25 amp 2pole breaker for water heater?

superrecordFebruary 29, 2012

Hello gardenweb, this is my first time posting on here so please bare with me.

I have an electric water heater that I need to connect.

The heater is 4500 watts 240 single phase

It requires an 84 foot from the sub panel.

The math I used looks like this.

4500w/240v = 18.7A

18.75A X 1.25(to maintain 80% of breaker capacity)= 23.4375Amp breaker minimum

Now according to the calculator on the below linked website I can use 10 awg copper and still be under 5% voltage drop.

Unfortunately nobody here in san francisco has a 2 pole 25A circuit breaker. If I bump up the breaker to 30A then I have to also bump up to an 8awg and that will cost me $90 which I don't really want to spend (and I don't want to snake 8-3mc through a wall for 84 feet). Is there a reason that nobody has 25A 2 pole breakers? I can find them online. I know they are made. I can't find any reasons as to why they are not existent. Are they illegal and I don't know it? Is there a reason for me not to use a 25A 2 pole breaker? This is in a commercial building which I don't believe makes a difference in this question.

Thanks for any advice on here..

(link to Voltage drop calculator that I used)

http://www.electrician2.com/calculators/vd_calculatoradv.htm

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bus_driver

The 30 amp is OK. The overcurrent device protects the conductors.

    Bookmark   March 1, 2012 at 7:30AM
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Ron Natalie

To amplify what Bus Driver is saying, the ampacity of the conductors is not affected by voltage drop issues. The wire is still capable of carrying the current even if the voltage drops to an unacceptable level by the time it reaches the load.

    Bookmark   March 1, 2012 at 8:32AM
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brickeyee

#10 is fine for a 30 amp breaker.

A 240 V 30 amp circuit using #10 wire is pretty 'standard' for residential water heaters.

Only one heating element is on at a time.

    Bookmark   March 1, 2012 at 10:09AM
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superrecord

Thank you so much for the help.

I am still new to this and have a couple more questions.

1: is it illegal to put a 30 amp breaker on a 10 wire of that length when you know it is rated to be over 5% voltage drop for that length.

2: If I pay $90 more right now for the 8 wire, will there be savings in the long run on the electric bill to keep this water heater always warm with less voltage drop due to the thicker wire.

Thanks again for the help on my newbie questions..

    Bookmark   March 1, 2012 at 1:50PM
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randy427

It's not illegal. The greater voltage drop will just mean that the WH element(s) won't get quite as hot and the water will take a bit longer to come up to temperature. I doubt that you would ever see a payback on the additional $90.00.
Voltage drop is mostly a consideration for motors, fluorescent lighting and electronics that can have their lifespan shortened under certain conditions.

    Bookmark   March 1, 2012 at 4:00PM
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petey_racer

I don't know what calculator you are using, but I come up with #12 for 18.7 amps @ 240v @ 84'.

Since a water heater must be considered a continuous load you must figure the 18.7A @ 125%. That is the only reason you need #10 and a 25 or 30A breaker.

    Bookmark   March 1, 2012 at 6:02PM
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bus_driver

petey, his calculations in the original post are the same as yours. I suspect a very hasty reading.

    Bookmark   March 1, 2012 at 6:34PM
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dennisgli

2: If I pay $90 more right now for the 8 wire, will there be savings in the long run on the electric bill to keep this water heater always warm with less voltage drop due to the thicker wire.

So if you have a 5% voltage drop; and you are paying $500/year to run the water heater; then you are paying $25/year to heat the wire - right?

But are you sure you are using that calculator correctly to get the 5% ?

    Bookmark   March 3, 2012 at 8:57AM
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bus_driver

No one said the voltage drop is 5%. I have not calculated it for this particular circumstance, but based on experience, my guess is that it will be less than 2%. Calculating it is not an interesting use of my time right now. I can say that for every voltage drop calculation I have ever done for properly sized conductors, lengths less than 100 feet do not have unacceptable voltage drop.
It is also to be noted that the use of #8 does not decrease the voltage drop to zero. Some voltage drop will still occur.

    Bookmark   March 3, 2012 at 9:11AM
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Ron Natalie

And to answer the question that we've already answered but he asked again
VOLTAGE DROP has no bearing on the ampacity of the wiring or the circuit breaker requirement. While the current draw increases the voltage drop, and may cause you to wish to increase the conductor sizes, the breaker is sized to the ampacity (again unaffected by the voltage drop unless there's something specific in the manufacturer's instructions that require a smaller breaker.

Further, if you want to put a 25A breaker in, fine. They're available for all the major panels. If you've got some goofball panel, maybe you have bigger problems.

    Bookmark   March 3, 2012 at 11:37AM
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