TABLE 310.16 - AMPACITY OF CONDUCTORS & TEMP RATING OF CONDUCTOR

[cdcengineer]

In referring to table 310.16 (now 310.15(B)(16) - how dare the code council change this on us), I have always used the 75 C column when calculating the max ampacity of conductors (when over 100A). However, when it comes to dollars and cents we're asked to look as hard as possible for ways to save money while meeting the intent of the code.

I notice that 500MCM XHHW CU is good for 380A & 430A at 75C & 90 C respectively. I would normally run 1200A feeder in 4 sets of 350 CU.

NEC 240.4 requires that for MOCP devices over 800A, the conductor be rated at or above that of the overcurrent protection device.

The question - we are running feeders through a parking garage, in a dry environment (re: table 310.104(A)), and they want to run (3) sets of 500MCM CU. Anyone have any issues with this?

I have never used the 90 C column, although I know it's there for a reason.

As always, input is greatly appreciated.

 

[jumper]

Unless you have all 90C terminations ,devices, etc.. I think you are still limited to 75C, or in some cases 60C, except for derating purposes.

 

[kingpb]

The problem is that for the conductors to get to 1200A, the temperature is going to have to rise above 75degC, unless you live in a part of the country, or the installation is not going to cause the ambient temp to go above 30 degC.

The fact that the terminations are good for 1200A, since that is what the breaker is good for @75deg C means you can run 1200A through the breaker. You will thusly exceed the temp limit of the terminations.

If the breaker is adjustable trip, then you may be able to use 240.6 (C) and turn the breaker trip down to 1140A, otherwise you are going to need more cable.

You may want to investigate cost of 4 // 350KCMIL Al.

 

[cdcengineer]

KING - 4x350AL is only good for 1000A, 240.4(C) requires that the conductors be rated for a min of 1200A..

 

[Dennis Alwon]

How is 3 sets of 500 kcm giving you 1200 amps. What insulation are you using. At 75C 380 amps for 500 KCM.

 

[cdcengineer]

Normally I use the 75C column, but the question is can they use 90C column rating if XHHW installed in dry location. I think the termination rating will deflate this request as pointed out by others above.

 

[charlie b]

I think the termination rating will deflate this request as pointed out by others above.

Correct. To be specific, the request would be deflated by 110.14(C)(1)(b)(2).

 

[Dennis Alwon]

Normally I use the 75C column, but the question is can they use 90C column rating if XHHW installed in dry location. I think the termination rating will deflate this request as pointed out by others above.

You can use the 90C for de-rating purposes as long as the final ampacity is not more than the 75C rating.

 

[beanland]

75C @ Terminations, 90C in Wire

75C @ Terminations, 90C in Wire

One option you have is to separate the feeder cable from the equipment terminations. Use the 90C rating in the conduit but then land the 90C cable on 90C terminal blocks and run wire from the terminal blocks to the equipment using the 75C rating. For example, run 3 500 CU in the conduit, using the 90C rating, but then use power blocks to increase to 4 500CU to the equipment, using the 75C rating.

Also, consider using more smaller wires. Ampacity is not linear with wire size, smaller wire has more amps/$ than large wire. Sometimes parallel small wire is cheaper than single large wire.

 

[kingpb]

The problem is that for the conductors to get to 1200A, the temperature is going to have to rise above 75degC, unless you live in a part of the country, or the installation is not going to cause the ambient temp to go above 30 degC.

The fact that the terminations are good for 1200A, since that is what the breaker is good for @75deg C means you can run 1200A through the breaker. You will thusly exceed the temp limit of the terminations.

If the breaker is adjustable trip, then you may be able to use 240.6 (C) and turn the breaker trip down to 1140A, otherwise you are going to need more cable.

You may want to investigate cost of 4 // 350KCMIL Al.

Opps, I meant 4//500KCMIL Al:slaphead:

 

[cdcengineer]

One option you have is to separate the feeder cable from the equipment terminations. Use the 90C rating in the conduit but then land the 90C cable on 90C terminal blocks and run wire from the terminal blocks to the equipment using the 75C rating. For example, run 3 500 CU in the conduit, using the 90C rating, but then use power blocks to increase to 4 500CU to the equipment, using the 75C rating.

Also, consider using more smaller wires. Ampacity is not linear with wire size, smaller wire has more amps/$ than large wire. Sometimes parallel small wire is cheaper than single large wire.

Bean - this could be a good solution. We need to verify the termination ratings of the cutler feeder breakers. I'm betting they're 75 C, but we'll see. The issue is that the first 100' of the run was already installed in an earlier contract. They installed (3) sets of 500CU as was the design on the original one-line. Not sure if they intended 90C conductor or if it was a mistake?

Thx for the responses

 

[beanland]

90C & 75C

90C & 75C

This might be a good way. Since the three 500 are alread in place, use power terminal blocks and go to four 500 to land on breaker.

There is no standard in NEC but you should have 2-4 feet of wire between the breaker and power blocks. It makes sure the 90C temperature dissipates before getting to the breaker.

Polaris IPLD is rated 90C; use an 8-port. Penn-Union NA500-8T. Burndy Unitap.

 

[augie47]

If the load allows, the least expensive route might be a 1100 amp rating plug for the breaker or 1100 amp fuses if applicable.

 

[skeshesh]

Polaris IPLD is rated 90C; use an 8-port. Penn-Union NA500-8T. Burndy Unitap.

I've had positive outcomes using the polaris blocks. Most important is to determine the termination ratings. Speculation and guessing is a part of the process but make sure to emphasis investigation. I'd love to hear about the outcome as this is an issue I will likely deal with sooner or later.

 

[RUWired]

Even if you upsized the 3 runs of 500 kcmil to 3 runs of 600 kcmil using the splice blocks, you would be adding 60 more termination to the run, adding 2 splice j-boxes and all the hardware and fittings etc., this would be less costly than re conductoring the existing run to 600 kcmil?

Rick

 

[cdcengineer]

Even if you upsized the 3 runs of 500 kcmil to 3 runs of 600 kcmil using the splice blocks, you would be adding 60 more termination to the run, adding 2 splice j-boxes and all the hardware and fittings etc., this would be less costly than re conductoring the existing run to 600 kcmil?

Rick

Definitely cheaper to find a solution without re-pulling wire. The CU at today's prices would be $15K.. I will post an update after we meet onsite this week to discuss.

Thx

DC

 

[cdcengineer]

So update,

The contractor opted to install splice blocks and call the wire 90 degree for the majority of te run, changing to 75 degree at each end for terminating. Definitely was cheaper in their eyes.

Thx to all for the responses

 

[mull982]

Can someone please explain the physics of why 75 deg wire rating needs to be used on a 75 deg rated breaker terminal? Does it have to do with the fact that if a 90 deg wire rating was used it would result in a smaller wire which whould have a larger resistance and thus a larger I^2R heat dissipation at the terminal? Or something similar to this?