Dis-similar Paralleled Conductors

[Hacker71]

Have a customer who wants to add a 350 run of copper to an exisiting 400 A residential service which currently is fed with as single run (3 conductors) of 500 MCM copper. These runs would be paralleled from the electric company's handhole up to the line side of the meter can. Will he have a problem with one run "hogging" the load or will the dis-similar conductors work fine? Obviously there is less resistance in the smaller wire. Thanks

 

[marcb]

Have a customer who wants to add a 350 run of copper to an exisiting 400 A residential service which currently is fed with as single run (3 conductors) of 500 MCM copper. These runs would be paralleled from the electric company's handhole up to the line side of the meter can. Will he have a problem with one run "hogging" the load or will the dis-similar conductors work fine? Obviously there is less resistance in the smaller wire. Thanks[/quote


What? Why do they want to change or add conductors. With paralleled conductors the conductors must all be the size size and type. 500cu is fine for 400 amp.

 

[ryan_618]

500cu is fine for 400 amp.

Not if the load is over 380A. 230.42, 215.2.

 

[marcb]

Not if the load is over 380A. 230.42, 215.2.[/quote

Sorry but you didn't read 310.15. This table is for dwelling unit feeders 120/2401-phase. Also see section 240.6 for equipment rated 800 amps or less. You can select the next larger size overcurrent device so 500cu is ok for a 400 amp panel.

 

[ryan_618]

Not if the load is over 380A. 230.42, 215.2.[/quote

Sorry but you didn't read 310.15. This table is for dwelling unit feeders 120/2401-phase. Also see section 240.6 for equipment rated 800 amps or less. You can select the next larger size overcurrent device so 500cu is ok for a 400 amp panel.

I understand that, but it doesn't change the fact that 500 is only a 380 amp conductor. Conductors must be sized to carry the load that they serve. 240.6 and 240.4(B) have nothing to do with the ampacity of the conductor, just the OCPD.

 

[hey_poolboy]

Yes, you can select the next higher OCPD, but the wire must be sized for the load. If the load is over 380, then you must increase wire size, or take some other action to handle the load.

Man, Ryan beat me!

 

[infinity]

I understand that, but it doesn't change the fact that 500 is only a 380 amp conductor. Conductors must be sized to carry the load that they serve. 240.6 and 240.4(B) have nothing to do with the ampacity of the conductor, just the OCPD.

I agree with Ryan and Poolboy. The rounding up provisions of the code don't permit you to have a connected load greater than the ampacity of the conductors.

 

[LarryFine]

Obviously there is less resistance in the smaller wire.

It's obvious to me that the larger wire has less resistance.

 

[iwire]

Have a customer who wants to add a 350 run of copper to an exisiting 400 A residential service which currently is fed with as single run (3 conductors) of 500 MCM copper. These runs would be paralleled from the electric company's handhole up to the line side of the meter can.

Assuming those conductors are under NEC control you can not do that.

It is a violation of 310.4

310.4 Conductors in Parallel.
Aluminum, copper-clad aluminum, or copper conductors of size 1/0 AWG and larger, comprising each phase, neutral, or grounded circuit conductor, shall be permitted to be connected in parallel (electrically joined at both ends to form a single conductor).

The paralleled conductors in each phase, neutral, or grounded circuit conductor shall
(1) Be the same length

(2) Have the same conductor material

(3) Be the same size in circular mil area

(4) Have the same insulation type

(5) Be terminated in the same manner
Where run in separate raceways or cables, the raceways or cables shall have the same physical characteristics. Conductors of one phase, neutral, or grounded circuit conductor shall not be required to have the same physical characteristics as those of another phase, neutral, or grounded circuit conductor to achieve balance.

Where equipment grounding conductors are used with conductors in parallel, they shall comply with the requirements of this section except that they shall be sized in accordance with 250.122.
Conductors installed in parallel shall comply with the provisions of 310.15(B)(2)(a).

Will he have a problem with one run "hogging" the load or will the dis-similar conductors work fine?

Yes one run would end up carrying more than 1/2 the load.

Obviously there is less resistance in the smaller wire.

Actually there is less resistance in the larger conductor and it would end up carrying more of the load.

If that would cause an overload of the larger set is hard to say but the NEC does not allow us to take that chance.

 

[Pierre C Belarge]

I am curious... what is the purpose of installing the additional wiring?

 

[cpal]

I am curious... what is the purpose of installing the additional wiring?

Ah that is the question

Charlie

 

[Dirk Diggler]

I understand that, but it doesn't change the fact that 500 is only a 380 amp conductor. Conductors must be sized to carry the load that they serve. 240.6 and 240.4(B) have nothing to do with the ampacity of the conductor, just the OCPD.

I think marb is correct. Look at table 310.15. The electrician could use 400 kcmil thhn if he wants. The table is for "single-family dwelling services and feeders. 120/240 only.

 

[hey_poolboy]

yes, but that does not negate the fact that you can not augment a set of conductors with a parallel set that does not meet the requirements for paralleling.

 

[iwire]

I think marb is correct. Look at table 310.15. The electrician could use 400 kcmil thhn if he wants. The table is for "single-family dwelling services and feeders. 120/240 only.

If the service fits the requirements for Table 310.15(B)(6) than I have to agree that does in fact allow 400 CU to be treated as a 400 amp conductor. We could load the service supplied by it to 400 amps non continuous.

This is a different from 240.4(B) that only changes the OCP not the ampacity.

If I recall Charlie the Utility guy had backed that up as well.

But.....no one has said this is a 240 service.

 

[Dirk Diggler]

I understand that, but it doesn't change the fact that 500 is only a 380 amp conductor. Conductors must be sized to carry the load that they serve. 240.6 and 240.4(B) have nothing to do with the ampacity of the conductor, just the OCPD.

Look at table 310.15. Single family services and feeders 120/240. The NEC allows 400 kcmil for a 400 amp service.

 

[iwire]

Look at table 310.15. Single family services and feeders 120/240. The NEC allows 400 kcmil for a 400 amp service.

Bill...opps....sorry, forgot you where 'undercover', Dirk no one has said this is a 240 service.

 

[Dirk Diggler]

Bill...opps....sorry, forgot you where 'undercover', Dirk no one has said this is a 240 service.

Bill? Undercover? Did you have a little too much to drink? The original post said residential. ****

 

[ryan_618]

Bill, er, I mean, incognito Bill, have you never seen a 3-phase residential service? Granted, they aren't as popular as single pahse, but they certainly exist.

 

[iwire]

The original post said residential.

Yes, I agree, but they did not say it was 240 volt.

Residential services can be single phase 208.

 

[iwire]

have you never seen a 3-phase residential service? Granted, they aren't as popular as single phase, but they certainly exist.

Agreed, also in my area in densely populated areas it is not uncommon for the utility to provide 208 single phase to each dwelling.

They run a 3 phase 4 wire network and balance the load of a bunch of dwelling off of that.