1. Member
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Originally Posted by JWB
ugly hackery. Takes the professionalism right out of what we do, surprised he didn't have a 7.5 (I have at least 4) but did have a 3mfd.....I may have 1 or 2......anyway, what is embarrassing is opening up a panel like that and the homeowner asks you if that mounting was 'correct'. My answer is no. I have strap and drill tips in my toolbag and will secure those even on a maintenance. I want them to know that I am there to make sure their system is ok. I work for them, ultimately. Nice picture.
So, you have four 7.5's on your truck at the beginning of the work day. By 3:00 you used all 4 of them, and on your last call, you need another one. Do you travel 60 miles to get one and return to put it in? And the customer is ok with paying an additional \$150 to \$200 for the same result? And you're ok charging the additional \$150 to \$200 even though paralleling 2 is acceptable?

2. Originally Posted by jodybhvac
ok, now I know it is ok to wire caps like that. Would like to see the formula for series and parralel wiring.
Wired in parallel, the value of the capacitors adds, so a 10µf + 10µf in parallel is 20µf.
This is how the Turbo capacitors from AmRad work.

Wired in series, the a reciprocal formula is used.
For 2 capacitors in series the easy formula is: Total µf= (µf1 x µf2)/(µf1 + µf2)
Two 10µf capacitors in series would be 5µf.
(10x10)/(10+10)
100/20 = 5

For more than 2 capacitors in series, you would need a different formula.
(1/µf1)+(1/µf2)+(1/µf3)=1/µfT
µf = 1/µfT

Three 10µf capacitors in series would be 3.33µf.
(1/10)+(1/10)+(1/10)=1/µfT
.1 + .1 + .1 = 1/.3
µf = 3.33

Typically there is not much, if any, use for capacitors in series in our industry.

3. Professional Member
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Mar 2011
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Thanks !

4. One thing does concern me about the 2 single caps in parallel: Are they 370V or 440V?

With a 120V blower motor, the working voltage (measure it while the motor is running) will not exceed 370... With a 240V fan motor, the working voltage may well exceed 370V, which means the caps may fail sooner.

Now if those single caps are 440V, well disregard what I just said...

5. Originally Posted by prorefco
So, you have four 7.5's on your truck at the beginning of the work day. By 3:00 you used all 4 of them, and on your last call, you need another one. Do you travel 60 miles to get one and return to put it in? And the customer is ok with paying an additional \$150 to \$200 for the same result? And you're ok charging the additional \$150 to \$200 even though paralleling 2 is acceptable?
No, I would wire something together. My point was that the install is really unprofessional and just plain disappointing. I observe a lot of 5mf's on rheem/ruud/weatherking units in place of the manufacturer's recommended 3 mf. Which leads me to assume that guys would rather throw a 35+5 in cause they have it handy then wire together a 35 and a 3. Mostly I don't see a lot of 3's around so I assume they don't bother stocking them.

And also, if my truck is out of stock, that is my problem, the customer at my company doesn't pay EXTRA for something I failed to stock. We eat the additional cost.

6. Professional Member
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Feb 2009
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If you have ever worked on old equipment manufacturers frequently used multiple capacitors. Of course they mounted them properly. As a refrigeration contractor I get the opportunity to replace capacitors frequently rarely are replacements the same physical size. Life is fun.

7. Turbo 2000 basically does the same thing... multiple caps in a single shell. However, professional mounting is a must. I too have joined capacitors in a pinch, but always mount them securely to the metal wall.

8. Originally Posted by mark beiser
For more than 2 capacitors in series, you would need a different formula.
(1/µf1)+(1/µf2)+(1/µf3)=1/µfT
µf = 1/µfT

Three 10µf capacitors in series would be 3.33µf.
(1/10)+(1/10)+(1/10)=1/µfT
.1 + .1 + .1 = 1/.3
µf = 3.33

Typically there is not much, if any, use for capacitors in series in our industry.
Actually Mark, you can still use the (C1XC2)/(C1+C2) for more than 2 in series. (10X10)/(10+10)=100/20=5 5 is the combination of cap 1 and cap 2. Now use the value of C1 and C2 (5) times cap 3 and do it again. (5X10)/(5+10)=50/15=3.33 Just makes for one less mathematical problem to remember.

As long as you continue using the calculated value for the next operation, you can keep going.

9. A supply house in my area closed last fall, bought a HUGE amount of parts for dimes on the \$\$\$... lots of caps cheap. For this year and maybe next; just get more from stock in my shop.

10. the duct tape is hackery.

wiring caps up to achieve what is needed.... been there, done that.

ther've been times when on call, i've put two 20's together to make a ten and vice versa.

11. Originally Posted by doc havoc
Actually Mark, you can still use the (C1XC2)/(C1+C2) for more than 2 in series. (10X10)/(10+10)=100/20=5 5 is the combination of cap 1 and cap 2. Now use the value of C1 and C2 (5) times cap 3 and do it again. (5X10)/(5+10)=50/15=3.33 Just makes for one less mathematical problem to remember.

As long as you continue using the calculated value for the next operation, you can keep going.
Interesting, I too was taught the reciprocal method for adding capacitance in series/resistance in parallel in my electronics and house wiring course in high school. This is an alternative I hadn't heard of before. Thank you.

12. Originally Posted by doc havoc
Actually Mark, you can still use the (C1XC2)/(C1+C2) for more than 2 in series. (10X10)/(10+10)=100/20=5 5 is the combination of cap 1 and cap 2. Now use the value of C1 and C2 (5) times cap 3 and do it again. (5X10)/(5+10)=50/15=3.33 Just makes for one less mathematical problem to remember.

As long as you continue using the calculated value for the next operation, you can keep going.
That works, but if/when I need to calculate something like that, I just use a scientific calculator and enter it all as one equation.

13. Originally Posted by mark beiser
That works, but if/when I need to calculate something like that, I just use a scientific calculator and enter it all as one equation.
I hear ya mark. I just like the idea of only having to remember 1 mathematic formula. Plus, it would take me a while to relearn a scientific calculator.

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