help with figuring wire size for tankless water heater

[Tim Fitzgerald]

wanting to install a rheem rte 13..., the specs call for 6 ga wire with 60 amp breaker (240)...
For some reason I bought a 50 ft roll of 8 ga wire thinking that would be sufficient. The extension whip on the product is 8 ga....

I can take the wire back and get 6 ga but that seems excessive to me plus more $$..., with that being said could someone pls explain the math of how they get to the 6 ga wire. I am learning and like to do things safely but at he same time I need to know how the answer was derived...? In my book it states that even 6 ga wire would not be big enough because it is rated for only 10,560 watts or 55 amps and even then that is only supposed to be 80% of that max which would be 8448 watts continuous...., just all confusing..., someone help please...

 

[alfredeneuman]

Use the #6 wire it calls for in the installation instructions.

#6 can be used the 60Amp breaker. 55Amp is the rating at 60°C, 65Amp at 75°C.
If it's Romex, which only is rated at 60°. you're allowed to use the next standard up breaker.
A 55Amp breaker doesn't exist, and 60 is the next size up.
If it's wire in conduit, it's the 75° rating

Not using it will void the warranty.
Too, all electrical products are to be used in accordance with the instructions by Code.

 

[FordsnFishin]

Depends on the type of wire, and if its being free aired or in a raceway.

 

[FordsnFishin]

Looking at my uglys book, it says #6 tthn in a raceway with not more than 3 current carrying conductors is good for 75 amps. #8 only being good for 55 amps

 

[Charles (in GA)]

I'm not sure how Rheem gets that one by the code. Code limits heating circuits (air,water,boilers,etc) to 60 amp max. That is why many heaters have multiple circuits supplying them. Then code says heating devices are continuous and as such cannot exceed 80% of circuit capacity, or a never exceed limit of 48 amps. This heater draws 54 amps per the instructions.

 

[BFBOB]

I'm not sure how Rheem gets that one by the code. Code limits heating circuits (air,water,boilers,etc) to 60 amp max. That is why many heaters have multiple circuits supplying them. Then code says heating devices are continuous and as such cannot exceed 80% of circuit capacity, or a never exceed limit of 48 amps. This heater draws 54 amps per the instructions.

It's a tankless heater - works on demand, not continuously?
My guess... normal water heaters work on a themostat, so aren't really continuous either, but will be on for fairly long periods. Maybe the tankless works for enough shorter intervals it isn't called continuous>

 

[larry_g]

It's a tankless heater - works on demand, not continuously?
My guess... normal water heaters work on a themostat, so aren't really continuous either, but will be on for fairly long periods. Maybe the tankless works for enough shorter intervals it isn't called continuous>

If you have a line break downline of the heater then you are in continuous mode. The code considers all cases out side normal, they just have to be possible. For me I use a water heater for hydronic floor heat. It can run continuously.

lg
no neat sig line

 

[Tim Fitzgerald]

Thanks for the help guys..., I will take the 8 ga back and get the 6. This will not be continuous demand and will only be used a few times per year but when it used I wanted it to be able to provide more than the 3 gpm per minute than he next step down that I could have used the 6 ga with, but I see the continuous load possibility with the line break as mentioned here as well.