480v 3 phase water heater calculation

[brother]

Im having a discussion with some fellow people about a 3 phase 480 v water heater. they say the specs 12.5 amps. 12,000 watt 480v . They were going to use #12 copper with 15 amp fuses.

I told them that water heaters branch circuit rating have to be calculated at 125% of the load, so at a minimum it would be 12.5 x 1.25=15.6 so a 20amp fuse would be needed if what they told me was correct. which seems odd for a 12,000 watt 480 water heater

I had more concerns when I finally saw the paper work , that says,
'Simultaneous wiring 4 wire 3 phase operation Full load current in amperes (terminals L3/Terminals L1 and L2) '21.7/12.5 element wattage 6000/6000

If I am understanding this correctly, these heating elements (480volt 2 hots feed) will be on at the SAME TIME and the total name plate? amps is 21.7. So it would be 12,000 watt divided by 480=25amps So calculated branch circuit rating wold be 25 X1.25= 31.25 amps So a 40amp branch circuit would be required if you did just the math, but if you use the 21.7X 1.25=27.125 amps so a 30amp branch circuit required. I would not go below 30amps regardless. So the fuses should be 30 at a minimum #10 copper. Ideally I would run a 40amp branch circuit with #8copper.

I told them after seeing the paper work that the minimum would be 30amps with #10 if they go off of the name plate of 21.7. That 12.5 amps is just for 1 heating element draw when you replace the heating elements.

Am I correct in this? they keep saying its only 12.5amps and they will do a 20amp circuit. I do not believe this right.

 

[winnie]

It sounds to me like there are two 6000W 480V elements wired 'open delta'.

2 terminals see 12.5A, the common terminal sees 21.7A. This would be wired as a 3 phase circuit.

If you could get suitable unbalanced OCPD you might be able to wire as a 30/20/20A circuit, but the gods of KISS would smack you.

You might have the option of wiring as a single phase circuit where you only have 2 terminals eac seeing 25A

Jon

 

[augie47]

It sounds as if it is configured so that you can wire it to 480 single phase or 3 phase (a lot of heat type appliances allow this) and the currents would be as the paperwork shows (the actual wattage me not be exactly 6000 but they use 'nominal' numbers)

 

[brother]

Thanks for the responses. after more research, it appears that #10copper 30 amp circuit is the best way to go and code compliant way.

I almost forgot, they wanted to use FUSES, but this would not be allowed according to 240.15(B). This must have a common trip 3 pole breaker not fuses nor handle ties as this is a 3 phase 480 volt (not 240v) water heater. Since 1 leg can blow a fuse and the others keep working, I can see the hazard.

Not sure why they would allow this in a 120/240 system, such as a residential water heater with a 240v circuit feed.

 

[augie47]

FYI: Others may, but I don't see where 240.15(B) disallows fuses.

 

[infinity]

A photo of the nameplate or make and model number be would be helpful.

 

[brother]

FYI: Others may, but I don't see where 240.15(B) disallows fuses.

I think I made a mistake with that one. As a practice we have used a common trip for conductors to the same equipment/appliance to ensure that all power was disconnected automatically in a fault occurrence, apparently this was only required in a breaker use, not based on the same conductors to equipment in 277/480 systems.

I have seen where 3 fuses were used and one fuse had blown but the others were still sending power the equipment that caused alot of damage on single phasing.