Secondary conductor sizing and Table 450.3(B)

[ShockedMonkey]

So I just watched Mike's DVD and am unsure of the code's intent with regard to T450.3(B)

Based on Table/DVD example:

75 KVA xformer delta/wye 3Ph 120/208vac secondary (480v Delta primary)

75,000/360v = 208.3 amps

208 amps x 125% = 260.4 amps i.e. have to jump to a 300 amp breaker in the 208 PB

conductors sized at 300 Amps 75c= 350KCM (seems crazy overkill to me if I only need a 225 amp panel installed)

1. Now my question does the code allow (and if so where) for the following example:

75 KVA xformer (same as above ex.)

120/208v Panelboard Calculated load (incl continuous loads etc) total = 180 Amps

180 Amps X 125% = 225 amps

conductors at 75C = 4/0 secondaries to the same 75KVA xformer

Thank you.

 

[augie47]

Certainly allowed. MH's example was for the maximum allowed secondary based on applying the 125% secondary protection rule.
There ares some restrictions on the minimum size such as the 10% rule in 240.21(C)(2), but what you propose is allowed.

 

[infinity]

1. Now my question does the code allow (and if so where) for the following example:

75 KVA xformer (same as above ex.)

120/208v Panelboard Calculated load (incl continuous loads etc) total = 180 Amps

180 Amps X 125% = 225 amps

conductors at 75C = 4/0 secondaries to the same 75KVA xformer

Thank you.

This is pretty standard for a 75 kva. You could even use a #3/0 with a 200 amp OCPD and the transformer would have a little extra headroom.

 

[ShockedMonkey]

Certainly allowed. MH's example was for the maximum allowed secondary based on applying the 125% secondary protection rule.
There ares some restrictions on the minimum size such as the 10% rule in 240.21(C)(2), but what you propose is allowed.

Thank you Augie. I just noticed the "Maximum" as I was fuming.

So to continue with the minimum based on 240.21(c)(2)(4)

Would this be correct:

75KVA (same as above)

75,000/831.3v = 90 amps

90a X 125% = 112amps

125 Amp primary side 480v Breaker

Primary to secondary ratio is 480/208 = 2.3 (unsure if this is correct)

125 x 2.3 = 287.5 amps

287.5 X 10% = 28.75 amps

Minimum possible wire size per 240.21(c)(2)(4) would be 30amps @ 60C = 10ga ?

Is that correct?

Thank you!

 

[ShockedMonkey]

Thank you Augie. I just noticed the "Maximum" as I was fuming.

So to continue with the minimum based on 240.21(c)(2)(4)

Would this be correct:

75KVA (same as above)

75,000/831.3v = 90 amps

90a X 125% = 112amps

125 Amp primary side 480v Breaker

Primary to secondary ratio is 480/208 = 2.3 (unsure if this is correct)

125 x 2.3 = 287.5 amps

287.5 X 10% = 28.75 amps

Minimum possible wire size per 240.21(c)(2)(4) would be 30amps @ 60C = 10ga ?

Is that correct?

Thank you!

Can anyone confirm this?

This is hypothetical, I'm just trying to make sure I am interpreting the code correctly regarding the minimum 240.21(c)(2)(4)

Thanks

 

[electrofelon]

Can anyone confirm this?

This is hypothetical, I'm just trying to make sure I am interpreting the code correctly regarding the minimum 240.21(c)(2)(4)

Thanks

First just a note on you first example, in case you were not aware: You would not need any secondary transformer protection at all if the primary protection was 125% or less. You didnt say what the primary protection was so throwing that out there. Note you still need to protect the Panelboard (if using one) and the secondary conductors so you will have some sort of OCPD on the secondary which, for typical installs, will "protect the transformer" anyway whether you need to or not.

Regarding the second question: Yes I also get 28.8 amps. Note that some of us think you can use the 90 degree ampacity for this value.

 

[ShockedMonkey]

First just a note on you first example, in case you were not aware: You would not need any secondary transformer protection at all if the primary protection was 125% or less. You didnt say what the primary protection was so throwing that out there. Note you still need to protect the Panelboard (if using one) and the secondary conductors so you will have some sort of OCPD on the secondary which, for typical installs, will "protect the transformer" anyway whether you need to or not.

Regarding the second question: Yes I also get 28.8 amps. Note that some of us think you can use the 90 degree ampacity for this value.

Thank you! Appreciate it.

I'm assuming your 125% is coming from table 450.3(B)

The cushion between the 75c and 90c table always gave me an unsubstantiated level of comfort for some reason. I'd have to hear more.

 

[charlie b]

Let me just comment on this part of your question:

120/208v Panelboard Calculated load (incl continuous loads etc) total = 180 Amps
180 Amps X 125% = 225 amps

If you calculated the load by properly taking continuous loads into account (i.e., included them at 125%), then you don't have to apply the 125% factor to the overall calculation. That is, you don't have to double-count the 125% factor.

 

[ShockedMonkey]

Let me just comment on this part of your question:If you calculated the load by properly taking continuous loads into account (i.e., included them at 125%), then you don't have to apply the 125% factor to the overall calculation. That is, you don't have to double-count the 125% factor.

Good to know. Thank you!