I am looking at a job that involves replacing a couple of roof top units. The existing wiring is #1 THHN running from the panel to each unit protected by 150A breakers.The new equip will be(per manf.cut sheet) minimum circuit ampacity 120A, maximum OCP 150A. Can the 90degree rating of the #1(150A) be used in this application or do I need to pull in 1/0 conductors to achieve the 150A @75 degrees? Thanks.
Conductor ampacity
- Thread starter nizak
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- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
#1 is good for 130 amps at 75C providing all terminations are at least 75C. Thus you should be good and you can still use 150 OCPD on #1 in this case. YOu don't need to achieve 150 just 120 amps with the conductor.
Are you saying that you can use the 150OCPD with the 130A #1 because 150A is the "Next size Up" from the ampacity of this wire?
- Location
- Connecticut
- Occupation
- Engineer
Dennis is saying that you can use #1 (ampacity 130) because the units needs a conductor with an ampacity of at least 120 (MCA.)
And you can use an OCPD rated at 150A because the feeder can be protected by an OCPD of 150A or smaller. (MOCP)
"Next size up" has nothing to do with it.
Yes but doesn't the #1 feeder have to be protected by something that will not exceed its maximum ampacity of 150A?
- Location
- Connecticut
- Occupation
- Engineer
I'm not sure I follow what you are asking.
If the unit had an MCA of 115, and an MOCP of 150, then you could feed it with #2AWG and protect it with a 150A C/B.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
No, the unit has overload protection that will protect the wire from being used above it's ampacity. The OCPD is only for short circuit and ground fault protection.
O.K. that makes sense. So basically the A/C manufacturer has done all of the leg work for you and is basically telling you what to use.
In general though for a feeder circuit lets say would a #1 conductor be able to be protected by a 150A breaker. Lets say in this case the calculated load is 130A.
Next Generation
Member
- Location
- Birmingham Alabama
They do not make a OCPD that is 130a The next size OCPD is a 150a. Only after 601a do you need to size your wire to your OCPD rating. anything below that you can round up.
- Location
- Massachusetts
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
In this case the calculated load of 130 is fine with #1 however if you go any higher then the next relevant size would apply. So, if the load was 135 amps then you need a 1/0 and you would put it on a 150 amp breaker. Now if the ac unit says we can use a 175 amp breaker then we can and still use 1/0.-- we are talking a/c units here...
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Yes, that is what I said in post #7
dana1028
Senior Member
- Location
- San Francisco Bay area
Rooftop temperatures
Rooftop temperatures
Doesn't conductor derating for heat come into play for this rooftop installation?
Rooftop temperatures
Doesn't conductor derating for heat come into play for this rooftop installation?
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Follow-up: A lot of RTU circuits are routed under the deck and enter the equipment directly from below. As such, rooftop derating is not applicable.It certainly can..
Also, where the run is exposed on the rooftop, we can resort to 310.15(A)(2) Exception if less than 10' or 10% of the run length, whichever is shorter, is all that is exposed.
dana1028
Senior Member
- Location
- San Francisco Bay area
Yes, "IF".
The OP made no mention of his raceway location and typically you guys jump on all the various possible scenarios and ask the OP for details. This time everyone jumped in with answers without asking about the raceway location, so I brought it up, as it could have a critical bearing on the answer.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Well, perhaps we assumed everything else was in order but without the OP giving the info we did our best. I have no idea whether or not the op is going thru a room that has a very high ambient temp or whether there are 20 ccc in the raceway, etc.
You can't expect us to answer with every scenario. But you are correct with a very likely scenario and I believe Smart gave a good answer for that. Just because it is run near the roof does not necessarily mean there are derating issues.
You can't expect us to answer with every scenario.
But you are correct with a very likely scenario and I believe Smart gave a good answer for that. Just because it is run near the roof does not necessarily mean there are derating issues.ricielectric
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thank you uuuuuuuuuuu
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