wire gauge size for DHW heater

[hemlock]

Hello,
I have recently bought a Marathon DHW heater. My electrical is a bit hazy - would I use 10/2 wire, and a double pole 30A breaker for the install? It is 4500W, 240VAC. Thanks.

 

[oldspark]

Hello,
I have recently bought a Marathon DHW heater. My electrical is a bit hazy - would I use 10/2 wire, and a double pole 30A breaker for the install? It is 4500W, 240VAC. Thanks.

4500 divided by 240 comes out to 18.75 amps, so 12 would do it with a 20 amp breaker unless you think you will need bigger wire later.

 

[Highbeam]

For a 30 amp service, yes, 10-2. That's what my electric water heater uses. See what the marathon requires just in case they want something special which would be silly since it would knock them out of the running for most retrofits.

 

[btuser]

Its a resistive load so you can go 100% on the circuit, but I always go 80%. Depending on the length of the pull you may find 10/2 will pay for itself due to lower resistance.

 

[begreen]

Yes, go with #10 and the 30 amp breaker. You need a margin of over protection. My NEC book is ancient, but it says 125% over nameplate rating for the HW heater. #12 on a 20amp breaker would not afford the required protection, especially if the voltage fluctuates down to say 230v.

 

[Highbeam]

I've never heard the bit about going 100% on a resistive load, is that actually code or is it just logic speaking? I have been told repeatedly to only use 75% of the breaker so in this case using 18+ amps of a 20 amp circuit is a no-no and you would bump to the 30 like about every water heater circuit ever put in a home.

 

[oldspark]

Yes, go with #10 and the 30 amp breaker. You need a margin of over protection. My NEC book is ancient, but it says 125% over nameplate rating for the HW heater. #12 on a 20amp breaker would not afford the required protection, especially if the voltage fluctuates down to say 230v.

If the voltage goes down the amps go down, ohms law. Forgot about the code, 20 amp breaker and #12 wire is plenty good enough except for the code wanting you to use 80% of wire capacity, 20 amp breaker would protect the heater better in case it starts to eat its pigs.

 

[Jags]

10/2 on a 20 amp breaker. Large wire - less resistance - less heat. 20 amp breaker will hold the normal operating environment but will protect the circuit quicker if a failure occurs. 10/2 wire will also hold any typical "upgrade" in the future with a simple breaker replacement.

Its just my opinion, but I don't like to have alot of "over capacity" on a specific load (i.e. a 30 amp breaker on an 18 amp load). Slow shorts cause the fires, not direct shorts (a direct short would pop a 30 amp breaker on 14 ga. wire without wire damage - as an example). That is the ultimate reasoning for different sized breakers. To protect the circuit to the closest value the circuit was designed for.

 

[hemlock]

Hello,
Thanks for all the advice. The manual says 25A breaker, 10 gauge wire. Does this sound good?

 

[Jags]

Hello,
Thanks for all the advice. The manual says 25A breaker, 10 gauge wire. Does this sound good?

Yep - do it to the book.

 

[oldspark]

Hello,
Thanks for all the advice. The manual says 25A breaker, 10 gauge wire. Does this sound good?

Perfect, you cant go wrong with the manual and then you should be covered as far as the code what ever that is in your area.

 

[Highbeam]

Have you ever seen a 25 amp breaker? I don't recall anything between the 20 and 30 on the shelf.

 

[oldspark]

Have you ever seen a 25 amp breaker? I don't recall anything between the 20 and 30 on the shelf.

They make them but I do not think you will find them easily.

 

[burnham]

Wired a hundred of them. 10-2, 30 amp breaker. If the panel is more than 50' (it may be something like 16 meters now, the code is going metric) away, or not within eye-sight, you are supposed to have a disconnecting means at the unit.

 

[begreen]

Have you ever seen a 25 amp breaker? I don't recall anything between the 20 and 30 on the shelf.

They are rare, but sometimes can be found. Square D and GE make them. Not sure about other panels.

 

[oldspark]

Wired a hundred of them. 10-2, 30 amp breaker. If the panel is more than 50' (it may be something like 16 meters now, the code is going metric) away, or not within eye-sight, you are supposed to have a disconnecting means at the unit.

I think something that runs at 18 amps does not get the protection it needs with a 30 amp breaker, I know it is SOP but just saying.

 

[Highbeam]

Breakers protect the wire, not the appliance. The appliance is able to pull 18 amps but it can easily start a fire if only pulling 1 amp.

 

[oldspark]

Breakers protect the wire, not the appliance. The appliance is able to pull 18 amps but it can easily start a fire if only pulling 1 amp.

I would not agree with that 100%, I have had circuit devices blow and or trip when the appliance has a problem and been able to correct the problem.

 

[seige101]

Those who said 10-2 with a 30a breaker are correct. You don't want to load up a circuit more than 80% of it's rated load (or take your load and multiple by 125%). So a 20 amp breaker and 12-2 should only be used for 16 amps. 25 amp breakers are made, an electrical supply house may have one in stock or special order from their nearest CDC.

 

[oldspark]

this is interesting, I dont know if I agree with the results though

http://www.csgnetwork.com/wiresizecalc.html

 

[oldspark]

And yet another interesting site with the fact that are breakers rated at 100% versus 80% useage.

http://ecmweb.com/mag/electric_sizing_circuit_breaker/

 

[Highbeam]

Breakers protect the wire, not the appliance. The appliance is able to pull 18 amps but it can easily start a fire if only pulling 1 amp.

I would not agree with that 100%, I have had circuit devices blow and or trip when the appliance has a problem and been able to correct the problem.

Well sure, a short in an appliance can blow a breaker but the breaker's job is to protect the wire. The appliance failed. This is a generality, not many circuits feed a single appliance. Obviously, the appliance short circuit resulted in less damage because of the popped breaker but that was good fortune. Your typical wall outlet circuit would be loaded with many many times the breakers capacity but since the wire is protected by the breaker, you can only pull so much current through those outlets.

 

[oldspark]

Breakers protect the wire, not the appliance. The appliance is able to pull 18 amps but it can easily start a fire if only pulling 1 amp.

I would not agree with that 100%, I have had circuit devices blow and or trip when the appliance has a problem and been able to correct the problem.

Well sure, a short in an appliance can blow a breaker but the breaker's job is to protect the wire. The appliance failed. This is a generality, not many circuits feed a single appliance. Obviously, the appliance short circuit resulted in less damage because of the popped breaker but that was good fortune. Your typical wall outlet circuit would be loaded with many many times the breakers capacity but since the wire is protected by the breaker, you can only pull so much current through those outlets.

A lot of appliances have there own circuit, yu must be talking about small amp draw devices, we were talking about a 18 amp heater at one time.

 

[sparke]

To be NEC code compliant the right answer is 25 amp breaker and 10 awg wire. This is why : Most hot water heaters are considered continuous load, this is why we use 125% rating on the circuit. 4500/240=18.75. 18.75 x 1.25=23.43. The closest sized listed breaker is 25 amps. If you are an electrician this breaker is easy to find at your local wholesale electrical warehouse... I would think most hardware stores / Home Depot would carry this breaker also.

On a side note - I see alot of 30 amp breakers on these hot water heaters, it is not a huge safety factor because the 10 awg supply circuit can handle that amperage however it is not code...


Also there are usually 2 individual circuits (one to each element) inside the hot water heater. These are usually wired with 12awg. These circuits are interlocked so only one element can come on at one time. I don't think it is intentional but ironically 12 awg is good for 25 amps although you can not use fuse it that high...