What size OCP are the rest of you putting on a water heater in a dewelling that has a total connected load of 3500w or 3380w @ 208?
OCP on Water Heater
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william1978
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You don't really have to jump right to the 150% maximum rating for sizing your OCP when you only need to go to 125% for a single continuous load.
This could possibly lead to a savings of some material costs on multiple installs, such as, through lower ratings for certain types of fuses for example.
Not necessarily in this case, but still good to keep in mind for future bids.
3380w/208vX125% =20.31A. Next standard size 25A.
3500w/208vX125% =21.03A. Next standard size 25A
This allows for a safe, compliant installation that is well below the 150% maximum OCP rating allowed in 422.11[E].
BTW, I have lost count of how many times I have seen electric water heaters wired with #12 connected to a 25A or even a 30A CB. Oy!
Problem I'm having is seeing a HW tank with only 3500 watts and assumed 220V since it also has a rating for 208V. That's about 16 amps and is within the range of a 20 amp breaker on #12 wire.
The HW tanks I'm most familiar with are 4500 watts at 220V, or just over 20 amps.
Do you install a disconnect at the HW tank where you are?
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You can if you'd like to but it's not required. If the panel is not within sight and 25ft you can install a breaker lock at the panel.
I believe the OP is referring to a standard 4500 watt water heater.
At 240 volts, 4500 watts = 18.75 amps. X 1.25=23.4 amps. In this case, I use #10's and a 25 amp breaker.
The same unit operated at 208 volts will consume 3380 watts, and draw 16.25 amps. 16.25 X 1.25 = 20.3. According to 220.5 (B), you're allowed to drop the fraction if it's less than 0.5 In this case I use a 20 amp breaker and #12's.
I was called on this once by a highly respected inspector, and had to provide code references, but I think he already knew the answer, and was just testing me.
Around here, no disconnect is required for anything except an outdoor HVAC unit on residential. On commercial/industrial, you'll need a disconnect if you can't see the panel from the water heater. I usually use the 30 amp pull-out A/C disconnects.
Rob
At 240 volts, 4500 watts = 18.75 amps. X 1.25=23.4 amps. In this case, I use #10's and a 25 amp breaker.
The same unit operated at 208 volts will consume 3380 watts, and draw 16.25 amps. 16.25 X 1.25 = 20.3. According to 220.5 (B), you're allowed to drop the fraction if it's less than 0.5 In this case I use a 20 amp breaker and #12's.
I was called on this once by a highly respected inspector, and had to provide code references, but I think he already knew the answer, and was just testing me.
Around here, no disconnect is required for anything except an outdoor HVAC unit on residential. On commercial/industrial, you'll need a disconnect if you can't see the panel from the water heater. I usually use the 30 amp pull-out A/C disconnects.
Rob
Unless it's within sight of the panel, I will normally put in a 4SQ box with a 30A/DP toggle switch and RS cover. It also makes it easier for me later as I sometimes get calls to service the units.
A lot of guys like to use a 30A non-fused AC disconnect as it is sometimes less expensive than a 30A/DP switch.
Micro, I don’t see a problem with how you’re doing it, especially since in this case that .3A is virtually nothing, but as usual, the code seems less than homogenous about this.
Some other less congenial inspector may call you on it again someday, as it could be argued that 422 replaces 220 for appliance branch circuits and the text of 220.5 that allows the dropping of less than .5A for branch circuit and feeder sizing is not mentioned for sizing appliance branch circuits in 422.10.
Also, since according to 422.11[e], the OCP is sized from the rating of the appliance, not the branch circuit, again that .3A would have to remain for minimum sizing of the OCP.
Since you would probably want to go to the next standard size up as allowed by 240.4, you would most likely end up with #10 and 25A OCP for the 208V water heater.
Anyway, keep your sword at the ready just in case.
Interesting. I don't use disconnects at HW heaters, but I sort of figure my circuits by using the ampacity of a 30 amp circuit, then build it. Heck, I don't know that I've lately seen a 25 amp double pole breaker, but I sure have seen plenty of 30 amp DP breakers.
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They have nothing to fail you on as long as it is not 12 gauge on a 30 amp breaker.
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The OP said 3500w @ 208v....
That is correct. I don't see how the wattage can be different for the same 208 voltage.
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