powering 2 condensing units with 1 feeder

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ksrm12

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There is an existing AC Condensing unit currently served by #4 copper conductors and a 70A,3p circuit breaker. This unit is being replaced by two smaller units each rated for (208V,3ph, MCA: 33).

I would like to reuse the existing feeder (#4 conductors) to serve both new condensing units by tapping it with two disconnect switches.

From what I understand, this is code compliant and Article 440 has to be followed to make sure the wire size is large enough, the breaker size and to size the disconnect switches.

I was a little confused while reading through article 440. Please verify my calculations below to size the wire, breaker and disconnect switches. Any comments/guidance would be much appreciated.

Since MCA is given I believe I can multiply by 80% to get the full load amps of each unit. 33X0.8=26.4A.

Feeder conductor size: 26.4X1.25 + 26.4 = 59.4. The #4 conductors are rated for 85A so it can be used.

Circuit breaker size: 26.4 x 1.75 = 46.2, round down to 45A OCPD
45A+26.4A=71.4A, round down to a 70A breaker. So the existing breaker can serve both units.

Disconnect for each unit: 26.4X1.15=30.36, next higher size is 40A, so 60A fused disconnect switch with 40A fuses.

MCA is 33A, so #10 conductors would be used to tap the feeder in order to serve each condensing unit.
 
You have the idea You must use the name plate data. Install a 70 amp main subpanel and two 40amp brakers to supply the units if the data works out. you'll need #8 to cover the 33 MCA
 
Thanks Cavie.

In this case, the wire I want to reuse is already there at the location of the condensing units. Can't I just keep the existing 70A breaker in the source panel (inside the building) and the one existing feeder (#4 conductors) and just tap it with #8 wire and a 60A Disconnect Switch fused at 40A for each condensing unit?

Or do I have to have a 70A subpanel at the roof with two (2) 40A breaker to serve the units?

Thank you very much for your help.
 
ksrm12 said:
Thanks Cavie.

In this case, the wire I want to reuse is already there at the location of the condensing units. Can't I just keep the existing 70A breaker in the source panel (inside the building) and the one existing feeder (#4 conductors) and just tap it with #8 wire and a 60A Disconnect Switch fused at 40A for each condensing unit?

Or do I have to have a 70A subpanel at the roof with two (2) 40A breaker to serve the units?

Thank you very much for your help.
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I think you can do what you stated by using art. 240.21(B)(2). I think what you have there fits #2
 
ksrm12 said:
Since MCA is given I believe I can multiply by 80% to get the full load amps of each unit. 33X0.8=26.4A.
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Thus is only true if the unit has only one motor. A typical condensing unit had a compressor plus an outdoor fan motor. For a typical small condensing unit the MCA is calculated as 125% of the compressor RLA plus 100% of the outdoor fan FLA. The actual calculated running load will be a bit higher than what you get by just dividing MCA by 125%. In this case the difference is probably small enough that it does not change the conductor or OCPD requirements, but that will not be true in all cases.
 
david luchini said:
Dennis, I'm not sure I follow this. I don't see anything in 240.4(G) that allows you to use the 75 deg column. Am I missing something.
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Assuming you are using conductors rated 75C or greater and the terminals are rated 75C then T.310.16 allows #12 to be used at 25 amps for certain situations. Check out the asterisk in T.310.16 for #14- #10 wire then go to the bottom where the asterisk directs us to 240.4(D). Art. 240.4(D) states that unless specifically permitted in 240.4(E) or (G) the OCPD.

240.4(G) mentions a/c units. Basically, IMO, this section gives permission to ignore the limitations set by 240.4(D1-D7).
 
Dennis Alwon said:
Assuming you are using conductors rated 75C or greater and the terminals are rated 75C then T.310.16 allows #12 to be used at 25 amps for certain situations. Check out the asterisk in T.310.16 for #14- #10 wire then go to the bottom where the asterisk directs us to 240.4(D). Art. 240.4(D) states that unless specifically permitted in 240.4(E) or (G) the OCPD.

240.4(G) mentions a/c units. Basically, IMO, this section gives permission to ignore the limitations set by 240.4(D1-D7).
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I see what you're saying. But you're assuming the terminals are rated 75C. If they're not, you'd have to use #8 for the 33MCA.
 
ksrm12 said:
There is an existing AC Condensing unit currently served by #4 copper conductors and a 70A,3p circuit breaker. This unit is being replaced by two smaller units each rated for (208V,3ph, MCA: 33).

I would like to reuse the existing feeder (#4 conductors) to serve both new condensing units by tapping it with two disconnect switches.

From what I understand, this is code compliant and Article 440 has to be followed to make sure the wire size is large enough, the breaker size and to size the disconnect switches.

.
Click to expand...

Sounds like you have the generic theory down pretty well, as some others have pointed out, maybe a slight error in the calculation, and depending on the temperature ratings of the equipment and terminations, will give you a precise answer.

The only thing I am curious about is if BOTH replacement units are going back into the same place as the old one was, or is one moving ??? I think the definition of "WITHIN SITE OF" has changed over the years and in the 05 cycle of the code is now worded to say something to the effect of "WITHIN 25 feet"). (this could also just be for some of the local jurisdictions I work in... I honestly can't say for sure).

the sub panel on the roof with 2 smaller breakers will suffice for OCP for the install, but if the 2nd unit moves in excess of the limitation, you will still be required to have a separate d/c (service switch) near or on the unit too far away to satisfy that portion of the code.

also, don't forget you will need to have a GFCI within a certain distance of the units as well.... the handy sub panel MIGHT be good for that too if one does not exist now to serve both units.
 
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david luchini said:
Of course, but the OP talks about reusing a 70A breaker. That breaker could be 50 years old for all we know, and may only be rated for 60 deg terms. I'm just cautioning against assuming that the 75 deg rating would be OK.
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I understand but I did caution. My post specifically says "assuming".
 
david luchini said:
Sure, but 110.14(C)(1) doesn't allow you to assume the terminals are rated for 75 deg.
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This is getting silly. We both understand the situation and I suppose the OP does also.

So Cavie is correct, unless you KNOW the terms are rated for 75 deg
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Well I can argue that Cavie isn't correct since he doesn't know if the terminals are 75C either.

This is not about being right or wrong it is about giving another possibility to the situation. You can not say that #10 is the wire no more than I can say #12 is correct-- but I did say if the terminals were 75C-- that's it. I am not assuming they are in the sense that I would not check. :) Assuming as in "If --- then". Need I go on-- I don't think so- I said my piece too many times. :D
 
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