90 amps in detached garage? Input please...

[StumpFJ40]

Hi guys,
Long time reader, sporadic poster looking for some advice.

I have a line on some 4 AWG THWN copper wire (approx. 800ft for $325) and have a plan to run it to my detached garage.
I have read tons of threads on here about power to detached garages and have soaked in all the information and I am caught in some analysis paralysis so I need some help/validation/criticism of my plan. I am anxious to pull the trigger an this project so I can do some real work in the garage (currently one 20amp 120v fused box providing power)

Load requirements: 24x24 2 car detached garage, 220v (15amp motor) 5hp 60gal compressor, Lincoln 135sp MIG, 220v 30amp plasma/TIG/stick welder), fluorescent lighting, various 120v saws, grinders etc. Very little muliple simultaneous tool usage (work alone).

I have 200 amp service to the house with 75*C breakers and a Square D homelite 100amp panel with 75*C breakers in the garage. Oh, My township follows the 2005 NEC.

My plan:
1: Install 90 AMP breaker in house main panel
2. Run 4 lengths of 4 AWG THWN (in conduit the entire run) through the basement and crawlspace, which is about 70 feet with one 90* bend then 18" underground for 50 feet through the yard to the garage. Total run in conduit: approx 130ft
3. Install 2 8' local ground rods for garage panel

Do you see any show stoppers here? If so, what are they and what would you do differently.

Cheers-
-Stump

 

[tfi racing]

Unless you have a large AC or space heating load,90A should be more than adequate.

 

[oleguy]

#4 thwn is only rated for 85 amps.

 

[StumpFJ40]

Tfi- Thanks for the response.
No electric heater... I may install a Mr. Heater (or equivalent) propane heater next fall, but that's it.

My main concern is the 4 gauge wire and 90amps.

 

[StumpFJ40]

#4 thwn is only rated for 85 amps.

Which means I can go up to the next larger breaker... correct?

Do I need to derate the wire as a result of the length of the run and the conduit?

 

[Torque1st]

Check this in the current code and check your feeder size to your present panel.

From 2002 NEC:
==============

Table 310.15(B)(6) Conductor Types and Sizes
for 120/240-Volt, 3-Wire, Single-Phase Dwelling
Services and Feeders.
Conductor (AWG or kcmil)
Copper ----- Al or Copper-Clad A -------- Service or Feeder
----------------------------------------------- Rating(Amps)
======================================
4 		2 				100
3 		1 				110
2 		1/0 				125
1 		2/0 				150
1/0 		3/0 				175
2/0 		4/0 				200
3/0 		250 				225
4/0 		300 				250
250 		350 				300
350 		500 				350
400 		600 				400

(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services
and Feeders. For dwelling units, conductors, as listed
in Table 310.15(B)(6), shall be permitted as 120/240-volt,
3-wire, single-phase service-entrance conductors, service
lateral conductors, and feeder conductors that serve as the
main power feeder to a dwelling unit and are installed in
raceway or cable with or without an equipment grounding
conductor. For application of this section, the main power
feeder shall be the feeder(s) between the main disconnect
and the lighting and appliance branch-circuit panelboard(s).
The feeder conductors to a dwelling unit shall not be required
to be larger than their service-entrance conductors.
The grounded conductor shall be permitted to be smaller
than the ungrounded conductors, provided the requirements
of 215.2, 220.22, and 230.42 are met.

 

[Aceman]

Why are you posting that table Torque? Are you wanting him to check the wire size to his homes service since that table doesn't apply to detached garage feeders...?

Which means I can go up to the next larger breaker... correct?

Do I need to derate the wire as a result of the length of the run and the conduit?

Yes, you can go to the next larger breaker provided the load is under 85 amps. And it looks like you're going to be well under that...

No, to your second question.

 

[nate379]

90 amps seems like alot of juice for a small garage. My whole house AND garage is on 100 amp service and really that is overkill.

That is a great price on the copper, but nothing that I know of stops you from putting a 50 or 60 amp breaker on it.

My old shop had 30 amps running to it and that was fine. I did have to run my welder off the house dryer outlet though.

 

[Norcal]

4 AWG with a 90 ampere overcurrent device is just fine, there would be be issues w/ a 100A breaker, since Table 310.15(B)(6) (As posted above) does not apply for a subfeeder.

 

[Torque1st]

There are many ongoing disagreements on the applicability of that table. I won't argue the subject here. It is his local inspector that would be the determining factor.

 

[Norcal]

Table 310.15(B)(6) Conductor Types and Sizes
for 120/240-Volt, 3-Wire, Single-Phase Dwelling
Services and Feeders.
Conductor (AWG or kcmil)
Copper ----- Al or Copper-Clad A -------- Service or Feeder
----------------------------------------------- Rating(Amps)
======================================
4 2 100
3 1 110
2 1/0 125
1 2/0 150
1/0 3/0 175
2/0 4/0 200
3/0 250 225
4/0 300 250
250 350 300
350 500 350
400 600 400

(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services
and Feeders. For dwelling units, conductors, as listed
in Table 310.15(B)(6), shall be permitted as 120/240-volt,
3-wire, single-phase service-entrance conductors, service
lateral conductors, and feeder conductors that serve as the
main power feeder to a dwelling unit and are installed in
raceway or cable with or without an equipment grounding
conductor. For application of this section, the main power
feeder shall be the feeder(s) between the main disconnect
and the lighting and appliance branch-circuit panelboard(s).
The feeder conductors to a dwelling unit shall not be required
to be larger than their service-entrance conductors.
The grounded conductor shall be permitted to be smaller
than the ungrounded conductors, provided the requirements
of 215.2, 220.22, and 230.42 are met.


I would say that art. is quite clear, a subfeed to a outbuilding is not permitted to use the reduced size conductors allowed by that article.

 

[StumpFJ40]

90 amps seems like alot of juice for a small garage. My whole house AND garage is on 100 amp service and really that is overkill.

That is a great price on the copper, but nothing that I know of stops you from putting a 50 or 60 amp breaker on it.

My old shop had 30 amps running to it and that was fine. I did have to run my welder off the house dryer outlet though.

I agree that 90 amps seems like a lot for a small shop, but... I want to ensure I have enough juice to be able to run my Plasma cutter 40A and compressor 15A at the same time with all the lights on and radio blasting. I'd hate to spend all the time upgrading and wind up needing more.

Right now, I have to push my belt sander to get it moving when I start it up (120v 20A power) and I'm over it.

Thanks all for helping me out with this decision! I feel better with my decision.

Cheers-
-Stump

 

[Torque1st]

Norcal- As I said, there are disagreements on the subject. You obviously interpret the code one way. His local inspector is the one to ask. The OP would only have to change out the breaker to go one way or the other. The difference is insignificant in this case. There is a big difference in a branch circuit or a feeder and the protections they require.

I am not an electrician. I have always used the various codes with the "engineering supervision" passages it contains. Most of the time I have been working with industrial machinery and MCC installations but the few times I have had to deal with inspectors they have been cooperative.

 

[walrus]

I would say that art. is quite clear, a subfeed to a outbuilding is not permitted to use the reduced size conductors allowed by that article.

While I agree its quite clear, I've seen inspectors in Maine let it go and allow #2 alum on 100 amp breakers. The load was nowhere near 100 though

 

[StumpFJ40]

Table 310.15(B)(6) Conductor Types and Sizes
for 120/240-Volt, 3-Wire, Single-Phase Dwelling
Services and Feeders.
Conductor (AWG or kcmil)
Copper ----- Al or Copper-Clad A -------- Service or Feeder
----------------------------------------------- Rating(Amps)
======================================
4 2 100
3 1 110
2 1/0 125
1 2/0 150
1/0 3/0 175
2/0 4/0 200
3/0 250 225
4/0 300 250
250 350 300
350 500 350
400 600 400

(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services
and Feeders. For dwelling units, conductors, as listed
in Table 310.15(B)(6), shall be permitted as 120/240-volt,
3-wire, single-phase service-entrance conductors, service
lateral conductors, and feeder conductors that serve as the
main power feeder to a dwelling unit and are installed in
raceway or cable with or without an equipment grounding
conductor. For application of this section, the main power
feeder shall be the feeder(s) between the main disconnect
and the lighting and appliance branch-circuit panelboard(s).
The feeder conductors to a dwelling unit shall not be required
to be larger than their service-entrance conductors.
The grounded conductor shall be permitted to be smaller
than the ungrounded conductors, provided the requirements
of 215.2, 220.22, and 230.42 are met.


I would say that art. is quite clear, a subfeed to a outbuilding is not permitted to use the reduced size conductors allowed by that article.

So... I gather the key word here is DWELLING and since the garage is an outbuilding this table does not apply.

I am amazed at how much is left to individual interpretation in a document (NEC) that is recognized as the definitive authority for electrical wiring.

 

[Torque1st]

Some see the key word as FEEDER...

As I said there are disagreements. You can find them with Google if you want.

The NEC is not the definitive authority on electrical wiring, never has been. Some people seem to think so. Localities are even free to modify it or to ignore it.