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Old 03-27-2016, 08:09 PM   #1
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Default Sizing Conductors for machine feeder.

We are getting ready to install a feeder for a machine in a factory. In house electrician designed the install and I'm not sure if I agree with him. Just wanted to get someone else's thoughts on conductor size before I try to more vigorously pursue it.

Here is the info:
Machine has a nameplate rating of 225A
Ambient temperature is 100 degrees F (highest reached during summer)
Only 3 current carrying conductors
Temp rating of the connectors is 75 degrees C
THHN Copper conductors
Square D I Line busway
480 volt 3φ
Distance from bus to machine is 30-40 feet (almost a straight drop)
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Old 03-27-2016, 08:11 PM   #2
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Hopefully I didn't forget anything.
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Old 03-27-2016, 08:21 PM   #3
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Is the machine motors or heat?
Honestly I would be OK with 4/0 copper but would probly run 250's if it is gonna run all day.
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Old 03-27-2016, 09:00 PM   #4
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It's a combo of motors and heaters. 4/0 is what I came up with in calculating it. And I was going to upsize to 250 as a compromise. He originally said 400. When I questioned it (before I sat down to do any calculations) he said it was because the terminations weren't marked so he was basing it on 60 degree C. But even if I calculate it for 60C I would still come up with 300.

I really respect the in house guy. I've known him for a long time and he has far more experience than me, but there are good reasons to challenge this if I know I can back it up. I'm looking to you guys to either set me straight or validate my point. Just wanna make sure I'm not missing something first.
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Old 03-27-2016, 09:04 PM   #5
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Also forgot to say that I questioned the 60C based on the wire termination size and the amperage rating of the machine and bus plug. Should be rated at least 75C if not marked.
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Old 03-27-2016, 09:06 PM   #6
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You would not use the nameplate rating but rather the sizing of the conductors would be based on the HP and Table 430.250
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Old 03-27-2016, 09:24 PM   #7
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Yeah, thought about that. What I'm basing it on is what the manufacturer who is installing it told us the feed should be fused and rated for (225A) and what the nameplate they showed me. Im just curious to what you guys would calculate the feeder conductor size for a 225A feed based on the info I gave you. Same info I have. They are supposed to have the machine on the ground ready to be fed tomorrow. I just want to make sure I'm not missing something. Just looking to learn something.
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Old 03-27-2016, 09:30 PM   #8
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Well I would not base my conductor on the highest temperature... This ambient temp. is a joke-- what is ambient temp???? It is the temp at any given moment, not an average and not the highest temp.
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Old 03-27-2016, 09:35 PM   #9
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4/0 is fine given the 100 degree ambient. Remember the temp rating of thhn is 90C so you can use that for de-rating purposes so a 225 amp overcurrent protective device would be ok with 4/0 copper
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Old 03-27-2016, 09:57 PM   #10
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Thanks Dennis, that's what I was thinking. I think he must be starting at the 60 degree column or just made a mistake. I wanted a different perspective just to make sure I wasn't making a mistake in my calculations. I will see him tomorrow and try to find out where he is coming from.
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Old 03-28-2016, 06:57 AM   #11
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Quote:
Originally Posted by nables30 View Post
When I questioned it (before I sat down to do any calculations) he said it was because the terminations weren't marked so he was basing it on 60 degree C.
Since the circuit is rated over 100A, you can use the 75C rating for unmarked terminations, 110.14(C)(1)(b)
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Old 03-28-2016, 10:45 PM   #12
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That's what I said. Today I talked with him and got it worked out. He was basing it on 60C terminations. I told him that if he wasn't comfortable with the Burndy lugs that were installed that I would install lugs with a known temp rating. He was satisfied with that.
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