question

[mrssb]

I have a question for somebody out there. If you have a 72kw electric heat strip package on a 460v three phase service what is your max fuse size, and the next question is how do you figure it. I have been given several formulas, and all have come up with different results. Thanks for your help guys...Mark

[wolfdog]

It was answered in the Pro section.

You want a formula or more code references or what?

[mrssb]

Wolfdog a formula would be helpful. I just now read your reply in the pro forum, how did you arrive at a 100 amp fuse. I have come up with a 125 max fuse size as the mac is rated at 122 by my calculations. thanks for your help.

[mrssb]

fuse is sized times 125%, and you would not divide by three....i think.

[klrogers]

For 3 phase

Amps = Watts /(volts * 1.732)

so
amps = 72000 / (460 * 1.732)

= 90.37 amps


The answer to fuse size depends on application, within unit, baseboard heat etc.


See wolfdog's answer here http://www.hvac-talk.com/vbb/showthread.php?threadid=74233

[Edited by klrogers on 04-12-2005 at 07:33 PM]

[wolfdog]

NEC article 424.22 would allow the over current to be not less than 100% of the load ,if over 50 KW and cerain conditions are met.

article 240.4 permits increasing to the next standard size overcurrent device.

That's why 90 amp was increased to 100 amp.


.....For the purposes of the example given.......


Smac - Perhaps more clearly stated would be "the load is fed from three phases".

[Edited by wolfdog on 04-12-2005 at 07:58 PM]

[mrssb]

Wolfdog....Thanks for all the help, but one further question.I found some more information from Trane, they write in their literature that a 72 Kw heat strip package @ 460 3 phase requires a max fuse size of 125Amps. Can you explain somehow the descrepancy. I arrive at the same numbers as everyone else, this is why i posted the question in the first place. Again thanks for the info.

[wolfdog]

Some parts of article 424.22 specify that the overcurrent "shall be sized at not less that 125%"

and another section gives the qualifier of "not less than 100%" that was mentioned above.

If you choose not to take the 100% rating mark down;



90.37 amps x 125% =112.96 amps

The next standard size overcurrent is 125 amps.

This is only a guess as to their thinking.

[key]

If you are looking for a text book formula it is as follows:

Amps=Watts/(volts x 1.73 x power factor)

[wolfdog]

Power factor has no relevance in a resistance load.

[key]

Wolfdog, thanks for clearing that up, I do however have a question you might be able to help me with though.

A 220 volt, 12 amp heater will draw how many amps at 460 volts?

[wolfdog]

I hope this is just a theoretical question.

220 x 12 = 2640 watts

2640 \ 460 = 5.74 amps

[key]

thanks Wolfdog

[klrogers]

Originally posted by wolfdog
I hope this is just a theoretical question.

220 x 12 = 2640 watts

2640 \ 460 = 5.74 amps



While I believe that you have good electrical knowledge based on your answers on this forum I respectfully disagree with this answer.

220v divided by 12amp equals a resistance of 18.3 Ohms

460v divided by 18.3 Ohms = 25.1 Amps or 11.5KW

[wolfdog]

Originally posted by klrogers
I respectfully disagree with this answer.




That answer doesn't deserve to be respectfully disagreed with.
It was a stupid answer, made in haste.

My apologies to Key.

Kudos to you for correcting the blunder.

[key]

thanks klrodgers. I looked at your formula and it is basic ohm's law

Howevever would wolfdog's formula work if the question read: 220 volt heater is drawing 12 amps, how many amps will it draw at 460 volts?

[Edited by key on 04-14-2005 at 07:47 AM]

[wolfdog]

The problem with my hasty reply was that watts are a result of amps X volts.

The watts ouput changes proprtional to the voltage applied.

In the example:

220 x 12 = 2640 watts

2640 \ 460 = 5.74 amps


The watts is a legitamate figure because P = VA.

The second equation is bogus. It is substition into an invalid equation, to get an answer.

We needed to know the resistance to calculate the amps.

[hvacker]

Three phase electric heaters can be hooked up eather delta or Y . The amp draw will be entirely different and so will the heat output.