I removed a old roof top unit 208v 3p on a 400a breaker. I will be using that existing breaker and the 3in EMT feeding up to the roof. It will now be feeding two RTU's with MCA's of 122a each and MOCP of 150a. I will split the wire at the two fused disconnects next to each other. Can i size the wire for 244a or must it be sized for 400a? john
wire size
- Thread starter jociha
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Re: wire size
my opinion (based on 430.62 & 440.22b). A 244 amp conductor would be o.k. if the breaker was sized at 300. But if you keep the 400 amp breaker the conductors would need to be rated above 350. I feel we will have amost interesting discussion here.
my opinion (based on 430.62 & 440.22b). A 244 amp conductor would be o.k. if the breaker was sized at 300. But if you keep the 400 amp breaker the conductors would need to be rated above 350. I feel we will have amost interesting discussion here.
- Location
- New Jersey
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- Journeyman Electrician
Re: wire size
The original feeder can remain on the 400 amp CB if it's conductors are rated for 400 amps. Or the 400 amp CB will need to be resized to protect the feeder if the conductors are sized for less than 400 amps. This would be providing that your fuse protection at the unit disconnects doesn't exceed 150 amps and your taps from the original feeder are 25' or less.
The minimum circuit ampacity already has an increase in size by 125% so you wouldn't need to increase the minimum size.
The original feeder can remain on the 400 amp CB if it's conductors are rated for 400 amps. Or the 400 amp CB will need to be resized to protect the feeder if the conductors are sized for less than 400 amps. This would be providing that your fuse protection at the unit disconnects doesn't exceed 150 amps and your taps from the original feeder are 25' or less.
The minimum circuit ampacity already has an increase in size by 125% so you wouldn't need to increase the minimum size.
Re: wire size
Jociha,
Using the same code articles as 'augie47' I read that you can wire for 244a and use 400c.b.
That aside,have you thought about bringing both 122a mca conductors thru the 3" pipe?? It may cut down on potential conflicts if wired as manafacturer has on name plate.
Jociha,
Using the same code articles as 'augie47' I read that you can wire for 244a and use 400c.b.
That aside,have you thought about bringing both 122a mca conductors thru the 3" pipe?? It may cut down on potential conflicts if wired as manafacturer has on name plate.
Re: wire size
Frank,
If the two nameplates call for 150 amp MOCP (sum of 300A) and the conductors are rated 244. how can we justify going above a 300 amp breaker?
No matter which calculation we use 440.22C says we can't exceed the mfg rating of 150..two 150's =300, a standard rating.
Frank,
If the two nameplates call for 150 amp MOCP (sum of 300A) and the conductors are rated 244. how can we justify going above a 300 amp breaker?
No matter which calculation we use 440.22C says we can't exceed the mfg rating of 150..two 150's =300, a standard rating.
allenwayne
Senior Member
Re: wire size
These are a/c units.What matters is what the new units data plate says.Min. ampacity tells us the wire sizing needsto be ( min .),our ocpd shall be what the unit has listed as max size throught
These are a/c units.What matters is what the new units data plate says.Min. ampacity tells us the wire sizing needsto be ( min .),our ocpd shall be what the unit has listed as max size throught
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