcowen674]

Originally posted by: SketchMaster
We are replacing our water heater, the old heater was a single element 3500W/240V. The new heater is a dual element 3500W/240V and my father and I are trying to figure out if the 12G wire that was being used with the last heater will be safe. The manual instructs to only use 10G wire but the odd thing is that the wire coming out of the heater IS 12G wire, so is it O.K to use the existing wire with this unit?

Do you want a 7000 watt fire?

Run 10 gage.

 

[SketchMaster]

Right, I'll pick up some 10G on the way home from work tomorrow.

Thanks for the help.

 

[tranceport]

You should run 10G wire. If your homeowners insurance found out you ran 12G and had a fire, they probably would not pay your policy.






edit: gammah

 

[LTC8K6]

I'm unclear on the total wattage of the new heater. Is it 3500W total, but with two elements instead of one, or does it have 2 3500W elements for 7Kw total?

3500W/240V=14.58A 12Ga. wire okay
7000W/240V=29.17A 10Ga. wire needed

 

[dmcowen674]

Originally posted by: LTC8K6
I'm unclear on the total wattage of the new heater. Is it 3500W total, but with two elements instead of one, or does it have 2 3500W elements for 7Kw total?

3500W/240V=14.58A 12Ga. wire okay
7000W/240V=29.17A 10Ga. wire needed

It's most likely two 3500 Watt elements, one 1/3 from the bottom of the tank, the other 1/3 from the top.

Looks like both elements may never be on at same time.

Then why the 10 gage requirement?

Hmmmmm

 

[woodie1]

On most electric water heaters only one element is 'hot' at any given time.

 

[dmcowen674]

Originally posted by: woodie1
On most electric water heaters only one element is 'hot' at any given time.

Interesting, so what is the advantage of a dual element unit?

 

[woodie1]

Originally posted by: dmcowen674

Originally posted by: woodie1
On most electric water heaters only one element is 'hot' at any given time.

Interesting, so what is the advantage of a dual element unit?

Demand for hot water. Under high demand the upper element is on, otherwise the lower element will heat the whole tank eventually.

 

[dmcowen674]

Originally posted by: dmcowen674

Originally posted by: woodie1
On most electric water heaters only one element is 'hot' at any given time.

Interesting, so what is the advantage of a dual element unit?

A quick search has the answer:

Which is better, one element or two on a water heater?"

Most two element systems work like this: The upper element goes on first and once it is hot enough on top then the lower element goes on.

If the water you are getting out of your heater comes from the top, why would anyone think that heating the bottom only is better? We think the only reason is to save manufacturing costs
=============================================
Poses an interesting question on the gage of wire if the two elements never are on at same time.

 

[mobobuff]

Originally posted by: dmcowen674

Originally posted by: woodie1
On most electric water heaters only one element is 'hot' at any given time.

Interesting, so what is the advantage of a dual element unit?

Depends on the spacing of the elements, I would guess? One may be higher and used at a full capacity, the other may be lower and used during a draining capacity.

Again I'm drawing on minimal knowledge, but I do like to hypothesize...

 

[dmcowen674]

Originally posted by: woodie1

Originally posted by: dmcowen674

Originally posted by: woodie1
On most electric water heaters only one element is 'hot' at any given time.

Interesting, so what is the advantage of a dual element unit?

Demand for hot water. Under high demand the upper element is on, otherwise the lower element will heat the whole tank eventually.

I've never run into a single element tank.

 

[woodie1]

If either of the switches that control the heating elements was to fail in the closed position then you would need 10 guage wire to handle the load safely. Also, I expect the building code of today in most USA locations will require 10ga.

 

[SketchMaster]

Info on how a dual element heater works.

 

[BigJ]

Another fun thing about electrical installations is that you are supposed to do them according to the manufacturer's instructions. A home inspector can and will fail that install if you don't abide by those rules.

On top of that, if something were to happen and your homeowner's insurance were to get involved, you're shit out of luck.

With water heaters, general rules is 10ga. You're going to be installing wire rated for at least 125% of the listed load generally.

So you're looking at 125% of ~14.5A. Which is roughly 18 1/4 A. 12ga wire is [INCORRECT] no good for that.[/INCORRECT]

EDIT: I misspoke for this. Had a brain fart and did a calculation of 0.8x20 and 1.25x15. Double dipped.

You can definitely use 12ga wire for a continuous load of 14.5A. For non-continuous loads, you can load it up to the max. As long as a continuous load is no greater than 0.8 x the Amperage of the Breaker/Wire you're fine.

 

[BigJ]

Originally posted by: LTC8K6
I'm unclear on the total wattage of the new heater. Is it 3500W total, but with two elements instead of one, or does it have 2 3500W elements for 7Kw total?

3500W/240V=14.58A 12Ga. wire okay
7000W/240V=29.17A 10Ga. wire needed

As a general rule, the actual amperage coming across the line should be only ~80% of what the wire is rated for. So while 12ga would be fine for a load of 15A, at a load of 29A, you're stepping down to 8ga wire.

 

[LTC8K6]

12Ga. wire would be fine for a 14.5A load, imo.

 

[LTC8K6]

at a load of 29A, you're stepping down to 8ga wire.

No, I'm not. You can if you wish. It certainly won't hurt anything.

 

[BigJ]

Originally posted by: LTC8K6

at a load of 29A, you're stepping down to 8ga wire.

No, I'm not. You can if you wish. It certainly won't hurt anything.

Do whatever the hell you want, but don't tell people they'll be OK installing 10ga wire for a load of 29A. If it's continuous, you can't. If it isn't, it's OK but you're not giving yourself much breathing room (and you give yourself the option of upgrading the circuit down the line if you use 8ga).

It doesn't matter what you think. It matters what 3 bodies think. The NEC, the local building code, and the inspector.

Also, below 120gal water heaters are considered continous loads. For wire ga caluclations, you would have to take that rating and multiply it by 1.25. So (for arguments sake we'll use the 29A figure) 29A at 1.25, you're now looking at 38A.

This is exactly why people tell you to multiply the amperage rating (of the wire/breaker) by 0.8. If it's not continuous, you can load up the line to it's maximum rating. Expect it to trip though. If it is continuous, it's against code if the continuous amperage rating x 1.25 exceeds the total amperage rating of the wire/breaker.

As for the OP, techincally, it's right below that 20A threshold. 3500W/240V = 14.583~ x continuous load factor of 1.25 = 18.22917. It's below the threshold of the wire and the breaker. So safety wise, you'd be fine. However, this goes back to what I said before.

Part of being in compliance with code is adhering to the manufacturer's instructions. If the manufacturer calls for 10ga wire, while technically you may be able to use 12ga wire concerning the amperage, you have to use 10ga wire.

 

[LTC8K6]

The manufacturer is telling him to use 10ga.

I am merely agreeing with the mfg.

 

[BigJ]

Brain fart, was double dipping.