wire guage

[pipehead]

Hello,
I am replacing my indirect fired oil DHW heater with a Marathon water heater (oil free at last!). The house has never had electric hot water. What wire guage should be used for a twin 30A breaker, 4500W draw elements? Thanks.

 

[jhoff310]

I am not an electrician by any stretch of the imagination, but I think you would be able to find your answer here

http://www.powerstream.com/Wire_Size.htm

 

[RacerX]

Look closely at the heater nameplate. Dual element heaters will usually operate with only one element on at a time so the nameplate should say something like:
Upper 4500W
Lower 4500W
Total Wattage 4500
Divide the total wattage by the voltage:

4500/240=18.75amps, Multiply the amperage by 125% since it considered a continuous load:

18.75 * 1.25=23.4 amps. This is your minimum conductor size.

So you would need a conductor rated for 25amps which for this application is #12 AWG on a 2 pole-25 amp circuit breaker. Since you've mentioned a 30 amp CB you would need #10 AWG cable.

 

[esshup]

Doesn't the distance that the current has to run come into the equasion?

 

[BlueRidgeMark]

Doesn't the distance that the current has to run come into the equasion?

Yes it certainly does!


This is no place for amateur hour, unless you think burning down your house is funny.


Get professional advice.

HINT: Anyone who tells you "23.4 amps. This is your minimum conductor size." doesn't know how much he doesn't know!

 

[RacerX]

HINT: Anyone who tells you "23.4 amps. This is your minimum conductor size." doesn't know how much he doesn't know!

OK then why don't you explain if if you know so much.

 

[jerryw66]

10 gauge nm-b type wire is what is commonly used to power a water heater. It's rated for the 30 amp breaker, orange in color, heavy enough for any normal domestic hot water heater, with runs under 100'.

 

[RacerX]

Look closely at the heater nameplate. Dual element heaters will usually operate with only one element on at a time so the nameplate should say something like:
Upper 4500W
Lower 4500W
Total Wattage 4500
Divide the total wattage by the voltage:

4500/240=18.75amps, Multiply the amperage by 125% since it considered a continuous load:

18.75 * 1.25=23.4 amps. This is your minimum conductor size.

So you would need a conductor rated for 25amps which for this application is #12 AWG on a 2 pole-25 amp circuit breaker. Since you've mentioned a 30 amp CB you would need #10 AWG cable.

There is an error in the above reference to using #12 AWG conductors for a 25 amp HWH load. Although a #12 AWG conductor is rated for 25 amps it would be limited to 20 amps according to 240.4(D) of the NEC for a water heater circuit. For some reason I was thinking of an air conditioning circuit or motor circuit would allow the 25 amp minimum circuit ampacity of the #12 AWG conductors.

The original statement "18.75 * 1.25=23.4 amps. This is your minimum conductor size" is still correct.


Also yes, voltage drop for long runs might be an issue. Not a code issue but a design issue. A voltage drop on a water heater circuit which would lower the current and lower the heater output. Not a great design but not a code issue either.

 

[pipehead]

Hello. Thanks, all. I overlooked it in the book. It asks for a twin 25A, 10 guage wire. Its not so much that I would call myself an amatuer, but most of my wiring experience comes from 3 phase 600V applications, not so much residential.

 

[BlueRidgeMark]

The original statement "18.75 * 1.25=23.4 amps. This is your minimum conductor size" is still correct.
.

No it is not. AMPS is not a conductor size. Amperage is a measure of current flow, from which the proper conductor size can be determined, if the length of run is also known.

Amperage alone does not equal conductor size.

 

[RacerX]

Hello. Thanks, all. I overlooked it in the book. It asks for a twin 25A, 10 guage wire. Its not so much that I would call myself an amatuer, but most of my wiring experience comes from 3 phase 600V applications, not so much residential.

Yup, for this application you would need a minimum of #10 AWG for a 25 amp CB.

No it is not. AMPS is not a conductor size. Amperage is a measure of current flow, from which the proper conductor size can be determined, if the length of run is also known.

Amperage alone does not equal conductor size.

OK, so you use the number from the calculation to determine the minimum conductor size. Semantics aside I thought that was obvious.

Is this better, the conductor must have an ampacity of 23.4 amps or greater.
(The NEC only cares if the conductor is large enough for the 23.4 amps.)

If he's worried about voltage drop then you can calculate to see if the conductor he chooses is large enough. If the VD is very large his current will be lower and the water won't heat up as fast.

 

[Valkyrie Rider]

10guage is good to 30A depending on the length of run. I believe ampacity chart in the code book is 316. something. spells it all out.

Try this link:
http://www.stealth316.com/2-wire-resistance.htm

 

[RacerX]

10guage is good to 30A depending on the length of run. I believe ampacity chart in the code book is 316. something. spells it all out.

Try this link:
http://www.stealth316.com/2-wire-resistance.htm

NEC Table 310.16 to be exact. Actually #10 AWG conductors (THHN) are rated for 40 amps. Due to the limitations of 240.4(D) you would normally use them as a 30 amp conductor.

 

[kawimudslinger]

usually 15amp is 14g, 20amp is 12g and 30amp is 10g

 

[greengoblin]

but but but, is it solid or stranded wire, lol

 

[RacerX]

usually 15amp is 14g, 20amp is 12g and 30amp is 10g

To keep it simple we can say that's true. But there are applications when doing wiring in your house where #14 AWG is good for 20 amps and #12 AWG is good for 25 amps. AC units is one application.

but but but, is it solid or stranded wire, lol

LOL! Like it matters.

 

[mtfallsmikey]

To keep it simple we can say that's true. But there are applications when doing wiring in your house where #14 AWG is good for 20 amps and #12 AWG is good for 25 amps. AC units is one application.




LOL! Like it matters.

Well, if you want some electrical fires, it does matter....where in the NEC does it say you can use #14 on a 20 amp circuit?

As far as the HWH goes, #10 with a 30 amp breaker works. A 4500 wt. heater element @ 240v. draws roughly 18 amps running.

 

[RacerX]

Well, if you want some electrical fires, it does matter....where in the NEC does it say you can use #14 on a 20 amp circuit?

As far as the HWH goes, #10 with a 30 amp breaker works. A 4500 wt. heater element @ 240v. draws roughly 18 amps running.

Look at Table 310.16:

60° C column:

#14-20 amps
#12-25 amps
#10-30 amps

75° C column:

#14-20 amps
#12-25 amps
#10-35 amps

The typical limits of #14-15 amps, #12-20amps and #10-30 amps are due to the limitations 240.4(D). Those limits do not apply to certain loads, such as AC units.

As an example the AC unit on the side of your house could have a minimum circuit ampacity of 20 amps and can be wired with #14 conductors. To take it a step further it could be protected by a circuit breaker greater than 20 amps depending on what the Maximum Over-Current Protection rating is of the unit.

Here's the 2008 NEC text:

(D) Small Conductors. Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed that required by (D)(1) through (D)(7) after any correction factors for ambient temperature and number of conductors have been applied.
(1) 18 AWG Copper. 7 amperes, provided all the following conditions are met:
(1) Continuous loads do not exceed 5.6 amperes.
(2) Overcurrent protection is provided by one of the following:
a. Branch-circuit-rated circuit breakers listed and marked for use with 18 AWG copper wire
b. Branch-circuit-rated fuses listed and marked for use with 18 AWG copper wire
c. Class CC, Class J, or Class T fuses
(2) 16 AWG Copper. 10 amperes, provided all the following conditions are met:
(1) Continuous loads do not exceed 8 amperes.
(2) Overcurrent protection is provided by one of the following:
a. Branch-circuit-rated circuit breakers listed and marked for use with 16 AWG copper wire
b. Branch-circuit-rated fuses listed and marked for use with 16 AWG copper wire
c. Class CC, Class J, or Class T fuses
(3) 14 AWG Copper. 15 amperes
(4) 12 AWG Aluminum and Copper-Clad Aluminum. 15 amperes
(5) 12 AWG Copper. 20 amperes
(6) 10 AWG Aluminum and Copper-Clad Aluminum. 25 amperes
(7) 10 AWG Copper. 30 amperes